Hvdc Transmission 5nu3t

HVDC Transmission Electrical Engineering 8th Semester Subject Code : EE801A

Lesson Plan Module

Content

Hours

1.

Introduction

04

2.

Analysis of HVDC Converters

06

3.

Control of HVDC Converters and Systems

08

4.

Harmonics and Filters

10

5.

Faults and Protection Schemes in HVDC Systems

04

6.

Multi-Terminal HVDC Systems

08

HVDC Transmission

2

Text & Reference Books  Text Books: 1. HVDC Transmission, S. Kamakshaiah & V. Kamaraju, Tata McGraw Hill Education. 2. HVDC Power Transmission System, K. R. Padiyar, New Age International. .Reference Books: 1. Direct Current Transmission (Volume I), E. W. Kimbark, Wiley-Interscience. 2. High Voltage Direct Current Transmission, J. Arrillaga, Peter Pregrinu. 3. High Voltage Direct Current Power Transmission, Colin Adamson and N. G. Hingorani, Garraway Limited, London. HVDC Transmission

3

Introduction  Introduction to DC Power Transmission Technology • War of Currents (Thomas A. Edison and George Westinghouse) • Limitations of Early Low Voltage DC Systems  High losses and voltage drop  Transformation of voltage required (Higher voltage means lower voltage drop)  Commutators of DC Machines impose limitations on the voltage, speed and size. (Voltage per bar < 21 V) high voltage per commutator → high number of bars → large diameter and low speed for less centrifugal force → heavier machines and more associated maintenance costs HVDC Transmission

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Introduction  Introduction to DC Power Transmission Technology • Rise of AC Power System  Advent of Transformers for stepping up and stepping down the voltage level  Development of Polyphase Induction Motors (simple, rugged, cheap and efficient)  Advancement of Synchronous Generators

• The Thury System (1880 - 1911)

HVDC Transmission

5

Introduction  Introduction to DC Power Transmission Technology • Major Problems in HVAC Systems  Insulation  Stability  Reactive Power and Line Loading  Ferranti Effect and Skin Effect  Power Control  Interconnection

HVDC Transmission

6

Introduction  Comparison of AC and DC Transmission • Technical Limitations & Economic Limitations Cheapest method by which a certain amount of power at a certain load factor can be transmitted reliably over a certain distance.

• Basis for comparison  Current Limit & Voltage Limit  Reactive Power & Surge Impedance Loading

 Economy & Terminal Equipment  Ground Return

 Stability  Control of Power

 Circuit Breaking  Short-Circuit Current

 Amount of Power Transfer

 Generating Units HVDC Transmission

7

 Comparison of AC and DC Transmission Basis for comparison Current Limit Thermal loading due to Ohmic losses depends on • Duration of Current Flow • Ambient Temperature • Conductor resistance Voltage Limit Switching and Lightning Surges

Remarks

HVAC Transmission

HVDC Transmission

• For Overhead (OH) Lines, • AC resistance is slightly • DC resistance is lower than Conductor temperature must not higher value due to skin effect. AC resistance. exceed permissible maximum temperature. • For same power, number of • For same power, number of (To avoid permanently increased sag) conductors required is more, conductors required is less, therefore losses are more. therefore losses are less. • For Underground (UG) Cables, Temperature of insulation in • Charging current in UG cables must not exceed permissible limits the maximum possible maximum temperature. load current. (To avoid insulation damage) • For OH Lines, maximum working voltage and minimum conductor size are limited by loss and radio interference due to corona. • For UG Cables, limiting factor is normal working voltage.

• Switching Surges ≈ 2-3 times normal peak voltage • Radio interference is increased in foul weather.

• Switching Surges ≈ 1.7 times normal voltage • Radio interference is decreased in foul weather. • Insulation must withstand direct voltage > peak value 8 of AC voltage

 Comparison of AC and DC Transmission Basis for comparison

Remarks

Reactive Power & Surge Impedance Loading (SIL)

• Voltage profile along line depends on inductive voltage drop and capacitive line charging, if sending and receiving end voltages are kept fixed.

Reactive power requirements of line heavily depend upon line length.

• For UG cables, the line charging current (proportional to line length) pose a serious problem.

HVAC Transmission

• For long and heavily loaded OH lines, excessive voltage drop (or rise) occurs at the middle portion of the line. • Line compensation must be employed to counteract the reactive power requirements.

HVDC Transmission

• No problem of reactive power and line charging, however converters at both sides of the line require huge amount of reactive power.

• Ferranti Effect Stability

• Loss of synchronism and hunting due to sudden loading / un-loading.

Steady state stability and transient stability • Critical power (or torque) angle

• Transient stability is lower than steady state stability. (Less than the half of thermal limit) • Maximum transferable power is dependent upon line reactance.

• Very high transient stability limit • Maximum transferable power is only limited by thermal loading. 9

 Comparison of AC and DC Transmission Basis for comparison Control of Power

Remarks

• Power control and power modulation is necessary for better performance and increased reliability of the system.

HVAC Transmission

HVDC Transmission

• Power control is not possible without additional components.

• Bi-directional control over power transmitted in either direction. Control of power is easy and rapid.

• Elimination of faults are complicated. Amount of Power Transfer

•Power transfer capability should be independent of line length. •In UG cables, reliable power transfer is dependent on permissible working stress.

• Power transfer capability is dependent upon power factor of operation (i.e. line losses).

• Fast control to limit fault currents. • For same amount of power transfer, DC lines are simpler, cheaper and require less right-of-way.

• Power transfer is complicated by line configuration, • In DC cables, permissible reactive power and right-of working stresses are higher way requirements. than that of the AC cables. • In AC cables, permissible working stresses are lower. 10

 Comparison of AC and DC Transmission Basis for comparison

Remarks

Economy and Terminal Equipments

• Total Cost = Fixed Cost + Running Cost

HVAC Transmission

• Initial cost is low but transmission cost increases tremendously with the • Cheapest method by which a certain increase in line length. amount of power at a certain load factor can be transmitted reliably • Right-of-way requirements over a certain distance. and cost due to conductor, tower erection and line commissioning is high.

HVDC Transmission

• Initial cost is high, primarily due to very expensive converter station equipments. • For same power, Right-ofway requirements and cost due to conductor, tower erection and line commissioning is low. • Transmission loss is low. Ideally suited for bulk power transmission.

Ground Return

• Use of ground as return is not • Ground can be used as possible due to very low return by using specially penetration. constructed ground electrodes. 11

 Comparison of AC and DC Transmission Basis for comparison Circuit Breaking

Short-circuit Currents

Generating Units and Tie-line Power Control

Remarks

• Circuit breaking is important for rapid fault clearing.

HVAC Transmission

HVDC Transmission

• Normal circuit breaking is possible due to presence of natural current zero.

• Circuit breaking is complicated due to absence of natural current zero in DC line current.

• Limited short-circuit fault level.

• Limited overload capability of the converter valves.

• Fault level increases with addition of new lines.

• Fault level remain unchanged with addition of new lines.

• Asynchronous operation is not possible.

• Asynchronous operation is possible. • Ideally suited for off-shore power generation. • Tie-line power control is easy and rapid.

• Tie-line power control is complicated

12

Introduction  Advantages of HVDC Transmission • Greater power per conductor and higher efficiency of transmission. • Simpler line construction and commissioning. • No problem of charging current and cables can be worked at higher voltage gradient. • Line power factor is always unity and line does not require reactive compensation. • Less corona loss and radio interference, especially in foul weathers. • Synchronous operation is not required and hence distance is not limited by stability. • May interconnect AC systems of different frequencies. • Low short-circuit current on DC line and does not contribute to short circuit current of AC system • Tie line power control is easy and rapid. • Ground return can be used. HVDC Transmission

13

Introduction  Limitations of HVDC Transmission • Converters and valves are expensive. • Converter stations require huge amount of reactive power. This reactive power needs to be supplied from AC side at both ends. • Converter stations generate harmonics on both AC and DC sides, which are to be eliminated using huge filters. • Converters have limited overload capacity. • Complexity of control. • Reliable multi-terminal DC systems are yet to be established because of lack of HVDC circuit breakers.

HVDC Transmission

14

Introduction  Applications of HVDC Transmission • For bulk power transmission over long distances by overhead lines. • For interconnection of systems using long cables, especially for sea water crossing (such as for feeding power to a far-off island using submarine cables). • For interconnection of AC systems having different frequencies or for asynchronous operation. • For supplying power to special types of load centers (such as congested urban areas) where it is difficult to acquire right of way for overhead lines and where the lengths involved make the AC cables impractical.

HVDC Transmission

15

Introduction  Reliability of HVDC Transmission • Reliability of HVDC transmission system is comparable to that of EHVAC   transmission system. • Reliability is rapidly increasing with the development of high quality valves and switches. • Main parameters for reliability –  1.  2.  3. Mean Time To Failure (MTTF)  4. Mean Time To Repair (MTTR)

• Monopolar operation is possible in the event of outage of one pole. HVDC Transmission

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