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GIS – Gas Insulated Substation Why GIS Types of Substation Switchgears [AIS/HIS/GIS] Space Reduction Analysis with AIS/HIS/GIS Specifications for GIS & AIS 400 kV / 220 kV GIS 400 kV side GIS Single Line Diagram & Components of GIS 220 kV side GIS Visual Comparison Between AIS & GIS Components Techno - Economic Comparison Between AIS & GIS Advancement in GIS Life Cycle Cost Analysis – [AIS/HIS/GIS] World Scenario -GIS

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Gas Insulated Substations (GIS) is a compact, multicomponent assembly enclosed in a ground metallic housing in which the primary insulating medium is compressed Sulphur hexafluoride (SF6) gas. SF6 acts as an insulation between live parts & the earthed metal closure.

The introduction of SF6 gas has revolutionized not only the technology of circuit breakers but also the layout of substations. The dielectric strength of SF6 gas at atmospheric pressure is approximately three times that of air. It is incombustible, non toxic, colorless and chemically inert. It has arc-quenching properties 3 to 4 times better than air at equal pressure. Space requirement is only 10 to 25 percent of what is required is a conventional substation.

GIS has small ground space requirements. Gas insulated Substations have easy maintenance( nearly zero Maintenance Less field erection time & less erection cost. For underground powerhouse of Hydro electric power project where space constraint is a major issue. For Fast Growing Major Cities where land availability is costlier. Non-Flammability & Non-Explosive , Oil-free & Less Pollution

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Surge Arrestor

Capacitor Voltage Transformer

Bus Post Insulator

Disconnector

Live Tank-Circuit Breaker

Current Transformer

Dead Tank- CB

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Surge Arrestor

Capacitor Voltage Transformer

Bus Post Insulator

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The weight and size of the GIS equipment do not change appreciably with the voltage class as the bulk of the current – carrying components and enclosures have identical dimensions for similar thermal and short time current. The additional insulation required for the next voltage class is achieved by increased gas density. Owing to these flexibilities, a few manufacturers offer the same equipment for two voltage classes (like 170/145 kV). Even when the GIS equipment is designed for an individual voltage class, the dimensions and weights of the equipment differ marginally. Table shows the dimensions, weight and floor loading for three voltage classes of GIS. Rated voltage

145 kV

170 kV

245 kV

Bay width, m

1.5

2.0

2.0

Bay Depth, m

3.3

3.35

3.4

Bay height, m

3.2

3.4

3.4

Floor area, sq.m

4.95

6.7

6.8

Volume, m3

15.84

22.78

23.12

Weight, kg

3800

5000

5700

Floor loading, kg/sq.m kg/sq.m

765

750

840

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132 Kv Clearances

220 Kv

400 Kv

765 Kv

Clearance Diff in Clearance Clearance Diff in Clearance Clearance Diff in Clearance for 132 Kv with Next for 220 Kv with Next for 400 Kv with Next Voltage class Voltage Class Voltage class

Mini.Clearance B/W Ph – Ph

1.22 m

0.84 m

2.06 m

1.94 m

4.0 m

3.6 m

7.6 m

Mini.Clearance B/W Ph - Ear

1.07 m

0.71 m

1.78 m

1.72 m

3.5 m

1.4 m

4.9 m

Sectional Clearance

3.50 m

0.78 m

4.28 m

2.22 m

6.5 m

3.8

10.3m

Ground Clearance

4.6 m

0.9 m

5.5 m

2.5 m

8.0 m

Earth Clearance: this is the clearance between live parts and earthed structures, walls, screens and ground. Phase Clearance: this is the clearance between live parts of different phases. Section Clearance: this is the clearance between live parts and the terminals of a work section. The limits of this work section, or maintenance zone, may be the ground or a platform from which the man works. 14

Minimum Clearance for Different Voltage Level [GIS] 145 Kv (132 Kv)

245 Kv (220 Kv)

420 Kv

800 Kv (765 Kv)

Clearance Diff in Clearance Clearance Diff in Clearance Clearance Diff in Clearance for 132 Kv with Next for 220 Kv with Next for 420 Kv with Next Voltage Voltage class Voltage class class Centre-tocentre distance of phases

0.37 m

0.09 m

0.46 m

0.20 m

0.66 m

0.15 m

0.81 m

Right-of way width

1.3 m

0.2 m

1.5 m

0.6 m

2.1 m

0.5 m

2.6 m

External diameter

240

310

470

620

Gas pressure at 20 C

420

420

420

420

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Specifications for 400 KV GIS (ELK-3) 1

Rated Voltage

420 Kv

2

Rated Power Frequency withstand voltage

650 Kv

3

Rated Lightning impulse withstand voltage

1425 Kv

4

Rated Switching Impluse withstand voltage

1052 Kv

5

Rated Frequency

50 Hz

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Busbar current

6300 A

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Feeder Current

4000 A

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Rated Short time Withstand Current

63 KA

Specifications for 220 KV GIS (ELK-14) 1

Rated Voltage

Upto 300 Kv

2

Rated Power Frequency withstand voltage

460 Kv

3

Rated Lightning impulse withstand voltage

1050 Kv

4

Rated Switching Impluse withstand voltage

850 Kv

5

Rated Frequency

50 Hz

6

Rated Continuous Current

4000 A

7

Rated Short time Withstand Current

50 kA 16

400 kV/ 220 kV Substation with 5 bays at 400 kV and 7 bays at 220 kV 2 nos. 315 MVA, 400/220/33 kV three phase Auto Transformers Gas Insulated Switchgear type ELK -3 at 400 kV and ELK-10 at 220 KV from ABB Switzerland Substation commissioned in 2007. Power received from ##### & ***** at 400 KV. Power supplied to Delhi Transco at 220 KV.

To ICT 1

To ICT 2

From # # #

Bus Coupler Bay

From * * * *

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1. Barrier insulator 2. Busbar Gas Compartment 3. Feeder Gas Compartment 4. CB Gas compartment 5. Voltage transformer

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Bus bars Circuit Breakers Disconnecting switches Earthing switches Current transformers Voltage transformers Cable and boxes Gas supply and gas monitoring equipment 1.Circuit Breaker 2.Operating mechanism (CB) 3.Current Transformer 4.Disconnector

5.Maintenance earthing switch 6.Fast acting earthing switch 7.Voltage transformer 22 8.SF6 Bushing

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A circuit breaker is an automatically-operated electrical switch designed to protect an electrical from damage caused by overload or short circuit. Its basic function is to detect a fault condition and, by interrupting continuity, to immediately discontinue electrical flow. Unlike a fuse, which operates once and then has to be replaced, a circuit breaker can be reset (either manually or automatically) to resume normal operation

Current interruption in a high-voltage circuit-breaker is obtained by separating two s in a medium, such as SF6, having excellent dielectric and arc quenching properties. After separation, current is carried through an arc and the arc is interrupted & cooled by a gas blast of sufficient intensity.

Each CB comprises three single-phase metal enclosed breaker poles. Each Pole consists of operating mechanism, the interrupter column with 2 interrupting chambers in series & the enclosure with the basic structure. To guarantee simultaneous interruption, the chambers are mechanically connected in series. One grading capacitor guarantees an equalized voltage distribution.

Assembly of Metal enclosed Breaking poles

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Making use of arc energy to produce the pressure necessary to quench the arc and obtain current interruption. Low current interruption, up to about 30% of rated short-circuit current, is obtained by a puffer blast. A valve between the expansion and compression volumes.

When interrupting low currents the valve opens under the effect of the overpressure generated in the compression volume. The blow-out of the arc is made as in a puffer circuit breaker by compression of the gas obtained by the piston action. In the case of high currents interruption, the arc energy produces a high overpressure in the expansion volume, which leads to the closure of the valve and thus isolating the expansion volume from the compression volume. 27 The overpressure necessary for breaking is obtained by the optimal use of the thermal effect.

Each Pole of the CB is equipped with the Hydraulic spring operating mechanism. It combines the advantages of both Hydraulic operating mechanism & Spring energy storage type. A Hydraulic pump moves oil from low pressure reservoir to high pressure reservoir side of the energy piston. Opening & Closing of CB is initiated by trip coil actuation.

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Current transformer (CT) is used for measurement of electric currents. Current transformers are also known as instrument transformers. Current transformers are commonly used in metering and protective relays in the electrical power industry.

When current in a circuit is too high to directly apply to measuring instruments, a current transformer produces a reduced current accurately proportional to the current in the circuit, which can be conveniently connected to measuring and recording instruments.

Voltage transformers (VTs), also referred to as “Potential transformers" (PTs), are used in high-voltage circuits. They are designed to present a negligible load to the supply being measured, to allow protective relay equipment to be operated at lower voltages, and to have a precise winding ratio for accurate metering

The single pole inductive voltage transformer is connected to switch gear with the connecting flanges with a barrier insulator. The primary winding is insulated with SF6 gas & connected to high voltage terminal. The primary winding is wounded on the top of the core & secondary windings. The secondary winding is connected to the terminals in the external terminal box through a gas tight multiple bushing.

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Disconnector and Earthing switches are safety devices used to open or to close a circuit when there is no current through them. They are used to isolate a part of a circuit, a machine, a part of an overhead line or an underground line so that maintenance can be safely conducted. The opening of the line isolator or busbar section isolator is necessary for safety, but not sufficient. Grounding must be conducted at both the upstream and downstream sections of the device under maintenance. This is accomplished by earthing switches.

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Disconnect switches are designed to continuously carry load currents and momentarily carry short circuit currents for a specified duration. They are designed for no-load switching , opening , or closing circuits where negligible currents are made or interrupted (including capacitive current and resistive or inductive current , or when there is no significant voltage across the open terminals of the switch. 35

Fast earth switch and maintenance earth switch are the two types of earth switches used for gas insulated sub-station systems. Fast earth switch is used to protect the circuit-connected instrument voltage transformer from core saturation caused by direct current flowing through its primary as a consequence of charge stored online during isolation / switching off the line. Use of fast earth switch provides a parallel (low resistance) path to drain the residual static charge quickly, thereby protecting the instrument voltage transformer from the damages that may otherwise be caused. The basic construction of these earth switches is identical.

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Wave Trap Capacitance Voltage Transformer Lightening Arrestor

Capacitor Voltage Transformers convert transmission class voltages to standardized low and easily measurable values, used for metering, protection and control of the high voltage system. Additionally, Capacitor Voltage Transformers serve as a coupling capacitor for coupling high frequency power line carrier signals to the transmission line. 37

Lightning Arresters or Surge Arresters are always connected in Shunt to the equipment to be protected, they provide a low impedance path for the surge current to the ground

Line trap also is known as Wave trap. It traps Hi-frequency communication signals sent on the line from the remote substation and diverting them to the telecom/ tele protection in the substation control room (through coupling capacitor and LMU). This is relevant in Power Line Carrier Communication (PLCC) systems for communication among various substations without dependence on the telecom company network. 38

SF6 – Air Bushings are used for connecting to open terminal equipment & Overhead transmission lines. SF outdoor bushings allow the enclosed switchgear to be connected to overhead lines.

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CABLE TERMINATION (Fig - a): High-Voltage cables of various types are connected to SF6 switchgear via cable connection assembly & also it enables the GIS & Cables to be tested separated

Fig - a

Fig - b

Transformer connection (Fig - b) consists of Oil/SF6 bushing, the enclosure, the main circuit end terminal & removable connection. For Hi-Voltage test on GIS, transformer is isolated from switchgear by dismantling the removable connection 40

The insulating and interrupting capability of the SF6 gas depends on the density of the SF6 gas . The pressure of the SF6 gas varies with temperature, so a mechanical or electronic temperature compensated pressure switch is used to monitor the equivalent of gas density. Gas Density Monitor is directly mounted on the enclosure. The gas pressure acts on metal bellows, with a reference volume for compensation of the temperature. In case of gas leakage a micro-switch is actuated. Thresholds for refilling (first stage) or lock-out alarm(second stage) can be mechanically set. The response character is shown in the Molier diagramme.

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Isolating Gas Pressure is generally 350-450 Kpa at 20 deg cel. Quenching gas pressure is about 600-700 Kpa Outdoor Equipment exposed to arctic conditions contains a mixture of SF6 & N2.

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An autotransformer is an electrical transformer with only one winding. The winding has at least three electrical connection points called taps. The voltage source and the load are each connected to two taps. One tap at the end of the winding is a common connection to both circuits (source and load). Each tap corresponds to a different source or load voltage.

2 nos. 315 MVA, 400/220/33 kV three phase Auto Transformers

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An autotransformer for power applications is typically lighter and less costly than a two-winding transformer, up to a voltage ratio of about 3:1 beyond that range a two-winding transformer is usually more economical. In an autotransformer a portion of the same winding acts as part of both the primary and secondary winding.

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To Line-4

From ICT 1

To Line-3

Bus Coupler Bay

To Line-2

To Line-1

From ICT 2

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(For Further Space reduction)

Gas Insulated Transformer (GIT) Instead of Oil Immersed Transformer (OIT). SMART GIS - Integration of Electronic CT’s & PT’s Combined Earthing Switch & Disconnector

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Natural Cooled type

Forced-Gas-Circulated , NaturalAir-Cooled

Forced-Gas-Circulated , Forced-AirCooled

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GIT

OIT

Medium

SF6

Insulating oil

Turn Insulation

PET film

Cellulose paper

Conservator

Not Required

Necessary

Pressure Relief Device

Not required

Necessary

Others

Fundamentally Same

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Nonflammability – Gas insulated transformers , using incombustible SF6 gas as insulation and cooling medium, enable to remove a fire fighting equipment from transformer room. Non Tank – explosion - Pressure tank enables to withstand the pressure rise in case of internal fault. Compactness – Since conservator or pressure relief equipment is not necessary, height of transformer room can be reduced approximately 2 – 2.5 meters. Easy installation – oil or liquid purifying process is not necessary in case of gas-insulated transformer. Easy inspection and maintenance work -Only SF6 gas pressure shall be basically monitored during periodically inspection.

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The Combined sensors are the Rogowski coil for current measurement and the capacitive divider for voltage measurement A combined current and voltage sensor has been developed to replace the conventional current and voltage transformers in GIS.

1. Serial Optic Link 2. Voltage sensor 3. Rogowski Coil 63

Why Combined Voltage & current Sensor ? Advanced CT’s without a magnetic core (Rowgowski coil) & Capacitive sensor have been developed to save space and reduce the cost of GIS. The output signal is at a low level, so it is immediately converted by an enclosure mounted device to a digital signal

Small size - Helps to optimize the use of space in the switchgear Lighter weight means less material usage and lower life cycle costs (LCC) Large dynamic range - permits minimization of number of sensor types needed and improvement of some protection functions. Protection and measurement functions combined. Lower losses mean lower LCC (Life Cycle Cost)

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Current Measurement – Current Sensor instead of Current Transformer The current sensor is based on a Rogowski coil (a coreless inductive current transformer). Voltage Measurement – Voltage Sensor instead of Voltage Transformer The voltage sensor is based on a capacitive electrical field sensor (Capacitive ring sensor). The capacitive ring, which acts as a voltage sensor, also has a linear characteristic and is very simple in of the insulation. 65

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1. Busbar with Combined DS & ES 2. Circuit-breaker 3. Current sensor (Rogowski coil) 4. Electro-optical voltage transformer 6. Make-proof earthing switch 7. Control cubicle

1. Busbar with Combined DS & ES 2. Circuit Breaker 3. Current Transformer 4. Voltage transformer 5. Combined DS & ES with cable sealing end 6. Hi-Speed ES 7. Control Cubicle

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The DSES incorporates the two functions of a disconnector and a maintenance earthing switch as a result saving the space in GIS. This is achieved by a sliding characterized by three defined positions: - disconnector open / earthing switch closed - disconnector closed / earthing switch open -disconnector open / earthing switch open The use of one moving for the disconnector and the earthing switch inhibits simultaneous closed position of both switches.

Busbar with Combined Disconnector & Earthing Switch

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Combined disconnector and earthing switch is mounted at the front, and acts via bevel gears and an insulating shaft on the three parallel pins. Depending on the direction of movement the s act as disconnector or earthing switch (maintenance earthing switch). By means of a crank handle, manual operation of the combined disconnector and earthing switch is also possible.

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(O & M)

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AIS

GIS

Planning and engineering

100%

80%

Real estate

100%

40%

Primary equipment

100%

120%

Secondary equipment

100%

100%

Earthwork, civil work, structures

100%

60%

Electrical assembly and erection

100%

70%

Maintenance

100%

50%

Outage

100%

50%

Life cycle costs after 10 years

100%

Max. 70%

Life cycle cost

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Optimizing of acquisition costs

Cost of Acquisition

Acquisition costs are mainly influenced by the layout, the redundancy concept and the requirement area for the installation. The optimization of the layout and the redundancy can be ed by LCC analysis which take into the different reliability figures of the different technologies. The goal is to achieve optimal availability of the substation with minimized costs. Optimizing of costs of ownership

Cost of Ownership

The costs of ownership are dominated by the maintenance strategy and the reliability of the switchgear. By means of LCC analysis, the different maintenance strategies can be simulated. Pure corrective maintenance can be compared with time-based or reliability – centered maintenance. Optimizing of renewal costs

Cost of Ownership

LCC calculations assist in determining the optimal date for replacing an existing substation. Aged equipment requires intensified maintenance, more specialized experts and spare parts. The effect of investment costs of new equipment with reduced maintenance costs can be balanced with the increased expenditure for intensified maintenance of the old equipment 80

Cost of GIS is high. The life of GIS is affected by certain factors such as: conductive particles, partial discharges and contamination (decomposition products, water, etc Gas Insulated Substations (GIS) can be used for longer times without any periodical inspections. However, conducting contamination (i.e. aluminum, copper and silver particles) could seriously reduce the dielectric strength of gas-insulated system. SF6 has been identified as a greenhouse gas, safety regulations are being introduced in order to prevent its release into atmosphere.

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The first GIS’s were put in operation in 1967 in Switzerland and . The GIS in is still in operation, whereas the GIS in Switzerland were recently decommissioned after 35 years of operation without major fault or gas leak. An assessment made on SF6 gas leakage over the lifetime of the first GIS and concluded that overall leakage rate was about 0.4% per year.

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