Grounding Fundamentals Course Presentation 694f3d

Grounding Fundamentals Instructor: Allan Bozek, P.Eng. www EngWorks ca www.EngWorks.ca

1. 1 5 EICCEUs

Introduction • •

•

• •

•

Introductions Please introduce yourself – name, job title and experience sheet circulated, everyone please sign in and return Emergency response requirements Please turn off all cell phones or turn to silent mode Washrooms and Breaks

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Grounding Fundamentals

2

Safety Topic Static Electricityy and Refuelling g

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Safety Topic Static Electricityy and Refuelling g 

Some statistics  Petroleum Equipment Institute reports 175 fires since 1992  50% of the accidents occurred when the refueler returned to their vehicle  Women for 75% of all static ignition fires



Safety Guidelines when refueling    



Turn off engine D 't smoke Don't k Never re-enter your vehicle while refueling. Do not overfill or top off your tank

If a fire starts  Do not remove the nozzle from the vehicle or try to stop the flow of g gasoline. Immediately y leave the area and call for help

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Learning Objectives 1. To understand why we ground 2 To describe the difference between grounding and 2. bonding 3. To apply the safety requirements as defined by the Canadian Electrical Code and the IEEE as they relate to grounding 4. To select the appropriate systems grounding scheme for an industrial facility  

Sizing of components How it impacts the overall design of a facility

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Learning Objectives 5. To implement a static electricity control and lightning g gp protection system y 6. To avoid the problems typically associated with the grounding g g of sensitive electronic systems y  

Ground loops Methods of noise mitigation

7. To design a ground grid for a high voltage industrial substation 

Concept of ground potential rise and touch and step potential

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Agenda Overview 1. History of Grounding 2 System grounding 2.  Generator and UPS systems grounding

3. 3 4. 5 5. 6.

Equipment bonding Static Protection Lightning Protection Grounding of Electronic and Instrumentation Systems 7. Station Ground Grid Systems Design 8 Tutorial 8. T t i l www.EngWorks.ca


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Introduction Section 1

Edison's Pearl Street Generation Station 

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Pearl Street Generation station was initially constructed in 1882 to provide DC current for lighting systems in New York's financial district


9

Edison’s Floating Approach to DC Systems y 



Original design used an earth ground for DC lighting systems Several incidents associated with “stray currents” forced Edison to revise his plan currents  One dead horse  Workers W k nearby b the th generating ti station t ti could ld ffeell the current Believed the there was a “devil devil in the wire wire”

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Edison’s Floating Approach to DC Systems y Current Flow + G Gen -

Intended Return Path

L

L

Unintended Return Path

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Human Sensitivity to Electricity 

Physiological y g Reaction to Electric Current  Range from minor muscular contraction to ventricular fibrillation  Function of body y weight g  Current magnitude  Current duration

H Human b body d can b be considered id d a 1000Ω resistor i t www.EngWorks.ca


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Human Sensitivity to Electricity Human Response

Direct Current (ma)

Alternating g Current (ma)

Men

Women

Men

Women

1

0.6

0.4

0.3

“Let Go” Threshold

6.2

3.5

1.1

0.7

Shock – Not Painful

9

6

1.8

1.2

Painful Shock – Muscular Control Loss

62

41

9

6

Severe Shock – Breathing Difficult

90

60

23

15

Slight Sensation on Hand

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Edison's’ Isolated 3 Wire System 

Edison later adopted a 3wire system y that did not rely on a earth path for return

Positive

+ 100V

G1

 Allowed two circuits to be run with three wires +  Circuit was isolated from ground 100V G2  All currents within the circuit could be measured and ed for

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L

L Neutral

L

200V

L

Negative

14

Shock Current Path 



A shock current path requires q two p points

Single point of

 One point for the current to enter and the second to exit  Voltage difference is required for current to flo flow An isolated system under normal operating conditions insures a single point of

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L

G1 N Neutral l

Isolated Ground S t System No Shock Current Exists


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Shock Current Path 

Under fault conditions, an isolated system ground creates a shock hazard

Single point of t t

Accidental A id t l Ground

L

G1 Neutral

Alternate circuit path leads to shock hazard

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Ground Fault Detection An isolated system cannot detect the presence of a ground fault

Circuit protection cannot detect the accidental ground

Accidental Ground

Fuse



L

G1 Neutral

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System Overvoltage and Surges 

An isolated system cannot dissipate a high voltage surge  Usually results in equipment damage

Equipment insulation is stressed as the high voltage surge finds its way to ground

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Fuse

Lightning Strike

L

G1 Neutral


18

The Intentional Grounding of Circuits 

Elihu Thompson  Founder of Thompson-Houston Industries  Later merged with Edison General and became General Electric

 Author of over 700 patents 

Advocated AC systems should be intentionally earthed  Proposed as a safeguard against a breakdown in insulation of a primary circuit conductor  Proposal created a large amount of controversyy

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Grounding Premise



An intentionally grounded circuit provides a circuit path back to the source in the event of an p accidental ground Allows the circuit p protective devices to function preventing the circuit from becoming a safety hazard

Low Impedance path to source allows fuse to operate

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Accidental Ground

Fuse



L

G1 Neutral


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History of Grounding 

Practice of earthing the secondary (neutral) conductor was banned by the New York Board of Fire Underwriters  Speculation p that Thomas Edison was behind the scenes with his patented 3 wire un-grounded circuit



AIEE (Precursor to the IEEE) recommended that low voltage AC systems be grounded where a reliable ground connection could be secured  Advocated a solid connection without a fuse on the neutral t l wire i

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History of Grounding 

NFPA later resolved that grounding the secondary circuit was the only way of absolutely insuring the safety f t off the th circuit i it  The debate continued from 1903 – 1913 when it was ed into law  Secondaries of all circuits 550V or less must be grounded  Recommended that all circuits 300V or less be grounded



Original rule has not been changed in substance since the original 1913 rule in the NEC  Section 10 of the CEC Part 1 also adheres to the fundamental premise of the rule

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Canadian Electrical Code - Part 1 CSA C22.1-06 

Minimum safety standards for installation and maintenance of electrical equipment  Compliance will ensure a safe installation



Section 10 deals specifically with grounding and bonding  Significant re-write in 2006  Minor updates p in 2009

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Canadian Electrical Code - Part 1 CSA C22.1-06 

Scope and object: Rules 10-000 and Rule 10-002  Protect life from the danger g of shock  Limit the voltage on a circuit  Facilitate operation of protective devices



System and circuit grounding: Rules 10-100 to 10-116  All circuits must be grounded with the exception of: Electric Arc furnaces Cranes installed in Class III locations Isolated systems in patient care areas Circuits less than 50V

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CEC Handbook 





Provides background information and commentary on the th Rules R l off th the Canadian C di Electrical Code, Part I Intended to provide a clearer understanding of the safety requirements of the Code I Incorporates t information i f ti on:  Rational  Intent  Field Considerations

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IEEE Standard 142 ((Green Book)) 

Recommended practices and methods associated with grounding     



Systems grounding Equipment grounding and bonding Static and lightning protection Grounding electrode design Grounding of electronic equipment

Applies to industrial and commercial power systems  Utility grounding methods are not covered

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Recommended Purchase Grounding Fundamentals

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Commonly Used Grounding and Definitions Neutral Conductor

Neutral Point

Neutral Ground Device Grounding Conductor Grounding Electrode www.EngWorks.ca

Metallic Enclosure Bonding Conductor

Stray Current

Earth Grounding Fundamentals

27

Earth 

Conducting body of varying resistance  Earthing – A connection to earth Interchangeable with the term ground

Earth www.EngWorks.ca


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Ground 

A conducting connection by which an electrical circuit is connected to earth  Grounding Electrode – a conductor buried in earth and used for collecting or dissipating ground current to earth  Grounding g Conductor – conductor used to connect the service equipment to a ground electrode

Grounding G g Conductor Grounding Electrode www.EngWorks.ca


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Bonding 

Low impedance path created by ing all noncurrent-carrying metal parts to ensure electrical continuity ti it  Bonding Conductor – conductor that connects the noncurrent carrying parts of electrical equipment, raceways or enclosures

B di Conductor Bonding C d t (Equipment ground conductor)

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Neutral Point 

The point of a symmetrical system which is normally at zero voltage g  Neutral Conductor – a system conductor, other than a phase conductor that provides a return path for current to the source Neutral Point

Neutral Ground Device

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Neutral Conductor


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Definitions 

Ground Fault Current – ground current resulting from anyy p phase-conductor-to-earth fault  Normal – brief flow of current that occurs until the protective device opens  Abnormal – continuous flow of current from a phase conductor to ground  Often referred to as the Zero Sequence Current



Neutral grounding devices include grounding resistors, grounding transformers, ground-fault ground fault neutralizers, reactors, capacitors, or a combination of these components

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Ground Fault Current

Neutral Ground Device

Metallic Enclosure Intended G Ground dF Fault lt Current Path Ground Fault

Earth www.EngWorks.ca


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Stray Current 

The uncontrolled flow of current over and through the earth  results in undesired safety and system performance characteristics

Stray Current Earth www.EngWorks.ca


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Systems Grounding Section 2

Purpose of a Systems Ground “System grounding, or the intentional connection off a phase h or neutral t l conductor d t to t earth, th is i for f the purpose of controlling the voltage to earth, or ground ground, within predictable limits” limits IEEE 142 Green Book 

Most system faults are ground fault related

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Systems Ground 

A systems ground will:  Control the voltage g to g ground to p prevent stressing g equipment insulation  Allow the operation of ground fault detection protection d i devices  Reduce the risk of fire and shock hazard to persons who might come in with live conductors  In some cases provide service continuity  Allow the ground fault to be isolated and repaired at a convenient time ti

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Concept of a System Ground 

A grounding system consists of all interconnected grounding connections in a specific power system and is isolated from adjacent; grounding systems through a high impedance  Isolation occurs via an ungrounded transformer winding connection

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Systems Ground 1

Y

Systems Ground 2

Y Y

Y

Systems Ground 3

Y

M


PP

M

M

Systems Ground 4

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Transformer Winding Connections 

∆ (delta) Connections  Isolates the p power system y from ground  Important is creating “zones of protection”



Y (wye) Connections  Y point provides a neutral point for managing ground faults  Opportunity for multiple voltages

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System Grounding Classifications

Ungrounded

Resistance Ground

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Solid Ground

Reactance Ground


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System Grounding Classifications Systems Grounding Ungrounded

Grounded

Impedance Grounded Resistance Low Resistance

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High Resistance

Solid Grounded Reactance Reactance

Tuned Reactance


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Ungrounded 

Historically was used on power systems where a high g level of p process continuity y was required q  Exists in many process facilities designed prior to 1980



Advantages g  Single ground fault does not does not allow current to flow  Allowed for a controlled shutdown for fault repairs

 Eliminates the need for f elaborate protection schemes  Grounding system cost is minimized A N G

B C

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Ungrounded 

Disadvantages  On a g ground faults,, the voltage g to g ground for the remaining g phases is elevated by 73%  Higher insulation rating required for system components

 Transient T i t overvoltages lt can be b a problem bl  Voltages up to 6X system voltage stresses insulation eventually leading to a second ground fault and subsequently a phase to phase fault

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Ground Fault Voltage Shift Normal Operating Conditions

A

A

VAG

IA

IA

N

IB

VCG

B IB CA

CB

C

C CC

IC

VCN

IC

B A

VAN N

VAG

G

VAB

VCA VCG

VBN

If CA = CB = CC then IA+ IB + IC = 0 www.EngWorks.ca

VBG


C

N G VBG VBC

B

44

Ground Fault Voltage Shift Ground Fault Phase C

A

IG VAG

A

IB

IA

IA

N

C G VCG=0

B IB

IG

CA

C

VBN

N

VAN

A

CB VCA

VCN

G

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B

VBG

IA + IB = IG

VAG

VAB

N

VCG=0

C G Grounding Fundamentals

VBC VBG

B

45

Intermittent Ground Faults  A IA

N B IB

IG G

CA

C

CB

Breakdown in insulation results in phase to phase fault IG = ISC

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Intermittent or restriking type yp g ground faults on isolated grounded systems can cause severe system overvoltages  Up to 6 or 8 times line to line voltage  Will eventually lead to an g in insulation failure resulting a phase to phase fault  Must be detected and corrected ASAP


46

Ungrounded System Ground Fault Detection Scheme 10-106 Alternating-current systems (see Appendix B) ((2)) Wiring g systems y supplied pp by y an ungrounded g supply pp y shall be equipped with a suitable ground detection device to indicate the presence of a ground fault.

Ground Fault

L

L

L

0V

Light Dims Or Extinguishes

Ground fault Detection Scheme www.EngWorks.ca


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Solid Ground 

A solid grounded system is one in which the neutral points have been intentionally connected to earth ground with a conductor having no intentional impedance  Often referred to as effective grounding

A N G

B C

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Solid Ground

Uniground System Used in Industrial Systems

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Multi-grounded System Used by Utilities in Rural Distribution Systems


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Solid Ground 

Advantages  Partially reduces the problem of transient over-voltages  Reduced R d d insulation i l ti llevell required i d

 Ground faults do not shift the system neutral  Simple ground relay schemes provide for circuit protection 

Disadvantages  Damage at the fault may be excessive  Arc flash hazard due to high ground fault current levels  Difficult to coordinate ground fault protection  Magnitude of the fault current is unknown

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CEC Definition 



Effective Grounding - a path to ground from circuits, equipment, q p , or conductor enclosures that is permanent and continuous and has carrying capacity ample to conduct safely any currents liable to be imposed upon it CEC Rule 10-500 in Appendix B states that the complete fault path of the circuit conductor conductor, together with the bonding return return, should have an impedance that allows at least five times the current setting of the overcurrent device to flow when a fault of negligible impedance occurs

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Solid Ground A N B C

O/C fuse may not clear arcing ground fault

IG G

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High Impedance Ground fault


VBN

N

VAN

VCN

52

High Resistance Ground 

System is grounded through a high-impedance resistor  High-impedance grounding typically limits ground fault current to 25 A or less  Typically used on low voltage (600V or less) systems under 3000 Amps A N 2 - 25A

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G

B C


53

High Resistance Ground Scheme

1000 KVA Xfmr 25kV – 600V 5.75% Z

AL

51G

Y

5 Amp NGR

NGR 5A

M 150HP Cooling Ground Fan

Fault www.EngWorks.ca

LP

U/H

45kVA

Lighting

30kW Unit Heater


Starter

75kVA

X2

HTP Heat Trace

M

M

75HP Recycle Pump

25HP Injection u p Pump

X2

54

High Resistance Ground 

Advantages  Allows system y to operate p under a g ground fault condition  Reduces arc flash energy associated with a ground fault  Insures a ground fault of a known magnitude  Aids in protective relay coordination and limiting equipment damage

 Reduces transient ground fault overvoltages  Allows easy identification and isolation of the ground fault location 

Disadvantages  Neutral shift on ground fault

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Low Resistance Ground 

System is grounded through a low-impedance resistor  Low-resistance grounding typically limits ground fault current to 400A or less for a short period of time (10 sec)  Typically used on medium and high voltage industrial power distribution systems A N 25 - 400A

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G

B C


56

Low Resistance Ground 

Advantages  Allows p protective relay y devices to q quickly y clear a g ground fault  Limits damage to equipment and reduces overheating and mechanical h i l stress t on conductors d t



Disadvantages  Neutral Ne tral voltage oltage shift of limited d duration ration

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Low Resistance Ground Scheme Trip Upstream Breaker 51 NGR 400A

Trip setting ~ 20% of NGR rating

Y

Trip Downstream Breaker

400A NGR 13.8kV

M

51 XFMR

SGR (Secondary Ground Resistor)

Y 5A NGR

600V

Alternate Arrangement M

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M

58

LR Grounding Resistor Connection to Neutral Point on Transformer

Connection to ground

Resistors Current Transformer

51 NGR 400A

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LR Grounding Transformer Ground resistor

51 XFMR

SGR (Secondary Ground Resistor)

Grounding Transformer

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Alternate Grounding Schemes 

Corner-of-the-Delta System  Applicable pp to low-voltage g systems  Not widely used in industrial systems t



A

I B C

I

Delta One Phase Grounded at Midpoint  Applicable to single phase 120/240V loads

240V 120V

120V G

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240V

61

Reactance Grounding 

Ground fault current is a function of the neutral reactance  Typically results in higher ground fault currents than a resistance grounded system  25 5 – 60% of o three t ee phase p ase fault au t current

 Primarily used by Utilities on multi grounded systems on multi-grounded systems above 5kV  Seldom used in industrial plant applications www.EngWorks.ca


N

51 Reactor

IG

62

Resonant Grounding 

Tuning reactor is used to ground the neutral point to ground  Reactor is tuned to match the system capacitance

N

 Results in a very low value of ground fault current  75% of line to ground faults are selfextinguishing

 C Complex l controls t l are required i d tto constantly match the reactance to the system capacitance  Primarily Pi il used d on overhead h d and d transmission lines above 15kV  Rarely used in industrial applications

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51 Reactor Ground Fault Neutralizer


IG

63

Grounding System Comparison Ung grounded

Solid Ground

Low Resistance

High Resistance

Immunity to transient overvoltages

Worst

Good

Good

Best

Arc Fault Damage Protection

Worst

Poor

Better

Best

Safety to Personnel

Worst

Better

Good

Best

Service Reliability

Worst

Good

Better

Best

Continued operation after initial ground fault

Better

Poor

Poor

Best

Not Possible

Good

Better

Best

Condition

Ground fault locating

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Ground Fault Sensing

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Ground Fault Sensing 

Ground Return  Ground fault current is measured in the neutral to ground connection  Applicable only at a source transformer or 51G generator g  Often used for ground fault alarm sensing on LV di t ib ti systems distribution t

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Phase A Neutral Phase B Phase C

66


Zero Sequence Relay  Measures zero sequence or ground currents by sensing the magnetic fields surrounding th phase the h and d neutral t l conductors


Should cancel under normal conditions

 Often used in motor protection and feeder breaker relays

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51G

67


Differential  Phase current and neutral current values are measured and ground fault current is calculated l l t d as th the diff difference  Used in applications where current transformers are required for phase overcurrent relays  High Hi h accuracy iin d detecting t ti ground faults

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51G

68

High Resistance Ground Detection Scheme

NGR 5A

Pulsing reading on phase indicates ground fault

AL

Y Pulsing Resistor

Clamp On CT

45kVA M

150HP Cooling Ground Fan

Fault www.EngWorks.ca

1000 KVA Xfmr 25kV – 600V 5.75% Z

5 Amp NGR

LP

U/H

51G

Lighting

30kW Unit Heater


Starter

75kVA

X2

HTP Heat Trace

M

M

75HP Recycle Pump

25HP Injection u p Pump

X2

69

High Resistance Ground fault Detection System y

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Ground Fault Relay Settings 

Alarm only on continuous rated ground resistor applications pp  Alarm setting at 80% of maximum current level allowed by ground resistor Above system charging current level



Trip on short time duty ground resistor applications  High resistance ground applications Trip at 80% of maximum current level allowed by resistor

 Low resistance g ground applications pp Trip at 20% of maximum current level allowed by ground resistor

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Low Resistance Ground Detection Scheme Trip p Upstream p Breaker 51G

NGR 400A

Trip setting ~ 20% of NGR rating

Y Trip Downstream Breaker

400A NGR

Trip

ZCT

51G

13.8kV Trip

Trip ZCT

51G

Trip ZCT

ZCT - Zero Sequence CTs

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51G

Trip ZCT 51G

ZCT

51G M

M

72

GF Relay Time Coordination Curves 



Settings for ground-fault relays can be determined during the relayy coordination studyy GF curves are plotted on the coordination diagrams  Set parameters include ti time and d currentt llevell Ground Fault coordination curves

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CEC Requirements Associated with Systems Grounding

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CEC Code Requirements 10-1102 – Installation of Neutral Grounding Devices 1) Neutral grounding devices can only be installed on systems where line to neutral loads are not served No single phase loads from a resistance grounded system

2) S Systems t with ith voltages lt > 5kV shall h ll b be d de-energized i d on detection of a ground fault a)) Electrical systems y operating p g at 5 kV or less are p permitted to remain energized if the ground fault current is controlled at 10A or less i.

Audible alarm is required

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CEC Code Requirements 3) Where line-to-neutral loads are served, the system must be de de-energized energized on occurrence of a: 1) Ground fault 2) Grounded neutral on the load side of the NGR 3) Break in the continuity of the conductor connecting the NGR to ground Apparent conflict between subsection 1) and subsection 3)

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NGR with Isolated System Neutral A Trip main breaker

HT Ckt

HT Ckt

HT Ckt

HT Ckt

HT Ckt

HT Ckt

N HT Ckt

B C

51

NGR IG

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Rule 10-1102 requires the system to g on detection of be de-energized ground current


77

Neutral Ground Devices 10-1104 NGRs must be approved for the application  CAN/CSA-C22.2 No. 0.4 – Bonding and Grounding of Electrical E i Equipment (P (Protective i G Grounding) di )  CAN/CSA-C22.2 No.14 – Industrial Control Equipment  CAN/CSA-C22.2 No. 94 – Special Purpose Enclosures 

Must be continuously rated where provisions are not made to interrupt the fault  Maximum temperature allowed is 375°C





Where not continuously rated, the time rating of the device y must be coordinated with the protective devices of the system Must have an insulation voltage equal to the line-to-neutral system voltage

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78

Location of Grounding Devices 10-1106 All live parts must be enclosed  Must be placed in a location accessible to qualified personnel  Must be placed in a location where it can dissipate the heat under ground fault conditions  Warning g signs g must be p provided indicating g the system is impedance grounded and located at:  Transformer or generator, or both  Consumers service switchgear  Supply authorities metering equipment

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Grounding Conductors 

System grounding conductors must be copper  Solid g grounded systems y sized as p per CEC Table 17  Based on the ampacity of the largest service conductor



No splicing is permitted

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CEC Code Requirements

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81

NGR Conductors 10-1108 conductors connecting the NGR to the Neutral point of the system p y must be identified as white or grey  Must not be grounded  Sized to conduct the rated current of the device  No less than #8 AWG

 Conductor connecting the NGR to the system ground electrode may be insulated green or bare  Made of copper pp

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82

NGR Conductors

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Bonding of Conduit Enclosing a Grounding g or Bonding g Conductor 

Magnetic effect of metal conduit can increase the impedance of the grounding circuit by a factor of 40!  Not an issue with PVC or aluminium conduits



Problem can be mitigated by bonding the grounding conductor to the metal conduit at both ends  Allow the metal conduit to carry a portion of the ground current  New CEC rule 10-806 makes this mandatory

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84

Sizing and Specification of Neutral Ground Resistors

NGR Sizing Criteria 

NGRs are sized based on the following criteria  Charging current HRG - Maximum ground current must be greater than 3X the charging current for the system LRG – Charging current not a factor

 Temperature rise Based on how long the fault is allowed to persist – Continuous – Extended E t d d titime (1 minute) i t ) – 10 seconds

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86

NGR Sizing Criteria RNGR =

51 NGR

IG

VLL √ G √3I

XCO RNGR ≤ 3 IG ≥ 3ICO WNGR = IG2RNGR RNGR = Resistor Size ((Ohms)) IG = Maximum Ground Current (Amps) ICO = System Charging Current (Amps) WNGR = Resistor Size (Watts)

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87

NGR Sizing Criteria Secondaryy Ground Resistor

51 XFMR

SGR

RNGR RSGR = N2 VLN(Pri) N= VLN(Sec) ISGR = NIG KVA = PNGR = IGVLN(Pri)

RNGR = Equivalent Primary Resistance (Ohms) RNGR = Equivalent Primary Resistance (Ohms) IG = Maximum M i G Ground dC Currentt (Amps) (A ) ISGR = Maximum Ground Current (Amps) N = Turns ratio PNGR = Resistor Power Rating (Watts) www.EngWorks.ca


88

Charging Current - Estimation 

Resistor must be sized to ensure that the ground fault current limit is g greater than the system's y total capacitance-to-ground charging current System Voltage

Charging Current (3ICO) Amps per 1000 kVA of System Capacity

480

0.1 – 2.0

600

0.1 – 2.0

2400

2.0 – 5.0

4160

2.0 – 5.0

13800

5.0 – 10.0

Typical Charging Currents based on Voltage Level www.EngWorks.ca


89

Charging Current – More Detailed Analysis System Voltage 600V

4160V

13800V

Component Cable

Type

Typical Charging Current

3/C - 250 – 500MCM

0.15A/1000ft

3/C - #1 – 4/0AWG

0.02A/1000ft

Transformers

0.02A/MVA

Motors

0.01A/1000HP

Cable

3/C - 500–1000MCM Shielded

0.58A/1000ft

3/C – 1/0 – 350MCM Shielded

0.23/1000ft

Non Shielded

0.1A/1000ft

T Transformers f

0 05A/MVA 0.05A/MVA

Surge Suppressor.

1.35A per Set

Motors

0.1A/1000HP

Cable

3/C - 600–1000MCM Shielded

0.65A/1000ft

3/C – 250 – 350MCM Shielded

0.75/1000ft

3/C - #1 – 4/0AWG Shielded

0.65A/1000ft

Transformers

0.05A/MVA

Surge Suppressor

2 25A per Set 2.25A

Motors

0.15A/1000HP

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90

Charging Current Calculation Example p ROT → IG ≥ 3ICO IG ≥ 3(4.78A) 3(4 78A) = 14.34A 14 34A Qty

Total Ch i Charging Current

Transformer

0.05A /MVA

17.5

500MCM Cable

0.58A /1000ft

4200 ft 2.43A

250MCM Cable

0.23A /1000ft

600 ft

Surge Suppressor

1.35A /Set

1

T t l Total www.EngWorks.ca

Y

12 MVA

10A NGR

0.875A

4160V 1500ft 500MCM

Charge C Current t

Surge Suppressor

1500ft 500MCM

Component

15A 5 NGR G more oe appropriate size

0.13A Y

2MVA

Y

2A NGR

1.35A

4 78A 4.78A

2MVA

600V

Y

1200ft 500MCM

600ft 250MCM 1 5MVA 1.5MVA

Surge Suppressor M

3000HP M


M

91

Charging Current Test Procedure 

A IA



N B IB CA

CB

C CC

IC



A Ammeter 0-10A 0 10A

 Ammeter will indication charging current (3ICO) 

G

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Connect an ammeter to ground g a resistance,, switch and through a fuse Increase the resistance to maximum level and close the di disconnect t Slowly reduce the resistance to zero All three phases should be g used measured and the average as the system charging current


92

Cable Insulation Ratings on Resistance Grounded Systems y 

Low Voltage Systems (≤ 600V)  100% % insulation rating g acceptable p for all applications pp  Refer to Standata CEC 12



Medium Voltage g Systems y ((IEEE Recommendations))  100% insulation level required where clearing time will not exceed one minute  133% insulation level required where clearing time will not exceed one hour  173% insulation level required where clearing time exceeds one hour

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93

NGR Ratings 

Based on the criteria defined in IEEE 32 - Standard Requirements, Terminology, and Test Procedure for Neutral Grounding Devices  Current Current through the device during a ground fault condition

 Voltage V = IR at 25ºC May need to be de-rated at elevations above 1000m

 Frequency Circuit Voltage of System

 Service NEMA Type 1 for Indoor Applications NEMA Type 3 for Outdoor Applications www.EngWorks.ca


94

NGR Ratings 

Basic Impulse Insulation Level System Insulation Class

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Class

BIL

1.2kV 2 5kV 2.5kV 5kV 8 7kV 8.7kV 15kV 23kV

45 60 75 95 100 150 Grounding Fundamentals

95

NGR Ratings 

Time Rating and Permissible Temperature Rise under fault conditions Permissible Temperature Rise (Rise Above 30ºC Ambient)

Time Rating Ten Seconds (Short Time) (NGRs

760ºC 760 C

used with Protective Relay)

One Minute (Short Time)

760ºC

Ten Minutes (Short Time) (seldom

610ºC

specified)

Extended Time

(GF

610ºC 610 C

allowed to persist > 10min)

Steady State (Continuous)

385ºC*

*CSA permissible rise is 375ºC over 40ºC Ambient www.EngWorks.ca


96

NGR Monitoring

Broken Spot Weld

NGR Thermal Failure Broken Resistor Wire www.EngWorks.ca


97

NGR Monitor 

The NGR monitor measures changes in NGR resistance, current in the neutral, and neutral-toground voltage g g  Anomalies are detected and an alarm or trip signal is activated

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98

NGR Sizing Tutorial

NGR Sizing Tutorial 

Modular Substation incorporating  5 kV Switchgear and MCCs  600 V Switchgear and MCCs



Grounding system consists of:  Power Distribution System Ground  5kV L Low resistance i t ground d system t  600V High resistance ground system



Objective  Size the grounding resistors for the 5kV LRG system and the 600V HRG system  Assume 1.5A charging current for the 600V System  Assume 8A charging current for the 5kV System

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100

Substation Single Line

LRG NGR

Y 5kV

M

HRG NGRY

M

M

600V ~=

M

M

=

~

UPS PP

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101

NGR Sizing Tutorial Answers

Sizing the NGRs RNGR =

51 NGR

IG

VLL √ G √3I

XCO RNGR ≤ 3 IG ≥ 3ICO WNGR = IG2RNGR RNGR = Resistor Size ((Ohms)) IG = Maximum Ground Current (Amps) ICO = System Charging Current (Amps) WNGR = Resistor Size (Watts)

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103

HRG Sizing ROT → IG ≥ 3ICO

ICO = 1.5A

IG ≥ 4.5 → Choose 5A as the HRG Current Rating RNGR = RNGR =

VLL √3IG 600V √3 x 5A

RNGR = 69.3Ω

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WNGR = IG2RNGR WNGR = 5A2 x 69.3Ω WNGR = 1733watts


104

LRG Sizing ROT → IG-Trip Setting ≥ 3ICO to avoid nuisance tripping

ICO = 8A

Trip T i setting i is i roughly hl 20% off the h LRG resistor i size i IG-Trip Setting ≥ 24A to avoid nuisance tripping IG ≥ 120A → Choose Ch 125A as the h LRG Current C Rating R i RNGR = RNGR =

VLL √ G √3I 4160V

√3 x 125A

RNGR = 19.2Ω www.EngWorks.ca

WNGR = IG2RNGR WNGR = 125A2 x 19.2Ω WNGR = 300kW Grounding Fundamentals

105

System Grounding Application Summary 

Solid Systems Ground  Industrial applications pp 208V or less  Commercial Applications 600V or less



High g Resistance Ground ((5-15A))  Industrial applications 600V or less  CEC allows HRG to be used on applications up to 5kV



Low Resistance Ground (100 – 400A)  Industrial applications 5kV – 34.5kV  Ground G d ffaultlt protection t ti provided id d by b Z Zero Sequence S CTs CT on individual equipment items  GF relays set to trip at 10 -20% of maximum ground fault current

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106

Obtaining a Systems Neutral Application of Grounding Transformers

Obtaining a Systems Neutral 

Often there are cases where a systems neutral point must be established for the purposes of:

Y

 Servicing line to neutral loads  Establishing a systems ground d point i t tto ground d th the system through a HRG, LRG or solid ground connection ti

13.8kV

M

 Example: Conversion of a isolated ground system to a high resistance ground system www.EngWorks.ca


108

Grounding Transformers 

Grounding transformers are the standard means of obtaining a systems neutral  Provide a low impedance path for ground fault currents



Zig-Zag transformer  Often Oft referred f d to t as a grounding di transformer t f  Specialized transformer with no secondary winding



Wye-delta Wye delta transformer configuration  Delta winding is left unconnected

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109

Grounding Transformer Schemes A B C

I

I G

Zig Zag Transformer

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I G

Wye-Delta Transformer


110

Transformer Connection 

The grounding transformer is connected to the main bus and serves as the return path for any unbalanced or ground fault currents

Y

 A NGR is then connected to th neutral the t l point i t off th the grounding transformer establishing a connection to ground d

13.8kV

M

LRG www.EngWorks.ca


111

Specifying a Grounding Transformer 

Parameters for specifying a grounding transformer  Primaryy Voltage g  BIL (Basic Impulse Level) rating  Defined by IEEE standards (refer to IEEE 141 Red book)

 Transformer impedance  Typically very high (up to 100%) to minimize magnetizing current flows

 Continuous neutral current rating  Applicable to four wire application

 Fault F l current and dd duration i  If a LRG scheme of limited duration is used, ( typically 10 – 60 seconds) the grounding transformer does not need a continuous duty rating www.EngWorks.ca


112

G Grounding di off Generators G t Section 3

Generator Grounding 

Generators differ from transformers in several ways  Less able to withstand the heating g and mechanical effects of a short circuit  Will have a higher initial ground fault current than three phase h ground d currentt  Can develop third harmonic voltages and currents  Less able to withstand voltage surges

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114

Objective of Generator Neutral Grounding 







Minimize the damage associated with internal ground faults Limit mechanical stresses in the generator for external ground faults Limit temporary and transient overvoltages on the generator t insulation i l ti system t Provide a means of system ground fault protection

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115

Systems Ground Incorporating Generation System Ground #1 NGR

Y 5kV

System Ground #2

M G

Y

G

600V Normal Bus

System Ground #3

M

M

G

600V Emergency Bus

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116

Generator Ground Fault IGF 400A NGR 2 X IGF IGF

400A NGR

Stator Ground Fault near Generator terminals Breaker Closed

Initial ground fault current results in 2 X 400A flowing into fault

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117

Generator Ground Fault IGF 400A NGR IGF 400A NGR Stator Ground Fault near Generator terminals Breaker Open

Upon breaker trip, ground fault current continues to flow due to the residual magnetism and inertia of the machine www.EngWorks.ca


118

Ground Fault Magnitude 

 

Magnitude of a ground fault is determined by the impedance p of the g generator or transformer winding g Maximum ground fault will occur on the system bus Maximum theoretical ground fault current in the generator will occur at the generator terminals  Closer the stator fault is to the generator terminals, the higher the fault



Resulting damage is a function of current and time

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119

Solid System and Generator Ground



NOT RECOMMENDED  Results in very high ground fault currents resulting in g extensive damage  Risk of abnormal third-harmonic currents when more than one generator is connected in parallel  Increased magnetic core losses in both generator and transformer

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120

Low Voltage Emergency Generator Scheme

Bonding Conductor

Normal Bus

Emergency Bus Gnd

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Gnd

121

Single Unparalleled Generator Grounding Solid Grounded with Neutral Equipment Ground conductor 3 pole Transfer Switch

Normal Bus

Neutral Conductor Zero Sequence CT is byed resulting in false ti trip www.EngWorks.ca

51G N

Neutral Connected To ground

Solid Neutral

Emergency Bus Gnd


N

Gnd

122

Single Unparalleled Generator Grounding Solid Grounded with Neutral 

Connection of the neutral to ground at the generator can cause problems p  Allows stray current to flow between the neutral and the ground conductors  Allow zero sequence (ground fault current) to flow in the neutral causing nuisance tripping of the main breaker  Prevent ground fault relays from detecting a ground fault



A neutral should not be connected to ground on the load side of a service disconnect

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123

Option 1 – Switch Neutral Equipment Ground conductor 4 Pole Transfer Switch

Normal Bus

Neutral Conductor

GFP N

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Neutral Connected To ground Neutral Switched With load conductors

Emergency Bus Gnd


N

Gnd

124

Option 2 – Connect Generator Neutral with Transformer Neutral Equipment Ground conductor 3 pole Transfer Switch

Normal Bus

Neutral Conductor Zero Sequence CT read full neutral current value l www.EngWorks.ca

51G N

Generator G t Neutral connected to transformer neutral in transfer switch

Emergency Bus Gnd


N

Gnd

125

Additional References 

IEEE 446 Orange Book  Provides application pp information for the system grounding and transfer switching of standby generators 600V or less

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126

Single Unparalleled Generator Grounding High g Resistance Grounded HRG Bonding Conductor

HRG

Normal Bus

Emergency Bus Gnd

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Gnd

127

HRG Source and Generator Grounding HRG HRG



Advantages  Ground fault current limited to a veryy low value



Disadvantage  Selective tripping on downstream breakers is not practical

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128

LRG Source and Generator Grounding LRG LRG



Advantages  Allows selective tripping of downstream feeders



Disadvantage  Damage can occur to the generator from high ground fault currents  Variations in fault current can cause relay coordination problems

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129

LRG Source and HRG Generator Grounding g HRG LRG



Advantages  Allows selective tripping of downstream feeders  Reduced R d d level l l off fault f lt currentt to t the th generators t minimizing i i i i d damage



Disadvantage  System is high resistance grounded when the generator is operating alone – makes selective tripping impossible

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130

Source and Generator Grounded with Artificial Neutral

LRG



Advantages  Allows selective tripping of downstream feeders  Allows All either ith source or generator t to t provide id power



Disadvantage  Damage can occur to the generator from restriking and intermittent ground faults

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131

LRG Source and Hybrid LRG/HRG Generator Grounding g LRG LRG



HRG

Ground fault will cause generator t breaker b k to t trip and open LRG circuit

Advantages  Allows selective tripping of downstream feeders  Allows generator to operate without the source transformer energized



Disadvantage  Additional complexity in the grounding and relaying system

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132

Unit Connected Generator Grounding HRG

LRG LRG



Advantages  Allows selective tripping of downstream feeders  Allows generator to operate without the source transformer energized



Disadvantage  Cost of the additional transformer

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133

E i Equipment tB Bonding di Section 4

System Grounding 



Grounding and bonding have distinct meanings within the context of the CEC Grounding refers to a conductive path direct to the grounding g g electrode  Low impedance path to ground  Conductors are sized to carry the expected fault current  Insure the operation of protective devices in the circuit should a fault occur

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135

Equipment Bonding 

Refers to the interconnection and connection to earth of all normallyy non-current carrying y g metal p parts  Insures that all metal parts remain at ground potential  Reduces the shock hazard to personnel  Provides a low impedance return path for ground currents  Allows the circuit protection device to operate

 Minimize the fire and explosion hazard  Reduce accumulated static charges

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136

Ground Return Path through Earth Insufficient current to operate protection device Line

~

S

Metallic Enclosure Ground Fault

V Neutral

Short circuit must take high impedance path to source High Impedance Ground Path

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137

Ground Return with Metallic Path High Current Operates Protection Device Line ~ Metallic Enclosure

S

Ground Fault

V Neutral

Low Impedance Path through Bonding Conductor High Impedance Ground Path

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138

Bonding Fundamentals 

To reduce electrical shock exposure:  the impedance p of the bonding g conductor must be capable p of carrying the fault current  Must provide a lower impedance than all other parallel paths th



For fire protection:  M Must st be able to conduct cond ct the available a ailable gro ground nd fa fault lt ccurrent rrent without excessive temperature rise or arcing  ts and connections are critical components



Overcurrent Protection Operation:  Provide a low impedance current path back to the source

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139

Bonding – CEC Requirements 10-400 

All exposed non-current carrying metal parts of fixed equipment q p  Supplied by a conduit wiring system  Supplied by a wiring system that contains a bonding conductor  Located in a wet location  In a hazardous location  Operates at more than 150V to ground



Examples  Distribution equipment, motor and generator frames  Lighting fixtures housings

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140

Bonding Methods  

Bonding conductor in a cable or raceway Rigid metal conduit  Bonding conductor is required if the conduit is in underground service or installed in concrete slabs



EMT conduit  Bonding conductor required if installed in concrete or masonry slabs





Sheath of a mineral insulated cable if manufactured of copper or aluminum CEC Not acceptable  Metal armor of liquid tight flex or cable assemblies  Conduit made of stainless steel

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141

Bonding Methods - Effectiveness Cable or Conduit

DC Resistance Ω/1000ft

Voltage Drop V/1000A/100ft

1-1/4” Rigid Steel Conduit

0.0108

11

1-1/4” EMT

0.0205

22

0.435

436

1-1/4” Flexible Conduit 3/C St Steell A Armored dC Cable bl

55

3/C Steel Armored Cable with Ground Conductor

11

3/C Aluminum Armored Cable

0.286

3/C Aluminum Armored Cable with Ground Conductor www.EngWorks.ca

151 12


142

Bonding Conductors 

Bonding conductors may be:  Be copper or other corrosion resistant material  Aluminium conductors are acceptable  May be insulated or bare  Insulated bonding g conductors shall be coloured g green

 

May be spliced or tapped as required If installed to supplementary bond a raceway:  Must be insulated  Must be run in the same raceway



M st be protected against mechanical inj Must injury r ifif:  Copper - Smaller than #6 AWG  Aluminum – Smaller than #4 AWG

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143

Bonding Conductors Equipment q p and Raceways y

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144

Bonding of Cable Trays 

Rule 12-2208 of the CEC requires that cable trays be bonded to ground g  If the metal s for cable tray are in good with the grounded structural metal frame of a building, the tray shall h ll b be d deemed d tto b be b bonded d d tto ground d  If not in direct , a bonding conductor must be installed and the tray bonded to the conductor at intervals not exceeding 15m  Sized as per CEC table 16 based on the largest ungrounded conductor in the tray

 A bonding conductor may also be required in the cases that the tray s single conductor cables of a three phase system www.EngWorks.ca


145

Bonding of Single Conductor Cables 



Separate ground conductor required to bond the metallic equipment at either end Must follow the same routing as the phase conductors

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146

Bonding Considerations 

Bonding connections require a clean surface  Paint must be removed from connection points



Connections between dissimilar metals should be avoided  Potential for deterioration of the connection due to galvanic action





Mechanical strength may often determine the size of conductor d t Electrical continuity of expansion ts  Cable tray connections

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147

Equipotential Bonding 

Practice of bonding all exposed and extraneous conductive p parts ((Ref CEC 10-406))  Purpose is to ensure that under fault conditions, all conductive parts remain at the same potential



Applies to     

Metallic water and sewer piping G piping Gas HVAC ducting Exposed metal equipment and structures Raised computer floors

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148

Equipotential Bonding 

CEC requires a minimum #6 AWG conductor

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149

Bonding of Portable Equipment 

Non-current carrying metal parts of portable equipment q p must be bonded when:    

Equipment is used in a hazardous location Equipment is used in wet or damp locations Equipment operates at more than 150V to ground When the equipment is provided with a grounding means  Three Th prong plug l with ith ground d

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150

Grounding of Portable Equipment 

Exceptions apply to double insulated equipment p products  Additional insulation barrier added to the electrical device  Will be marked with a double insulated symbol



Ground may omitted if a Class A ground fault circuit interrupter is used

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151

GFCI Schematic





Designed to provide protection against electric shock from leakage current flowing to ground Provide supplementary pp yp protection but are not a substitute for insulation and grounding protection

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152

Ground Fault Circuit Interrupters 

GFCI Class A  Primarily used for personnel protection  Typically trip at 5ma  Time to trip based on the formula T=



20 I

1.43

T in seconds I fault current between 4mA and 260 mA

GFCI Class B (Ground Fault Equipment Protectors)  Used for equipment protection  Heat trace circuits in hazardous locations

 30ma trip level

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153

GFCI – Where Required    



Outdoor receptacles Wet locations Health care facilities s supplying power for buildings or projects under construction Heat trace systems

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154

Static Grounding Section 5

Did the Cellphone Cause the Ignition?

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156

Static Hazards in Industry 

Aviation Industry  Static charges are built up during flight and on the ground



Manufacturing  Paper and Printing  Power P and d conveyor b belts lt moving i over pulleys ll

 Paint operations  Transfer of fluids 

Coal, Flour and Grain Industry  Movement and accumulation of dust and particles



Petrochemical P h i lP Processing, i R Refining fi i and d Transportation  Movement of materials

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157

Reasons for Static Grounding   



Reduce the risk of fires and explosions Improve process and quality control Reduce the operating costs associated with storing flammable materials Minimize the potential for damage to sensitive electronic equipment q p  Loss of electronic data





Comply p y with hazardous g goods transport p and storage g regulations Reduce the cost of insurance

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158

Energy from a Static Discharge CH4/Air

Ignition Energy (mJ))

10

H2/Air

E = CV2 X 10-9

Typical range of spark discharge energy from a human body

Where C = Capacitance in pF V = Voltage in V E = Energy in mJ

1.0

Stoichiometric CH4/Air Mixture 0.274 mJ

0.1

Stoichiometric Air/H2 Mixture 0.017 mJ 0

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60 20 40 Fuel (% Volume)

80

Material

Dust Cloud

Dust Layer

Coal

60 mJ

560 mJ

Grain

30 mJ

-

Sulfur

15 mJ

1.6 mJ

Energy Required for Dust Ignition Grounding Fundamentals

159

Typical Values of Static Voltages and Capacitances p Equipment

Voltage

Object

Capacitance

Carpet Walk

12 kV

Human Being

200 pF

28.8 mJ

Fabric on Fabric

25 kV

Automobile

500 pF

312.5 mJ

Tank Truck

25 kV

Tank Truck

1000 pF

625 mJ

Tank Truck

25 kV

3.6m Tank with Insulated Lining

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100000 pF


Energy

62,500 mJ

160

Static Charge Generation 

Static electricity is generated by the movement of dissimilar poor conducting materials in close Non conductive fluids or powders in motion are a frequent cause of static



Static charge increases as the velocity of movement is increased.  Anything which generates eddies, turbulence or discontinuities in flow Filters Changes in piping cross sectional area

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161

Static Charge Generation 

Triboelectric Effect  electrification in which certain materials become electrically charged when coming into with another and are then separated

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162

Electrostatic Charge Dissipation 

Electrostatic charges continually leak away from a charged g body y  Termed electrostatic dissipation  Determined by a materials conductivity  Measured in pS/m (picosiemens per meter) for petroleum products

 Electrostatic charges accumulate when they are generated at a higher rate than they are dissipated  Function of the relaxation time constant  Time required for a charge to dissipate to approximately 37% of its original i i l value l

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163

Conductivity and Time Constants for Typical yp Materials Product

Conductivity (pS/m)

Relaxation Time (Seconds)

Benzene

0.005

>>100

Toluene

1

21

Gasoline

10 – 3000

0.006 - 1.8

0.5 – 50

0.36 - 36

50 - 1000

0 018 – 0.36 0.018 0 36

Crude Oil

> 1000

< 0.018

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Diesel Fuel Oil

164

Static Discharge 

For an electrostatic charge to be a source of ignition, four conditions must be present: p  A means of generating an electrostatic charge  A means of accumulating an electrostatic charge capable of producing an incendiary spark  A spark gap  An ignitable vapor vapor-air air mixture in the spark gap

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165

Static Charge Generation

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166

Static Discharge Spark discharges occur between conductive objects that are at different voltages g  Brush discharges can occur between a grounded conductive object j and a charged g low conductivity y material  Incendive discharge is a discharge that has enough energy to cause ignition 

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167

Industrial Materials Prone to Static Electricityy Nonconductive glass  Nonconductive conveyor belts  Rubber  Plastic resins  Dry gases  Paper  Petroleum fluids 

 Oil water mixtures

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168

Sources of Static Electricity Dry materials handling equipment  Flammable liquid pumps and handling equipment 

Charge Separation in a Pipe

 Multiphase flow enhances charge generation

Liquid filling operations  Plastic piping systems  Conveyor Belts  Liquid motion in tanks 

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169

Sources of Static Electricity

API 2003 www.EngWorks.ca


170

Spark Promoters 

A spark promoter will provide the necessary conditions for a spark gap to occur  Loose floating conductive objects  Conductive downspouts  Gage tapes, thermometers or sample p containers lowered into a tank  “tank gauging rod, high-level sensor,, or other conductive device that projects into the cargo space of a tank truck” API 2003

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171

Static Sparks in Kanses

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172

Static Control Ignition hazards from static sparks can be eliminated by y controlling g the g generation or accumulation of static charges  Static removal involves recombining g separated p charges 

 Usually met by bonding all electrically conducting parts

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173

Methods of Static Control 

Piping Systems  Keep fluid velocities low  Max 15 ft/sec



Filling Operations  Eliminate splash filling and free fall of materials  Reduce filling velocity to less than 3 ft/sec



Fluid Storage  Non Non-conductive conductive material storage containers are not allowed for NFPA Class I, Class II and Class III materials

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174

Methods of Static Control 

Humidity Control  65% or higher will prevent static discharge



Antistatic treatments  Addition Additi off carbon b bl black k tto materials t i l



Use bonding and grounding to prevent build-up build up of potential differences between conductive parts  Small gauge conductors generally sufficient to prevent the b ild p of static build-up

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175

Static Grounding

Vehicle Connected to Ground

Vehicle Bonded Together

Vehicle Bonded together and To Ground

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176

Static Grounding

Drum Container Storage Scheme

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177

Static Grounding

Bulk Fluid Transfer Operation

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178

Static Grounding

Bonding connections should be less than 10Ω for static control www.EngWorks.ca


179

Railcar Loading Bonding Scheme

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180

API RP 2003 







www.EngWorks.ca

Provides guidance on how to protect against hydrocarbon ignition from static, lightning and stray current discharges Discusses how static charges are accumulated and how they can be safely dissipated Lightning protection for metallic tanks equipment and structures Identification and mitigation of stray currents resulting from fault currents and cathodic protection applications


181

NFPA 77 



www.EngWorks.ca

Applies to the identification, assessment,, and control of static electricity for purposes of preventing fires and explosions Provides guidelines for controlling t lli static t ti electricity l t i it iin selected industrial applications


182

Lightning Protection Section 6

Lightning Strikes

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184

Lightning 

No such thing as a standard lightning strike  Highly g y complex p p phenomenon  Described by statistical means + + + + - - - - - -

+

Charge Separation in Cloud

High electric field causes ionization of air Corresponding charge + Induced in ground + Current flow in metallic pathways + + + + +

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185

Lightning Strike Initiation

+ + + + + + + + + + + + + + - - - - - - - - - - - - - - - - - Downward leader --Upwards Charge flows - leader meets - to ground + downwards leader - through Upward leader + structure + + + + + + + + + + + + + + + + - + -

+

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186

Lightning Discharge 

Direct effects of lightning

Lightning Stroke

 Heat energy and large mechanical h i l fforces  Direct ignition of flammable materials

Indirect effects of lightning    

Incendive sparks Electromagnetic pulse Earth current transients Bounded charges

Subsequent kA

98%

95%

4

80%

50%

20 46 4.6

5% 90

12

Typical Lightning Current Value

C Current



First negative kA

Cumulative Frequency

Time Typical Lightning Discharge www.EngWorks.ca


187

Difference between Lightning and High Voltage Electricity Factor

Lightning

High Voltage

Energy Level

25 kA typical, millions of volts

Usually much lower

Time of Exposure

Brief, instantaneous

Prolonged

y Pathway

Flashover,, orifice

Deep, p, internal

Burns

Superficial and minor

Deep with major injury j y

Cardiac

Primary & secondary arrest, asystole

Fillibration

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188

Incidence of Lightning 

Lighting varies with    

Terrain Altitude Latitude Time of the year

Number of flashes per square kilometre per year

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189

Lightning Protection 

Lightning strikes cannot be stopped but their energy can be diverted in a controlled manner  Strike frequency goes up with the square of the height above the average terrain  Damage g is caused by y the lightning g g energy gy taking g a random – high g impedance path to ground



3 components to a lightning protection system  Air terminal or electrode the intercepts the surge  Low impedance conductor system to ground  Ground electrode to dissipate the energy



If all equipment within an elevated potential area is bonded together, the potential for damage is minimized

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190

Inherent Grounding 

Inherent grounding  Metallic equipment, tanks and structures in direct with the ground do not require additional grounding if:  The thickness of tanks, vessels and process equipment is greater than 5mm and are capable of withstanding a direct lightning strike without damage  Indirect with the ground (self grounded)  Sealed to prevent the escape of liquids, vapours or gas





Mostt petrochemical M t h i l ffacilities iliti are iinherently h tl grounded d d and d require no additional lightning protection Equipment that may require special consideration  Open floating roof tanks  Tank farms incorporating a containment liner

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191

Bounded Charge Dissipation 

Bounded Charges  Occurs when a storm cell induces an electrical charge on everything beneath it  Consideration with open floating roof tanks

--------------Teflon seal isolates + roof from tank + + + + +

Bounded Charge

++++++++++++++ Flammable Product

Floating Roof tank + + + + + +

+++++++++++++++++++++++++++++ www.EngWorks.ca


192

Bounded Charge Dissipation

-

Bounded Charge

++++++++++++++ Flammable Product

Floating Roof tank -

Incendive discharge to ground

--------------------------------------------------- -

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193

Protection against Lightning 

Floating Roof Lightning Protection  Floating roof cable connection  Grounding Shunts (Not Recommended)

-------Cable connection to floating roof

Bounded charge Dissipated with Lightning strike Grounding shunt

Flammable Product

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194

Methods of Lightning Protection 

Conventional air terminal  Provides a low impedance path to ground

+ + + + - - - - - --

+

 Lightning rods (sometimes called Franklin rods)  Conducting masts  Overhead wires

RA = 0.84 x

h0.6

x

RA

I0.74

RA = Attractive Att ti radius di iin meters t h = height of lightning mast in meters I = Peak lightning current in kA

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RA


+

+

+ + + +

+

195

Dissipative Array System (DAS) 

Claim of the technology is to dissipate a charge before a lightning strike occurs  No scientific proof that this in fact occurs  Renamed the Charge Transfer System (CTS) technology in recent years  Still considered ineffective

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196

Early Stream Emission Air Terminals 

Consist of lightning rods incorporating a device that triggers the early initiation of a lightning strike  Effectiveness is also questioned

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197

Lightning Surge Protection 

Transient Overvoltages can damage electrical equipment  Result in insulation breakdown and eventual failure



Mitigated g by y  Surge arrestors  Equipment insulation standards

Lightning Strike EMF Travelling Wave

Surge Voltage Wave

Line www.EngWorks.ca


198

Surge Protection

Diminished Surge Voltage Wave 35kV

Transient Voltage Surge 70kV

13 8kV 13.8kV 25kV

Surge Arrestor

Is Surge Voltage is Surge suppressor Induced on secondary reduces surge g voltage g winding by capacitive to below BIL of transformer coupling effect

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199

Equipment Insulation Voltage Withstand Requirements q 



Basic Impulse Level (BIL) is used to describe the insulation class of electrical equipment q p Based on the voltage rating of the equipment  Based on specified p crest value kV  Specified in the various equipment standards

Surge Voltage and Current Wave www.EngWorks.ca


200

Surge Arrestors 



Surge arrestor must have a high resistance i t under d normall conditions diti and a very low resistance under surge conditions Metal oxide arrestor is the industry standard  Consist of a series connection of zinc oxide elements

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201

Surge Arrestors 

Class of Surge Arrestors (IEEE Std C62.11)     

Station Class Intermediate Class Distribution Class – Heavy duty Distribution Class – Normal duty y Secondary

600V Secondary Surge Arrestor www.EngWorks.ca

Distribution Class Surge Arrestor


202

Surge Arrestor Installation Considerations 









Should be mounted as close as possible to the transformer bushings Arrestor must be coordinated with the BIL of the equipment it is protecting A dedicated “down down lead” lead conductor to ground required for each arrestor Down lead conductor should be mechanicallyy and thermallyy capable of handling the surge voltage to ground Down lead should be as short as possible with no changes in direction  Minimum radius of 200mm  No bends greater than 90º

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203

Lightning Arrestors – CEC Requirements 10-1000 Lightning Arrestors on Secondary Services 1) Grounding conductor shall be as short (and straight) as possible 2) The lightning arrestor grounding conductor may connected to the: a) b) c) d)

Grounded service conductor Common grounding conductor Service equipment grounding conductor Separate grounding conductor

Common ground conductor Grounded service conductor

Service i equipment i grounding di conductor d www.EngWorks.ca


204

Lightning References 





NFPA 780 “Standard for the Installation of Lightning g g Protection Systems” provides detailed guidance on the design of lightning protection systems” API 2003 “Protection against I iti Ignitions A Arising i i outt off St Static, ti Lightning and Stray Currents” IEC 61024 “P “Protection t ti off St Structures t Against Lightning – Part 1”

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205

NFPA 780 



Installation standard for lightning protection systems for building structures and facilities handling flammable vapors p g gases and liquids q Does not apply to electric generating, transmission and d di distribution t ib ti systems t

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206

Electronic Equipment Grounding

Section 7

History 

Grounding principles for communication systems were developed to meet the operational characteristics h t i ti off th the equipment i t  Early telegraph systems used a two wire circuit path  Later systems used the earth return as the signal path

Morse Landline Telegraph System www.EngWorks.ca


208

History 

Earth return offered several advantages  Iron wire was used for telegraph conductors  The use of an “earth” ground doubled the distance a circuit could be run

 Eliminated one wire from the circuit 

Problems endured:  Quality of the signal was effected by weather  Leakage current to ground during wet weather  Resistance of the return ground path varied with soil conditions  Presence of “foreign” voltages

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209

History 

Early development of the telephone system also relied on the th solid lid DC reference f ground as the return path  lines were p particularly y noisy, y, picking up electrical noise from power lines, adjacent telephone lines, telegraph g p lines, streetcars, and machinery



The grounded system was later replaced with a system employing two wires per telephone line eliminating most of the noise

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210

Electronic Equipment Grounding 

Signal Common Grounding  referred to as the “DC Signal Common”  Zero reference system for data lines  Very sensitive to transient voltages



DC Power Supply Reference Ground Bus  -ve terminal on a DC power Supply



Equipment Ground Bus  Used for equipment chassis bonding  Often referred to as the safety ground bus



Variety of other used (depending on manufacturer)  AC Safety Ground, Computer Reference Ground, DC Signal Common, Earth Common, DC Ground Bus

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211

Electronic Systems Grounding 

Most electronic computer systems employ a DC reference ground  Required for logic circuits



Problems occur when the DC reference ground is tied to the AC safety ground  With the logic circuits referenced to the equipment chassis ground, any small amount of chassis potential caused by current flow in the grounding of the device could cause reference error in the equipment.

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212

Noise in Sensitive Circuits 

Errors result when the noise is greater than the actual signal g  Results in parity check errors  signal is ignored if check fails

30-50V L i Logic Signal Noise does not impact signal Noise cause parity check errors 3-5V Logic Signal

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213

Circuit Noise Sensitivity Measurement 

Signal to Noise Ratio



 Measure of the interference in a communications circuit  Measured in dB SNR = 10log S dB N Bit Error Rate  Measure of the number of bits received to those in error Bit E Error Rate

10-66 10-7 10-88 10-9

0 www.EngWorks.ca

10

20

30

40

50

60

SNR (dB)


214

Categories of Noise 

Traverse Mode Noise  A disturbance that appears pp between two active conductors in an electrical system  Measurable between two line conductors or from line to neutral t l  Originates from within the power system A B

V

C N G

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215

Categories of Noise 

Common Mode Noise  Appears pp simultaneously y in each active conductor  The term "common" refers to the fact that identical noise appears on both the active and neutral wires  Generally involves the ground conductor A B C

V

N G

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216

Typical Problems Associated with Electronic Systems y Grounding g Electronic Equipment Symptoms

Electrical Condition

Temporary or chronic data hang-ups Slow data transfers, multiple retries I/O Damage

Different signal reference levels I d Induced d currents t on cable bl

Intermittent lock-ups Corrupted Signals I/O damage

Transient voltages and currents

Random data errors Slow transfer in analog circuits

Stray currents and common mode noise in equipment gro nding cond grounding conductor ctor

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217

Electronic Equipment Grounding 

Computers require a “quiet” ground where no voltage transients or electromagnetic g noise occurs  Stabilize input voltage levels  Act as a zero voltage reference point for circuits





Led to the practice of installing an “Isolated Ground” system specifically for electronic equipment This practice was in direct conflict with the CEC which requires that all grounding systems be i t interconnected t d  CEC is concerned with safety – not with performance

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218

Principles of Noise Mitigation 

For noise to be a problem  Requires q a noise source of sufficient magnitude g  Some means of communicating the unwanted noise to the electronic circuit  Galvanic coupling  Electrostatic / Capacitive coupling  Magnetic g or Inductive coupling p g



Solving the problem involves either reducing the amplitude of the noise voltage or effectively isolating the circuit from the noise source

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219

Source of Electrical Noise 

Motor starts  High g current in-rush is impressed p on the communications circuit



Fluorescent lighting  High frequency noise associated with the ballast operation



Switching power supplies or VFD systems  High frequency noise associated with switching power supplies



High Hi h voltage lt surges d due tto lilightning ht i strikes t ik and d electrical faults

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220

Galvanic Coupling 

Occurs when two circuits share a common conductor  Examples: Telephone circuits that used the same common return t path th as DC ttram lilines iin the early days



Easily solved by separating the circuits by using separate return conductors

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221

Electrostatic/Capacitive Coupling 



Form of coupling that is proportional p p to the capacitance between the noise source and the signal wires Function of:  Distance from the noise source to the signal wires  Length g of the signal g wires  Strength of the noise voltage  Frequency of the noise voltage

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R1 I1 C1 I2 R2 Noise source


I3

RL

C2 E

dV dT

222

Electrostatic/Capacitive Coupling 

Mitigation     

Shielding g of the signal g wires Separating the source from the noise Reducing the amplitude of the noise voltage Reducing the frequency of the noise voltage Twisting of the signal wires

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223

Shielding 

Conductor shield provides a lower impedance path for the noise current to flow I2 R1

Copper braid (85% coverage) provides a noise reduction ratio of 100:1 Aluminum Mylar tape with drain wiren provides a noise reduction ratio of 6000:1

C3

I3 I5

I4

RL

C4

R2 I1 C1

C3 and C4 are 1/100 C1 and C2

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Noise source E


C2 dV dT 224

Shield Grounding 



Shielding on instrumentation and communication circuits eliminates the electrostatic induction into wires carrying low signal voltages Shielding g method may y be  Braided copper wire  Metalized foil, with a copper drain wire  Metal conduit (if steel conduit, this also serves as a magnetic shield)



Shields must be grounded  One end only for frequencies up to 1 Mhz  Two or more locations for frequencies > 1 Mhz

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225

Magnetic or Inductive Coupling  Depends of the rate of change and the mutual inductance between the source of noise and the signal g wires  Influenced by:  Magnitude of the noise current  Frequency of the noise current  Area A enclosed l db by the h signal i l wires  Distance between the noise sources and d th the wires i www.EngWorks.ca


226

Magnetic or Inductive Coupling 

Mitigation  Twist the signal g conductors  This results in lower noise due to the smaller area for each loop. This means less magnetic flux to cut through the loop and consequently a lower induced noise voltage  Noise voltage that is induced in each loop tends to cancel out the noise voltages from the next sequential loop

Inductive coupling is reduced by ratios varying from 14:1 for a fourinch lay to 141:1 for one-inch lay

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227

Magnetic or Inductive Coupling 

Mitigation  Enclose the signal g wires with a magnetic g shield  The magnetic flux generated from the noise currents induces small eddy currents in the magnetic shield which then create an opposing magnetic flux Ø1 to the original flux Ø2  Galvanized steel conduit is an effective magnetic shield Placing parallel (untwisted) wires into a steel conduit will provide a noise reduction of approximately 22:1

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228

Physical Segregation 

Separate the noise sources from the noise sensitive equipment q p  Cable spacing based on susceptibility levels defined by IEEE 518  Level 1 – High: Analog signals less than 50V and digital signals less than 15V  Level 2 – Medium: analog signals greater than 50V  Level 3 – Low: Switching signals greater than 50V  Level 4 – Power: Voltages g 0 – 1000V;; Currents 20–800A

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229

Physical Segregation Level Level

Separation

1

2

2 – 30mm 30

1

3

3 – 160mm

1

4

4 – 670mm

Cables Contained in Separate Trays

Level Level Separation 1 2 2 – 30mm 1 3 3 – 110mm 1 4 4 – 460mm One Cables in Conduit and the other In Trayy

Level Level Separation 1 2 2 – 30mm 1 3 3 – 80mm 1 4 4 – 310mm Both Cables in Conduits www.EngWorks.ca


230

Electrical Segregation

Y

Y

Y Shielded Isolation Transformer

M

M

M

M

UPS M

M

~

Y

Sensitive Loads

Worst Case Sensitive Loads are subject to voltage fluctuations caused by motor loads

=

=

~

Better Best

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231

Separately Derived AC Power Distribution System y using g an Isolation Transformer 

Isolates power to the control system from the rest of the AC distribution system y  Provides good line regulation and transient filtering



Transformers should be of a shielded design g  Provide superior noise isolation using the same concepts used for shielded cables





Input power to the transformers should be sourced from the highest line voltage available K factor f t transformers t f should h ld be b considered id d if th the control system load employs a large number of switching power supplies

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232

Isolation Transformer Grounding

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233

Separately Derived AC Power Distribution System y using g an UPS 



Provides a continuous power supply to the control system y in the event of a p power interruption p Protects the control system from power system surges g  Isolation transformers cannot prevent surge events from being transmitted to the load without additional surge protection



Provides a “conditioned” AC power supply to the control system  Completely disconnects the control system power supply from the source providing superior isolation from power system transients and noise

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234

1 Phase UPS

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235

3 phase UPS

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236

UPS Grounding



UPS configuration with common source for UPS and by yp circuit  Does not meet the definition of a separately derived circuit  Common mode noise attenuation may be a problem

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237

UPS Grounding



Addition of a by transformer meets the definition of a separately derived source  Improved common mode noise attenuation  Neutrals in UPS and by transformer are connected  Power distribution center must be within 15m of the UPS

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238

UPS Grounding



Best configuration for common mode noise attenuation  No restriction on distances  Allows more flexibility in UPS voltages

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239

Multiple UPS Grounding Scheme

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240

Ground Loops 

Occurs when there is more than one ground connection path between two pieces of equipment  ground current may take more than one path to return to the grounding electrode  form the equivalent of a loop antenna which very efficiently picks up interference currents  Conductor lead resistance transform the currents into voltage fluctuations



Consequences  Ground reference in the system is no longer a stable potential  Signals ride on the noise  Noise becomes part of the program signal



Example  Audible 60hz noise in your stereo system

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241

Ground Loop U it A Unit Internal Connection

+ Output -

Unit B

Communication Cable

+

Input -

Power Ground

Power Ground

1A Current Flowing 0.1V

0.2V Low Resistance 0.1Ω

Stray Current in Ground Causes Current to Flow in communication conductors

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242

Ground Loop – Motor Start Unit A Internal Connection

+ Output -

Communication Cable

Unit B +

Input -

Power Ground

From Electrical Sources

Power Ground

M Motor Start

Motor frame bonded To ground www.EngWorks.ca


243

Ground Loop Mitigation 

Add one or more separate grounds  Not N t CEC code d compliant li t Unit A Internal Connection

+ Output -

Communication Cable

Unit B +

Input -

Power Ground

C Current t Fl Flowing i Mi Minimized i i d High Resistance

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Separate p Instrumentation Ground


Motor frame bonded 244 To ground

Ground Loop Mitigation 

Interrupting the continuity of the grounding conductor  Shielded communication cables

Unit A + Output -

Communication Cable

Unit B +

Input -

Power Ground

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Interrupt ground path here

Power Ground


245

Ground Loop Mitigation •

Control the path of the ground current • Use an insulated ground receptacle Unit A + Output Power Ground

Unit B

Communication Cable

+

Input -

Isolated Instrument ground

Power Ground

Insulated ground conductor Single point ground

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246

Isolated Ground Receptacle 



Helps to limit electrical noise introduced into a circuit via the grounding conductor Establishes a dedicated ground path connected to ground at one point only

Conventional Receptacle www.EngWorks.ca

Isolated Ground Receptacle Grounding Fundamentals

247

Isolated Ground Receptacle

Power Transformer

Branch B h Ci Circuit it board Junction Box

NEMA IG#5-15R2 Isolated Ground Receptacle

Metal Device Box

System Ground

www.EngWorks.ca

Insulated Isolated Ground wire

Conduit or Cable

Bare Bonding Conductor or Conduit


248

Concept of a Single Point Ground System 



Poor or faulty grounds are the most common causes of control system y faults The best way of insuring the performance and reliabilityy of a control system y is to employ p y a single g point ground network system  Consists of an organized system of ground wiring that t terminates i t in i a single, i l d dedicated di t d point i t on th the plant l t ground d grid  Provides a clean reference for control signals

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249

Single Point Ground System

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250

Single Point Ground System Multiple Enclosures

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251

Instrument Tri-Ground System Main Transformer

Ground C d t Conductor NGR

Lightning Arrestor Connection to system ground may be temporarily disconnected to isolate ground loop

System Ground AC Ground

Instrument Tri Ground www.EngWorks.ca


252

Intrinsically Safe Circuit Grounding H Hazardous d A Area

Field Device IS Apparatus

Non Hazardous Area IIntrinsic i i Safe Barrier

Controll C System Interface

Associated pp Apparatus Interconnecting Wiring System

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253

Intrinsic Safety – Simple Field Devices Non Hazardous

Hazardous Location Th Thermocouple l

Controller Simple Device

Internal Fault Controller

Internal Fault Controller

Explosion IS Barrier

IS Ground www.EngWorks.ca


Device is Considered Safe under Fault C di i Conditions 254

Intrinsic Safe Barrier Circuit Protects Zener from Destruction Limits input current

Limits the output currentt

Hazardous Area

Safe Area Fuse

Control System Interface

Zener Z Diodes

Limits the output voltage lt www.EngWorks.ca

Current Limiting Resistor

Field Device

IS Ground


255

Intrinsic Safety - Grounding

Incorrect Ground Scheme

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Correct Ground Scheme


256

Intrinsic Safety - Grounding   

Extremely important for the safe operation of an IS wiring systems M t be Must b visibly i ibl id identified, tifi d secure and d accessible ibl Must be capable of carrying the maximum fault current  #12 AWG minimum conductor size

 

Total resistance must not exceed 1Ω Must be insulated from f ground in all places except at the point of connection to the ground electrode  Duplicate up ca e g ground ou d co conductors duc o s required equ ed



Aluminium must not be used as a ground conductor material  Potential for electrolytic corrosion

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257

IEEE Standard 1100 ((Emerald Book)) 

Recommended engineering principles and p practices for p power and g grounding g sensitive electronic equipment  Provides consensus in an area where conflicting information has prevailed  Excellent reference that describes the many challenges associated with grounding electronic equipment  Power related noise control  Signal related noise control

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258

Station Electrode Design Section 8

Ground Grid Design

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260

Ground Grid Design Fundamentals 

In the event of a fault or transient phenomena ((lightning g g or switching g transients)) the g ground g grid must  Ensure personnel safety  Protect equipment against damage



Design Considerations  Grid must be able to withstand the maximum ground current without damage  Limit the ground potential rise between two points to a safe value

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261

CEC Code Requirements 

Section 10 “Grounding and Bonding” addresses grounding g g electrodes for facilities operating p g at less than 750V to ground  Requirements are minimal



Section 36 “High Voltage Installations” addresses the grounding of facilities operating at more than 750V to ground d  Requirements are in addition to those defined in Section 10  More substantial in nature and therefore require a deeper understanding of ground electrode theory

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262

CEC Section 10 Requirements 

CEC Section 10 does not specify a minimum ground resistance for a g g grounding g electrode but specifies the acceptable methods of obtaining a grounding electrode 

NEC specifies a ground resistance of 25Ω or less

10-700 Grounding G Electrodes shall consist off a) Manufactured grounding electrodes b) Field Fi ld assembled bl d grounding di electrodes l t d c) In-situ grounding electrodes

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263

Manufactured Ground Electrode 

Must be certified to CSA C22.2 No.41 “Grounding and Bonding g Equipment” q p  Rods must be driven to their full length and separated by a minimum of 3m  Connected by a bonding conductor sized by Table 17

R d El Rod Electrode t d www.EngWorks.ca

Plate Electrode Grounding Fundamentals

264

Field Assembled Ground Electrode 

Min 6m bare copper conductor buried or encased in concrete

 conductor must be encased within the bottom 50 mm of a concrete foundation footing, with the footing  in direct with the earth,, at not less than 600 mm below finished grade Field Assembled Ground Electrode www.EngWorks.ca


265

In-situ Ground Electrode  



Copper water pipe Metal reinforcement of concrete slabs, concrete pilings, and concrete foundations Iron pilings, when they are in significant with earth 600 mm or more below finished grade

In-situ Grounding Electrode www.EngWorks.ca


266

10-700 Grounding Electrodes (5) Where local conditions such as rock or permafrost prevent a rod or g p grounding gp plate from being g installed at the required burial depth, a lesser depth shall be permitted

Horizontal Ground Rod Installation www.EngWorks.ca


267

Other Section 10 Requirements 



Lightning rod systems must be connected to ground using g a separate p g grounding g electrode that is not used as the grounding electrode for any other system Where a facilityy incorporates p more than one g ground electrode for lightning, communication or other systems  Must be separated by a minimum of 2m  Bonded together by a minimum #6AWG conductor

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268

CEC Section 36 Requirements 36-302 Station ground electrode Every outdoor station shall be grounded by means of a station ground electrode that shall meet the requirements of Rule 36-304 and shall a)

consist of a minimum of four driven ground rods not less than 3 m long and 19.0 mm in diameter spaced at least the rod length apart and, where practicable, located adjacent to the equipment to be grounded;

b) have the ground rods interconnected by ground grid conductors not less than No. 2/0 AWG bare copper buried to a maximum depth of 600 mm below the rough station grade and a minimum depth of 150 mm below the finished station grade; and c) have the station ground grid conductors in Item (b) connected to all non-current-carrying non current carrying metal parts of equipment and structures www.EngWorks.ca


269

Distribution Utility Standard Ground Electrode Design g

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270

Distribution Utility Standard Ground Electrode Design g

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271

CEC Requirements for Station Ground Resistance 36-304 Station ground resistance (see Appendix B) 1) The maximum permissible resistance of the station ground electrode shall be determined by the maximum available g ground fault current injected j into the ground by the station ground electrode or by the maximum fault current in the station, and the ground d resistance i t shall h ll b be such h th thatt under d allll soilil conditions that exist in practice (e.g., wet, dry, and frozen conditions) …..

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272

CEC Section 36 – Ground Potential Rise 36-304 Station ground resistance ….the the maximum ground fault current conditions shall limit the potential rise of all parts of the station ground g g grid to 5000V 2) In addition to subrule (1), the touch and step voltage at the edge, within, and around the station grounding electrode…..shall not exceed the tolerable values specified in Table 52

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273

CEC Section 36 – Tolerable Touch and Step p Voltages g

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274

IEEE Standard 80 

IEEE Standard 80  Defines the safe limits for touch and step potentials  Provides guidance on the design of ground systems for outdoor substations  Primarily Pi il used db by utilities tiliti for grounding on high g substations voltage

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275

Ground Potential Rise (GPR) 

Ground potential rise is a function of the current g injected j into the earth and the soil magnitude resistivity  Measured with respect to a remote point  May vary from a few meters to several hundred meters away



5000V criteria specified in the CEC is based on the maximum GPR communication circuits are d i designed d tto h handle dl

GPR = IG X Rg

IG = Maximum Grid Current Rg = Grid Resistance www.EngWorks.ca


276

Ground Potential Rise 3 wire i ttransmission i i with ith no Metallic return path

G

∆

A

∆

N

N G

B C

Generator Transformer

Ground Fault

IG

NGR

EG

GPR

E th Earth Rg Ground Path

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277

System Ground Fault Return Path Non Metallic conduit With no bonding conductor

Y

0.1Ω 500V

Ig=5kA Ground Return Path

Lack of bonding conductor forces ground fault return path through the earth creating personnel hazard www.EngWorks.ca


278

System Ground Fault Return Path Metallic conduit with bonding conductor Low Impedance Ground Return Path

Y Ig=5kA

High Impedance Ground Return Path

Bonding conductor provides low impedance path to source: Stray current is minimized with improved safety www.EngWorks.ca


279

Current GF Path with Local Source

Y

Y

Multiple low impedance ground paths limit the ground potential rise within th station the t ti www.EngWorks.ca


280

Current GF Path with Remote Source

Overhead ground wire current path

Y

Y

Stray Current Paths

Multiple high impedance ground current paths back to source www.EngWorks.ca


281

Ground Potential Rise (GPR) 

Grid system must limit the ground potential rise (GPR) between two points to a safe value  GPR can cause hazardous voltage in the form of Step & Touch Potentials  May occur in location remote to the actual fault

 Safe values of GPR, Touch and Step Voltages are determined by the human tolerance to shock currents  Function of current magnitude, g , duration and frequency q y

GPR = IG X Rg IG= Maximum Grid Current Rg = Grid Resistance

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282

Step Voltage ESTEP

Potential Rise above remote earth during short circuit R1

IF

ESTEP

IF RF RK

R2 R0

RF

RK

R1 www.EngWorks.ca

R2

R0 Grounding Fundamentals

283

Touch Voltage ETOUCH

Potential Rise above remote earth during short circuit

IF

ETOUCH

IF

RF

R1

RK

RF/2 RK R0 RF/2

R1 www.EngWorks.ca

R0 Grounding Fundamentals

284

Touch and Step Potential IEEE 80

ρ = resistivity of earth beneath surface ρs = surface material resistivity (Ω . m) hs = thickness of surface material in m www.EngWorks.ca


285

Relationship between GPR, Touch and Step p Potentials

Y

Es

Et

Emesh

GPR

Remote Earth

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286

Mesh Voltage 

Defined as the maximum surface voltage g p potential difference between a grid conductor and and a point between two grid conductors  Th Theoretical ti l maximum i touch voltage found within a ground grid

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287

Touch and Step Potential  1 1.

2.

Two ways of making a grounding system safe Minimize the touch and step voltages that may appear at any point within the substation and around its perimeter p Increase the tolerable touch and step voltages by placing a high resistivity material over rough grade  



Asphalt Crushed rock

Both methods are typically used together

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288

Ground Surface Potential Gradients

High ground resistance increases step potential

10kA

Ground Rod

R1 Re1

Gn nd Surface Potential P

30kV

R = R1+Re1 = 3 ohm 0 Volts Infinite Earth

8kV Step Potential 0

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Distance from Rod Grounding Fundamentals

289

Ground Surface Potential Gradients Multiple ground rods reduce the ground surface potential low resistance surface reduces step potential

10kA

Gn nd Surface Potential P

R1+Re1 = 3 ohms

R1 4kA

R2

0 ohms

Re1

6kA

Re2

R2+Re2 = 2 ohms 0 Volts Infinite Earth

12kV 0

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Infinite Earth (0V) Grounding Fundamentals

290

Symmetrical Grid Current 





The current that causes the ground potential rise in a grid is from a remote source g Only a portion of the current is responsible for the ground p g potential rise Multiple return paths include  Overhead ground conductors  Cable shields



The current flowing into the ground that is responsible for the GPR is adjusted by a split factor to incorporate the effect of the multiple paths

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291

Symmetrical Grid Current Ig = Sf x If Ig = RMS Symmetrical Grid Current Sf = Split factor (Current Division Factor) If = RMS value of the symmetrical ground fault current Sf may be estimated using computer programs or by graphical analysis  Typically ranges between 10 – 70% of If  Refer to IEEE 80 for more information www.EngWorks.ca


292

Split Factor Split factor s for the multiple current paths that will occur in a fault situation Overhead ground wire current path

Y

Y

If

Ig = Sf x If

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293

Split Factor Graphical Estimate IEEE 80 Annex C Curve most likely to be used for a single circuit customer owned substation

28%

The symmetrical grid current (Ig ) would be approximately 28% of the total fault current for a substation with a 2.5Ω grid resistance

2.5Ω Substation www.EngWorks.ca


294

Grid Current for Design IG = x Df x Ig IG = Maximum Grid Current = Estimated growth factor during station life span = 1 ffor zero growth g Df = Decrement factor for the duration of the fault Ig = RMS value off the symmetrical y ground g fault f current

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295

Decrement Factor Decrement factor s for the total asymmetric fault current flowing between the grounding system and the surrounding earth Df Fault Duration tf Sec 0.008 01 0.1 0.25 0.5

Subtransient S b i T Transient i Network N k Network (0-5 cycles) (5-30 cycles) www.EngWorks.ca

Cycles 0.5 6 15 30

1.65 1 25 1.25 1.1 1.0

S Steady State S Network (>30 ( 30 cycles))

Industrial Power System Protection and Control

296 296

Design Information Provided by Utility Current values to be used in the design of the station ground d grid id

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297

Ground Resistance  

The “WildCard” in the grounding design Ground grid resistance varies with soil conditions and may change over time  Changing Ch i water t ttable bl Resistivity of the ground will change in drying or drought conditions

 Chemical content of soil Presence of salts decrease resistivity

 Frozen ground or permafrost conditions Consideration in all Canadian g grounding g situations www.EngWorks.ca


298

Soil Resistivity (Ω . cm) = 100 x (Ω . M) Medium

Resistivity (Ω . cm) Minimum

Surface Soil, Loam

100

Clay, Shale, Gumbo

300

Sand and Gravel Limestone

Average

Maximum 5,000

4,000

20,000

5 000 5,000

10 000 10,000

500

400,000

Granite, basalt

1,000,000

Low Hills, Rich Soil

3,000

Medium hills, Medium Soil

20,000

St Steep Hills, Hill R Rocky k S Soilil

50 000 50,000

Sandy, dry coastal country

30,000

50,000

500,000

Freshwater Lake

10,000

20,000

20,000,000

Sea water

2,000

10,000

20,000

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299

Effects of Moisture on Resistivity Moisture Content 0 2.5 5 10 15 20 30

www.EngWorks.ca

Resistivity Ω Ω-cm cm Top Soil Sandy Loam > 109 > 109 250,000 150,000 165,000 43,000 53,000 18,500 19,000 10,500 12,000 6,300 6,400 4,200


300

Effects of Temperature on Resistivity for Sandyy Loam

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Temperature ºC 20

Resistivity Ω cm Ω-cm 7,200

10

9,900

0 (Water)

13,800

0 (Ice)

30,000

-5

79,000

-15

330,000 Grounding Fundamentals

301

Seasonal Variation in Earth Resistance

19mm Rod in Stony Clay Soil Curve 1 – 1m below surface C Curve 2 – 3m 3 b below l surface f

Moisture and temperature p is more stable at g greater depths p below the surface

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302

Effect of Chemicals on Earth Resistivity for Sandyy Loam Effect of Salt on Resistivity of Soil ((Moisture 15% Temp p 17ºC)) Added Salt % by Weight of Moisture

Resistivity (Ω-cm)

0

Effect of Temperature on Resistivity of Soil with Salt (20% Moisture 5% Salt) Temperature C ºC

Resistivity (Ω-cm)

10,700

20

110

0.1

1,800

10

142

1.0

460

0

190

5

190

-5 5

312

10

130

-13

1,440

20

100

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303

Electrode Resistance

ρ(Ω.cm) Ω Rg (rod) = 335 cm Applies to 3m ground rod and is accurate within 15% www.EngWorks.ca


304

Multiple Ground Rod Resistance 

Resistance of a grounding system of 2-24 rods placed on rod length p g apart p will p provide a g grounding g resistance divided by the number of rods multiplied by the factor F taken from Table 14 IEEE Std 142

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305

Alternate Formulas for Ground Rod Resistance resistance of one ground rod ρ 4L - 1 R= Ln X 2πL a ρ = Soil resistivity in Ω Ω-cm cm L = rod length in cm a = rod diameter in cm

Ground Rod Separation D = 2.2 X L

resistance of multiple ground rods Rn =

R n

X 2 – e-0.17(n - 1)

n = number of ground rods

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306

Smoking Ground Rod 

Current loading capacity of a ground rod is a factor  Current p ing g through g an electrode will have a direct impact on the temperature and moisture conditions immediately surrounding the ground rod  Must M t be b checked h k d I=

34,800 X d X L

√ ρXt

I = Current loading gp per foot of rod length g d = rod diameter in meters L = Length in meters ρ = ohm meter t = seconds (3 (3.0 0 seconds is the value recommended by IEEE) www.EngWorks.ca


307

Ground Rod Resistance to Earth Tutorial

Ground Rod Resistance to Earth Tutorial 1. Determine the resistance to earth for a ground rod system y consisting g of Qty y 4 – 10 foot long g 5/8” (16mm) ground rods spaced at 10’ intervals and interconnected connected together and placed in clay soil 2. Calculate the resistance to earth for a ground rod system t consisting i ti off Qty Qt 4 – 20 ffoott 5/8” (16mm) (16 ) ground rods interconnected together and placed in clay soil 3. Calculate the current loading capacity of the system

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309

Ground Rod Resistance to Earth Tutorial Answers

4 - 10’ Ground Rods ρ(Ω.cm) Ω Rg (rod) = 335 cm Rg (rod) =

Average ρ for clay soil = 4000 Ω·cm

4000 Ω.cm = 11.94 Ω 335 cm

For 4 ground rods 11.94Ω 11 94Ω Rg (4 rods) = 4

X 1.36 = 4Ω

IEEE 142 Table 14

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311

4 – 20’ Ground Rods resistance of one ground rod ρ = Soil resistivity in Ω-cm ρ 4L - 1 L = rod length in cm R= Ln X 2πL a a = rod diameter in cm 4000 4(610) - 1 R= Ln X = 6.649Ω 1.6 2π(610) resistance of multiple ground rods Rn = Rn = www.EngWorks.ca

R n

X 2 – e-0.17(n - 1)

6.649 4

X

n = number of ground rods

2 – e-0.17(4 - 1) = 2.32Ω


312

Current Carrying Capacity of Ground Rod System y I=

34,800 X d X L

√ ρXt

I = Current loading of the ground rod system d = rod diameter in meters L = Length in meters ρ = ohm meter t = seconds (3.0 seconds typical value)

34,800 34 800 X 0.016 0 016 X 3.048 3 048 X 4 = 620 Amps 4 – 10ft Ground Rods I = 40 X 3

√

4 – 20ft Ground Rods I =

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34,800 X 0.016 X 6.1 X 4

√ 40 X 3 Grounding Fundamentals

= 1240 Amps

313

Primary and Auxiliary Ground Electrodes 

Primary Ground Electrode  Installed specifically p y for g grounding gp purposes p  Ground rods  Interconnecting wire mesh



A ili Auxiliary G Ground d El Electrode d  Installed for purposes other than grounding  Typically T picall ha have e limited ccurrent rrent carr carrying ing capacit capacity  Examples  Steel building gp piles  Steel reinforced concrete foundations  Rebar grounding

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314

UFER Ground 

First used by the US Army to ground a series of bomb storage vaults in the vicinity of Flagstaff, Arizona  Dry desert conditions made for a very poor ground electrode system  Herbert Ufer developed an alternate electrode system based on using the steel rebar used to reinforce concrete



Concrete is inherently alkaline and hydroscopic (absorbent) in nature  The high pH provides a supply of ions to conduct current  soil around concrete becomes “doped” by the concrete

 has an effective resistance of 3000 Ω-cm 

Bases for the development of concrete encased electrodes

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315

Concrete Encased Electrodes 

Can be used as part of an effective low resistance grounding system  Will typically lower the overall resistance of the ground



Very cost effective!  Adds very little cost to the installation  Reduces the amount of buried conductor required for an installation  Aids in reducing the amount of construction re-work  Buried B i d ground d conductors d t are a popular l it item tto dig di up d during i ttrenching hi operations



The 2006 NEC requires that rebar encased in concrete be incorporated into the system ground  No equivalent CEC requirement

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316

Methods of Connecting Rebar to Building Steel 

Option 1 – Connect structural steel and rebar using ground wire  Requires electrical trade to be on on-site site during pouring of foundations

Bolted Connection to Steel C Copper Wi Wire

Ground Well

Grounding Compression p Connection or Cadweld

To ground grid

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317

Methods of Connecting Rebar to Building Steel 

Option 2 - Tie the vertical rebar to anchor bolts and the steel columns are g grounded through g the bolts and nuts

Rebar welded To anchor bolts

Ground Well

To ground grid

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318

Rebar Grounding Installation Considerations 

  



Rebar must be bare or zinc coated Minimum length – 6m Minimum diameter 13mm I t ll d iin a minimum Installed i i off 50mm of concrete

Surge Current Conductivity of Rebar in Concrete Rebar Diameter (in.)

Surge Ampere per Foot

0 375 0.375

3400

 Preferably y located near the bottom of the foundation

0.5

4500

Concrete must be in direct with earth

0.625

5500

0.75

6400

10 1.0

8150

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319

Ground Conductors 

For a given application, ground rods are more effective than g ground g grid conductors  Ground rods will penetrate the frost level injecting current into unfrozen ground



Basic Requirements for the selection of a Ground Grid conductor  H Have sufficient ffi i t conductivity d ti it  Resist fusing and mechanical deterioration under fault conditions  Be mechanically reliable and rugged  Resist corrosion

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320

Ground Grid Conductor Options 

Copper    



Highest g conductivity y Highest cost Subject to theft Commercial hard drawn specification most often used

Copper-clad steel  Good option where theft is a problem



Aluminum  Not recommended as is subject to corrosion



Steel  Poor conductivity limits use

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321

Sizing of Grid Conductors 

Based on the design ground fault current and the fault duration time

Akcmil = I · Kf √tc Akcmil = area of conductor in kcmil I = Fault current in kA tc = current duration in seconds (IEEE recommends 3.0 seconds) Kf = constant based on the material (Refer to table 2 IEEE 80)

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322

Material Constants ((Excerpt p from Table 2 IEEE 80))

Material

Conductivity

Kf

Copper, Annealed Soft Drawn

100%

7.00

C Copper, C Commercial i lH Hard dD Drawn

97%

7 06 7.06

Copper Clad Steel Wire

40%

10.45

Aluminum 6201 Alloy

52.5%

12.47

Steel 1020

10.8%

15.95

Stainless Steel 304

2.4%

30.05

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323

Grid Conductor Sizing Example  



Fault current is estimated at 6kA Commercial hard drawn copper selected as the grid conductor What size of grid conductor is appropriate to handle the maximum fault current for 3.0 seconds? Akcmil = I · Kf √tc Akcmil = 6 · 7.06 √3.0 Akcmil = 73.37kcmil → #1 AWG

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324

Grid Conductor Sizing Table Current Carrying Capacity (kA) kcmil

AWG

pp HD Copper

Copper pp Clad Steel Wire

Steel 1020

500

-

40

27.62

18.1

250

-

20 4 20.4

13 81 13.81

9 05 9.05

212

4/0

17.34

11.71

7.67

133

2/0

10.88

7.35

4.81

83.7

#1

6.84

4.62

3

66.4

#2

5.43

3.67

2.4

Based on a 3.0 Second Fault Duration

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325

Grounding Connections 





All grounding connections must be selected to withstand the short circuit forces and heating effects associated with an extended groung fault Resist the effects of corrosion High pullout resistance Time-current curves for ground grid conductors and connectors

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326

Typical Connections found within a Grid Design g

Conductor to Ground Rod

Conductor to Conductor www.EngWorks.ca

Substation F Fence to t Grid G id Conductor Grounding Fundamentals

Equipment to G id C Grid Conductor d t 327

Cadweld Connections  Thermite welding process is used to fuse the connection  Suitable for high current applications  Will meet the requirements of IEEE 837 ““IEEE Standard for Qualifying Permanent Connections Used in Substation Grounding Grounding”

Cadweld Connection

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328

Grounding Connections 

Compression Connections  Acceptable p alternative to Cadweld connections in substation applications

Compression Tool www.EngWorks.ca


329

Corrosion Considerations 

A basic corrosion cell consists of the following:  Anode – An electrode losing g metal  Cathode – An electrode gaining metal  Electrolyte – chemicals in solution in with the anode and cathode 0.78V  Connecting conductor -

V

+

IRON Anode

Earth

COPPER Cathode

G l Galvanic i C Cellll www.EngWorks.ca


330

Corrosion Considerations 

When iron or steel is connected to copper with a low impedance p conductor,, it corrodes  Rate of corrosion is dependent on the current flow  Each ampere-year of current flow will result in 20lbs of steel being lost μ –5 μA 5A + A

IRON IRON Anode

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COPPER Earth

Cathode

Soil Corrosivity Ohm-Cm Corrosivity <2000 Very High 2-5000 High 5-10000 Moderate 10 25000 Mild 10-25000


331

Corrosion Mitigation 

Use a UFER ground system whenever possible  Do not use an UFER g ground system y in conjunction j with a copper ground rod and grid system in the same vicinity  Will result in deterioration of the concrete rebar



IIn situation i i where h a grid id system iis iin close l proximity i i to large amounts of steel and corrosive soil conditions exist  Consider using galvanized ground rods and insulated ground conductors g  Conductor insulation should have a high resistance to chemical degradation



Consider installation of a cathodic protection system

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332

Other CEC Requirements 36-312 Grounding of metallic fence enclosures of outdoor stations (1) The fence shall be located at least 1 m inside the perimeter of the station g p ground electrode area. (2) The station ground electrode shall be connected to the fence by a tap conductor at each end post, corner post, and gate post, and at intermediate posts at intervals not exceeding 12 m by a conductor of not l less th than N No. 2/0 AWG copper

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333

Grounding of Fence Enclosures

Grounding Detail Gate

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Grounding Detail Fence


334

Other CEC Requirements 36-304 Station ground resistance ((4)) After completion p of construction,, the resistance of the station ground electrode at each station shall be measured and changes shall be made if necessary to and ensure that the maximum permissible resistance of Subrule (1) is not exceeded Exception p  Station phase to phase voltage is less than 7500V  Ground surface has a 150mm layer of crushed rock or asphalt

(5) ground d surface f covering i layer l shall h ll extend t d att lleastt 1 m beyond the station grounding electrode area on all sides.

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335

Measurement of Ground Resistance 









Current IA is ed through auxiliary probe A Voltage between L and P is measured R is then calculated based on IA and VLP Several ground resistance measurements t are ttaken k and d the results are averaged Moisture and temperature data should also be recorded

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IA

1 – 10A

V

L

P

A

IEEE Std 81 addresses Ground Resistance testing


336

IEEE Standard 81 

Describe the techniques used d tto measure ground d resistance and impedance  Factors that impact earth resistivity  choice of instruments and techniques  purpose of the measurement  accuracy required  Potential sources of error

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337

Design of a Station Ground Grid 





Typical plant ground grid should have a resistance of 10Ω or less For satisfactory lightning protection, grounding network resistance must be less than 5Ω IEEE 80 Guidelines  1Ω or less for transmission substation  5Ω or less for distribution substation

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338

Options Available for the Design of a Station Ground Grid 1. Design the GPR for the station to be less than the Touch Potential Specified p in the CEC  

 

Suitable for situations where the ground fault current is of limited value Station resistance is determined by selecting the appropriate number of ground electrodes using the formulas provided in IEEE 142 Green Book Ground electrodes are then connected together using 4/0 AWG grid conductors Results in a very conservative design

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339

Options Available for the Design of a Station Ground Grid 2. Use the publication “Simplified Rules for Customer Owned Substations” (CEA Report 249 D541)  Publication is referenced as a referenced in CEC Section 36-304(3) and appendix B  Outlines a procedure for design of a pre-approved pre approved station electrode design  Method 1 based on simple calculations, tables and curves – Takes into frost penetration in winter

 Method 2 based on design curves – Valid only for fault durations less than 0.5 seconds and where frost penetration is negligible p g g

 Examples are provided  Was developed primarily to simplify the ground grid design process prior to the availability of computer aided ground grid design programs www.EngWorks.ca


340

Options Available for the Design of a Station Ground Grid 3. Use the procedure outlined in IEEE 80  Calculation intensive but will result in an appropriate and safe design  Suitable for the design of Utility and Large scale customer owned substations where ground fault current levels are high  Most computer aided ground grid design programs use the methods outlined in IEEE 80

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341

Station Ground Grid Design GPR < Touch Voltage Method

Relationship between GPR, Touch and Step p Potentials

Premise of the method Is to reduce the GPR To less than the tolerable Touch voltage

Y

Es

Et

Emesh

GPR

Remote Earth

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343

GPR < Touch Voltage Premise Premise of the method is to reduce the GPR to less than the tolerable touch voltage as defined in Table 52 of the CEC

Y

Estep max

Etouch max

Emesh

GPR = IG X Rg

GPR Remote Earth

IG= Maximum Grid Current Rg = Grid Resistance www.EngWorks.ca


344

Step 1 Ground Fault Current 1. Determine the maximum ground fault current (IG) that might g be injected j into the station g ground electrode 

Reference CEC 36-304  



Specifies the maximum ground fault current or: The maximum fault current for the station

Information is typically provided by the Utility

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345

Step 2 Ground Resistivity 2. Determine the resistivity of the ground in the area of the substation 

Should be determined by test  



Often done as part of the geotechnical survey for civil works Ground resistivity under all conditions (wet, dry, frozen earth)

Determine the type of surface layer to be used in the vicinity of the substation 

150mm of crushed stone generally used

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346

Step 3 Tolerable Touch Voltage Touch Voltage Criteria

3 Determine the tolerable 3. touch voltage from CEC table 52 Typical Value Used based on Table 52 Note 2

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347

Step 4 Required Ground Resistance 4 Calculate the required ground resistance for the 4. station ground electrode 

The use of 150mm of crushed stone over the surface of the substation will help raise the Etouch and the overall required ground resistance and simplify the design

R = Etouch/IG R = Grid Resistance IG = Maximum Grid Current Etouch = Tolerable touch voltage from CEC table 51

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348

Step 5 Number of Ground Rods 5. Estimate the number of ground rods required to obtain the specified p g ground resistance  

Estimate the ground resistance of one ground rod Divide out the resistance of one ground rod by the number of rods required to achieve the target station ground resistance IEEE 141

ρ(Ω.cm) Ω Rg (rod) = 33 cm 335

Valid for 5/8” 10 foot ground rods www.EngWorks.ca


349

Step 6 Ground Rod Layout 6. Space out the ground rods in a symmetrical pattern tt throughout th h t the th substation  



Substation fence Min. 1 rod length

Minimum spacing p g of one ground rod distance apart Ground rods on the peripheral are more effective than ground rods in the interior of the substation Grid design should extend beyond the fence of the substation a minimum of 1 meter

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1m Min

350

Step 7 Interconnect Ground Rods 7. Interconnect the ground rods using a minimum 2/0AWG bare copper ground wire 

Conductors shall be buried to a minimum depth of 150mm below finished grade to a maximum of 600mm below the rough station grade

→ Go G to t Detailed D t il d d design i



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2/0AWG CU Wire


351

Station Ground Grid Design IEEE 80 Design Procedure

IEEE 80 Ground Grid Design 

Appropriate for both Utility and Large customer owned substation facilities  Large generation facilities should reference IEEE 665 “IEEE Guide for Generator Station Grounding”  Generating station typically cover a much larger physical area and have numerous large buried structures and foundations  Workers generally work indoors and are not in direct with the earth or layer off crushed gravel



Defines the safety criteria which establishes the basis for design and then provides a procedure for the design of a practical grounding system

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Step 1 Field Data 



Obtain a field map of the location and determine the area that may be used for the installation of a ground grid Conduct a soil resistivity test  Determine the soil resistivity profile of the area in concern  Select and determine the resistivity of the surface layer material to be used in the design of the substation

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Step 2 Conductor Size 

Select the ground grid conductor material and calculate an appropriate conductor size

Akcmil k il = I · Kf √tc Akcmil = area of conductor in kcmil I = Maximum 3 phase fault current in kA tc = current duration in seconds (IEEE recommends 3.0 seconds) Kf = constant based on the material (Refer to table 2 IEEE 80) 

Maximum 3 phase fault current value is provided by the Utility

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Step 3 Touch and Step Criteria 

Calculate the tolerable touch and step p voltage g criteria for the station

ρ = resistivity i i i off earthh beneath b h surface f ρs = surface material resistivity (Ω . m) hs = thickness of surface material in m

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Step 4 Initial Design 

Preliminary design should incorporate a conductor loop surrounding the available area with cross conductors to provide convenient access for equipment grounds

Recommend ground rods be placed around the perimeter of the grid

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Step 5 Grid Resistance 

Estimate the resistance of the initial design Rg = ρ 1 LT

1 1 + 1+ √20A 1+h√20/A

Rg = Substation resistance in Ω ρ = Soil resistivity in Ω . m A = Area occupied by the grid in m2 h = Depth of the grid in m LT = Total length of conductors and rods in m

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Step 6 Grid Current 

Determine the maximum ground fault current expected within the substation  This value can usually be obtained from the Utility  If not, calculate the fault current using the following:

IG = x Df x Ig IG = Maximum Grid Current = Estimated growth factor during station life span = 1 for zero growth Df = Decrement factor for the duration of the fault Ig = RMS value of the symmetrical ground fault current

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359

Step 7 GPR < Touch Voltage? 

Determine if the GPR is less than the acceptable touch voltage for the station

GPR = IG X Rg < Etouch IG = Maximum Grid Current Rg = Grid Resistance Etouch = Etouch50 or Etouch70  If YES → Grid Design is complete  Go to Detailed Design

 If NO → go to step 8

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360

Step 8A Calculate Mesh Voltage 

Calculate the MESH voltage for the grid design Em =

ρ · Km· Ki · IG LM

EM = Mesh Voltage ρ = Soil resistivity in Ω·m Km = Geometrical correction factor for grids of varying dimension Ki= Correction factor for grid geometry IG = Maximum grid current LM = Effective length of grid conductors and ground rods in m

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Km = Geometrical correction factor 1

D2 Km = · ln l 16 · h · d 2·π

+

D+2·h2 8·D·d

-

h 4·d

+

Kii Kh

8 · lln π(2 · n – 1)

For grids with ground rods along the perimeter and thoughout the grid area: Kii = 1 For grids with no ground rods: Refer to IEEE 80 Formula 82 for calculation of Kii 1+ h ho = 1m (grid reference depth) Kh = ho

√

D = Spacing between parallel conductors in m h = Depth of ground grid conductors in m d = Diameter of grid conductor in m n = Effective number of parallel conductors in a given grid

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Km = Geometrical correction factor n = na · n b · n c · n d na =

LC = is the total length g off the conductor in the horizontal grid in m Lp = is the peripheral length of the grid in m

2 · LC Lp

nb = 1 for square grids nc = 1 for f square andd rectangular t l grids id nd = 1 for square, rectangular and L – shaped grids Otherwise

nb =

Lp 4 · √A

nd =

Dm 2

√Lx · Ly

2

nc =

Lx · Ly

0.77 ·A 0 A Lx · Ly

A

Ki= Correction factor for grid geometry Ki = 0.644 0 644 + 0.148 0 148 · n www.EngWorks.ca


363

LM = Length of Grid Conductors and Ground Rods For grids with no ground rods or very few rods scattered throughout the grid – but none on the corners or on the perimeter

LM = LC + LR LC = is the total length g off the conductor in the horizontal grid g in m LR = is the total length of all ground rods For grids with ground rods in the corners, as well as along the perimeter and throughout the grid

Lr LM = LC + 1.55 + 1.22 √Lx2 · Ly2 √

LR

Lr = is the length of each ground rod in meters Lx = is the maximum length of the grid in the x direction in m Ly = is i the th maximum i length l th off the th grid id in i the th y direction di ti in i m www.EngWorks.ca


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Step 8B Calculate Step Voltage Es =

ρ · Ks· Ki · IG LS

LS = 0.75 · LC + 0.85 · LR

KS =

1 π

Es = Step Voltage ρ = Soil resistivity Ks = Spacing p g ffactor for f stepp voltage g Ki= Correction factor for grid geometry IG = Maximum grid current LS = Effective buried conductor length m LC = is the total length of the conductor in the horizontal grid in m LR = is the total length of all ground rods

1 1 1 n-2 2 1 – 0.5 + + 2·h D+h D

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D = Spacing between parallel conductors in m h = Depth of ground grid conductors in m d = Diameter of grid conductor in m n = Effective number of parallel conductors in a given grid


365

Step 9 Em < Etouch?

If YES go to step 10 If NO → Modify design

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Step 10 Es < Estep?

If YES go to Detailed Design If NO → Modify design

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Detailed Design of the Station Ground Grid

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Station Ground Grid Detailed Design 1. Connect all non current carrying metal equipment to the station g ground g grid   

Two 2/0 AWG connections for electrical equipment apparatus Overhead transmission ground wires Metal structures  Pedestals  Security fence  Substation building steel



Underground metal pipes and other metallic structures ing through the station  Intervals not exceeding 12m

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Station Ground Grid Detailed Design 2. Connect the neutral ground resistor to the station ground grid using a conductor sized to maximum current of the resistor 

Provide for the inspection of grounding connections

3 Connect the lightning arrestor to the station ground 3. grid using a minimum 4/0 AWG conductor 

Must be short,, straight g and direct as p possible

4. Interconnect adjacent substations and the reinforced steel of adjacent plant structures using 4/0 AWG intergrid i t id conductors d t 5. Connect isolated instrument ground to the station ground grid at one location only www.EngWorks.ca


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Intergrid Conductors Reinforced Steel of Building or Plant Substation Fence

Buried Substation Ground Electrode

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Intergrid conductors


371

Step by Step Instructions for the Design of a Station Ground Electrode 6. the following design parameters   

GPR Touch and Step voltages Current loading capacity of the ground rod

7 Test 7. T t the th resistance i t off the th station t ti ground d electrode l t d  Procedures outlined in IEEE 81  Tests are not required if the phase to phase voltage is less than 7500V and a 150mm surface layer is installed – See CEC 36-306

8. Modifyy the design g as required q  

Install additional ground rods or use longer ground rods to reduce the station resistance Modify the resistance of the soil using chemical salts

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Grid Design Documentation 

Soil resistivity data     



Dates the measurements were made Temperature of the air and soil Wetness of the earth Measurement methodology Material used for earth surface covering layer

Value assumed for the maximum fault current injected into the station ground grid  IIndicate di t if value l was calculated l l t d or obtained bt i d ffrom another th source

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Grid Design Documentation 

Indicate what soil model was used and how it was obtained  Worst case scenario



Indicate the maximum GPR, touch and step voltages predicted for the system  Fault duration assumed





Incorporate the results of ground resistance testing of the grid Incorporate p scale drawings g of the g ground rod placement and ground conductor interconnections  Show all fences, underground pipes, intergrid conductor connections

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Grounding Tutorial Section 9

Substation Ground System 

Modular Substation incorporating    



5 kV Switchgear and MCCs 600 V Switchgear and MCCs UPS power distribution system DCS Control System

Grounding system consists of:  Power Distribution System Ground  5kV Low resistance ground system  600V High resistance ground system

  

Equipment Ground Instrumentation ground Lightning surge arrestor ground

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376

Substation Single Line Overhead line With no Neutral

LRG NGR

Surge Arrestor

Y 5kV

M

HRG NGRY

M

M

600V ~=

M

M

=

~

UPS PP

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Substation Layout 600V MCC

600V Swgr

DCS

UPS

5kV MCC

5 kV Swgr

HRG NGR 600V XFMR

LRG NGR

5kV XFMR Surge Arrestor

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378

Tutorial Objectives 

Use the HRG and LRG NGRs sizing from the previous tutorial  5A NGR on the 600V System  125A NGR on the 5kV System



Design a station ground grid system based on the following information  Maximum ground fault current = 0.4kA (Provided by utility)  Assume a 150mm crushed rock surface layer over a clay subsurface layer with a resistivity of 6000 (Ω . cm) under normal conditions



the GPR, touch and step potentials for the design

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Grounding Tutorial Answers

GPR < Touch Voltage Method Step 1 – Determine the Maximum Ground Fault Current  

3phase fault current given as 5kA Maximum ground fault current injected into the grid is given as 0.4kA

Step 2 – Determine the resistivity of the soil in the vicinity i i it off th the substation b t ti  

Resistivity of Clay given as 6000 (Ω . cm) or 60 Ω·m 150mm Surface layer of Crushed stone specified

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Step 3 Touch Potential Step 3 - Ground grid must be capable of handling 0.4kA and limit the touch and step pp potentials as specified in CEC table 52 Use llower value U l as the th Ground grid voltage Rise Criteria

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Step 4 Required Ground Resistance 

Target grid resistance is based on limiting the station GPR to less than the maximum touch voltage g

R = Etouch/IG R = Grid Resistance IG = Maximum Grid Current Etouch = Tolerable touch voltage from CEC table 51

885V M i Maximum required i d resistance it off groundd grid id = = 2.21Ω 2 21Ω 0.4kA

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Step 5 Number of ground rods required 

Resistance of one ground rod ρ(Ω.cm) Ω Rg (rod) = 335 cm 6000Ω.cm = 17.91Ω Rg (rod) = 335 cm 

Estimate 16 ground rods to start Rg XF n 17.91Ω X 1.92 Rn = 16 2 14Ω Rn = 2.14Ω

Rn =

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Step 6/7 Station Ground Grid 3.3m

3.3m 600V MCC

600V Swgr 5A NGR

33m groundd rods d (16 in total)

DCS

UPS

5kV MCC

Ground rods with Inspection well

5 kV Swgr Bare 2/0 AWG

600V XFMR

5kV XFMR

125A NGR Surge Arrestor

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385

IEEE 80 Design Calculation 

Step 1 Field Data  

 

Maximum 3 p phase fault current given as 5kA Maximum ground fault current i j t d iinto injected t th the grid id iis given i as 0.4kA Resistivity of Clay given as 6000 (Ω . cm) or 60 Ω·m 150mm Surface layer of Crushed stone t specified ifi d

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386

Step 2 Conductor Size 

Select the ground grid conductor material and calculate an appropriate conductor size

Akcmil = I · Kf √tc Akcmil = area of conductor in kcmil I = Maximum 3 phase fault current in kA tc = current duration in seconds (IEEE recommends 3.0 seconds) Kf = constant based on the material (Refer to table 2 IEEE 80)

Akcmil = 5 · 7.06√3 = 61.14 kcmil 61.14 kcmil → Minumum #2 AWG 2/0 AWG Selected for Mechanical Strength www.EngWorks.ca


387

Step 3 Touch and Step Criteria

ρ = resistivity of earth beneath surface ρs = surface material resistivity (Ω . m) hs = thickness of surface material in m

0.09 1 -

CS = 1 -

60 3000

2(.15) ( ) + 0.09

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= 0.773


388

Step 3 Touch and Step Criteria 116 + 0.174(0.773)(3000) ETouch = = 735V √0.5

EStep =

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116 + 0.696(0.773)(3000) √0.5

= 2446V


389

Step 4 Initial Design 

Preliminary design should incorporate a conductor loop surrounding the available area with cross conductors to provide convenient access for equipment grounds

10m

14m

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13 - 3m 3 ground d rods d = 39 39m 5x14m + 5x10m = 120m #2/0 AWG Area = 10m X 14m =140m2 Grid depth p = 450mm


390

Step 5 Grid Resistance Rg = ρ 1 LT

1 1 + 1+ √20A 1+h√20/A

Rg = Substation resistance in Ω ρ = Soil resistivity in Ω . m A = Area occupied by the grid in m2 h = Depth of the grid in m LT = Total length of conductors and rods in m

1 1 1 Rg = 60 + 1+ 159 √20(140) 1+0.45√20/140 Rg = 2.48Ω

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Step 6 Grid Current 

Maximum ground fault current injected into the grid is given as 0.4kA

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392

Step 7 GPR < Touch Voltage? 

Determine if the GPR is less than the acceptable touch voltage for the station

GPR = IG X Rg < Etouch IG = Maximum Grid Current Rg = Grid Resistance Etouch = Etouch50 or Etouch70

GPR = 400A X 2.48Ω = 992V GPR = 992V > Etouch = 734V  Go to step 8 and Calculate Mesh Voltage www.EngWorks.ca


393

Step 8A Calculate Mesh Voltage 

Calculate the MESH voltage for the grid design Em =


=

60 · Km· Ki · 400 LM

EM = Mesh Voltage ρ = Soil resistivityy in Ω·m Km = Geometrical correction factor for grids of varying dimension Ki= Correction factor for grid geometry IG = Maximum grid current LM = Effective length of grid conductors and ground rods in m

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394

Km = Geometrical correction factor 1

D2 Km = · ln l 16 · h · d 2·π

+

D+2·h2 8·D·d

-

h 4·d

+

Kii

8 · lln π(2 · n – 1)

Kh

For grids with ground rods along the perimeter and thoughout the grid area: Kii = 1

Kh =

1+ h √

ho

ho = 1m (grid reference depth) =

1 + 0.45 = 1.2 √

1

D = Spacing between parallel conductors in m = 3.5m h = Depth of ground grid conductors in m = 0.45m d = Diameter of grid conductor in m = 0.0093m n = Effective number of parallel conductors in a given grid

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Km = Geometrical correction factor LC = is the total length of the conductor in the horizontal grid in m = 120m Lp = is the peripheral length of the grid in m = 48m A = Area of Grid = 140m2

n = na · n b · n c · n d na =

nb =

2 · LC Lp

=

Lp

2 · 120 48 =

=5

48 = 1.01 1 01 4 · √140

4 · √A nc = 1 for square and rectangular grids nd = 1 for square, rectangular and L – shaped grids

n = 5 · 1.01 · 1 · 1 = 5.05 Ki= Correction C ti factor f t for f grid id geometry t Ki = 0.644 + 0.148 · n = 0.644 + 0.148 · 5.05 = 1.39 www.EngWorks.ca


396

LM = Length of Grid Conductors and Ground Rods For grids with ground rods in the corners, as well as along the perimeter and throughout the grid

Lr LM = LC + 1.55 + 1.22 √Lx2 · Ly2

• LR

Lr = is the length of each ground rod in meters Lx = is the maximum length of the grid in the x direction in m Ly = is the maximum length of the grid in the y direction in m LC = is i the th length l th off conductors d t in i the th grid id in i m LR = is the length of rods in the grid in m

LM = 120 + 1.55 + 1.22

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3 √102 · 142


• 39 = 181.46

397

Mesh Voltage 1

3.52 Km = · ln + 16 · 0.45 · 0.0065 2·π

3.5 + 2 · 0.45 2 8 · 3.5 · 0.0065

0.45 4 · 0.0065

-

1

8 + · ln π(2 · 5.05 – 1) 12 1.2

Km = 0.70 0 70 Ki = 1.39 LM = 121.57 121 57 Em =

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=

60 · 0.7· 1.39 · 400 181.46


= 128.7V

398

Step 8B Calculate Step Voltage Es =


Es = Step Voltage ρ = Soil resistivity = 60 Ω·m Ki= Correction ffactor for f grid g geometry g y = 1.39 IG = Maximum grid current = 400A LS = Effective buried conductor length m Ks = Spacing factor for step voltage

LS = 0.75 · LC + 0.85 · LR LC = is the total length of the conductor conducto in the horizontal ho i ontal grid g id in m = 1200 LR = is the total length of all ground rods = 39m

LS = 0.75 0 75 · 120 + 0.85 0 85 · 39 = 123.15 123 15

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399

Step 8B Calculate Step Voltage KS =

1 π

1 1 1 n-2 1 – 0.5 + + 2·h D+h D

D = Spacing between parallel conductors in m = 3.5 h = Depth of ground grid conductors in m = 0.45 d = Diameter of grid conductor in m = 0.0065 n = Effective number of parallel conductors in a given grid = 5.05

1 KS = π

Es =

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1 1 1 5 05 2 5.05-2 + + 1 – 0.5 2 · 0.45 3.5 + 0.45 3.5


=

= 0.513

60 · 0.513 · 1.39 · 400 123.15


= 139V

400

Step 9 Em < Etouch? Emesh = 128.7V < 734V = Etouch YES go to step 10

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401

Step 10 Es < Estep? Es = 139V < 2446V = Estep YES go to Detailed Design

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402

Detailed Design 2/0 AWG Insulated Gnd Wire

3.3m

3.3m

600V MCC DCS Eqpt Gnd Bus Isolated Inst. Gnd Bus

600V Swgr 5A NGR

33m groundd rods d (16 in total)

600V XFMR

UPS

5kV MCC

Ground rods with Inspection well

5 kV Swgr

5kV XFMR Surge Arrestor

Intergrid Grounding Conductors Bare 2/0 AWG

125A NGR 2/0AWG to Xo Terminal

Gnd Conductor as short as possible www.EngWorks.ca


403

Substation Ground System 

Instrumentation Ground  Designed to insure that all components of the control system operate at the same potential Eliminate potential ground loops Isolate system noise on the ground system Allow ground to be accessible for disconnect to assist in isolating ground loops

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404

Control System Grounding Scheme SWGR

UPS

UPS PNL

N

Main Transformer

Ground Conductor Lightning A Arrestor t

Instrument Bonding Conductors

Bonding Conductor

NGR

System Ground

Isolated Ground Bus

Insulated Instrument Ground conductors

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Computer Analysis GPR < Touch Summer Conditions

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Computer Analysis GPR < Touch Summer Conditions

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Computer Analysis GPR < Touch Winter Conditions

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Computer Analysis IEEE 80

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Computer Analysis IEEE 80

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Computer Analysis Optimized

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Computer Analysis Optimized

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Summary and Wrap up

Section 10

Learning Objectives Review 1. To understand why we ground  Protect life from the danger of shock  Limit the voltage on a circuit  Facilitate operation of protective devices

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Fuse

Low Impedance path to source allows fuse to operate

Accidental Ground

L

G1 Neutral


414

Learning Objectives Review 2. To describe the difference between grounding and bonding g  System grounding refers to the intentional connection of a phase or neutral conductor to earth for the purpose of controlling t lli th the voltage lt tto ground, d within ithi predictable di t bl lilimits it  Bonding or equipment grounding refers to the interconnection and connection to earth of all normally non-current carrying metal parts  Insures that all metal parts remain at ground potential  Reduces the shock hazard to personnel  Provides a low impedance return path for ground currents – Allows the circuit protection device to operate

 Minimize the fire and explosion hazard www.EngWorks.ca


415

Learning Objectives Review 3. To apply the safety requirements as defined by the Canadian Electrical Code and the IEEE as theyy relate to grounding 

CEC Section 10   



Defines when a system should be grounded and when equipment should be bonded Describes the acceptable methods for grounding and bonding and stipulates the size of grounding and bonding conductors Defines what an acceptable grounding electrode shall be

CEC Section 36 

Describes the grounding and bonding requirements for high voltage substations – –

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GPR < 5000\v Touch and Step potential as per table 52 Grounding Fundamentals

416

Learning Objectives Review 3. To apply the safety requirements as defined by the Canadian Electrical Code and the IEEE as theyy relate to grounding 

IEEE 142 (Green Book) 



IEEE 1100 (Emerald Book) 



Recommended Practice for the Grounding of Industrial and Commercial Power Systems Recommended Practice for Powering and Grounding Electronic Equipment

IEEE 80  

Guide for Safety in AC Substation Grounding Primarily concerned with outdoor AC substations

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417

Learning Objectives 4. To select the appropriate systems grounding scheme for an industrial facilityy Ungrounded

Solid Ground

Low Resistance

High Resistance

Immunity to transient overvoltages

Worst

Good

Good

Best

Arc Fault Damage Protection

Worst

Poor

Better

Best

Safety to Personnel

Worst

Better

Good

Best

Service Reliability

Worst

Good

Better

Best

Continued operation after initial ground fault

Better

Poor

Poor

Best

Not Possible

G d Good

B tt Better

B t Best

Condition

Ground fault locating g

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418

Learning Objectives 5. To implement a static electricity control and lightning g gp protection system y  Static Control  Bond together and to ground

 Lightning Protection  Lightning strikes cannot be stopped but their energy can be diverted in a controlled manner  Requires a low impedance path to ground

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419

Learning Objectives 6. To avoid the problems typically associated with the grounding g g of sensitive electronic systems y  

Ground loops - use the single point grounding concept Methods of Noise Mitigation    

Physical Separation Electrical Segregation Harmonic Filtering Shielding or screening of noise sources

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Learning Objectives 7. To design a ground grid for a high voltage industrial substation  Limit the ground potential rise between two points to a safe value  Limit the touch and step potentials to a safe value  Must be able to withstand the maximum ground current without damage  Important part of a safe and reliable electrical systems design

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