High Voltage Transmission System

HIGH VOLTAGE TRANSMISSION SYSTEM BEEDEE 709/ MPSDEE 709

VII SEMESTER B.TECH./ M.TECH. (Integrated)

SASTRA UNIVERSITY SCHOOL OF ELECTRICAL & ELECTRONICS ENGINEERING

DEPARTMENT OF ELECTRICAL & ELECTRONICS ENGINEERING

COURSE INSTRUCTOR: Dr. S. VENKATESH/ SAP-SEEE

1

COURSE OBJECTIVES • Provide insights into modelling and design of EHV lines which cater to the following:

Computational methods to obtain the line inductance and capacitance for various configurations

Various factors that affect the design of line parameters

• Present an overview of HVDC converter operation and strategies for control of power flow in DC lines

Generic Converter Configuration

Graetz Bridge Converter Operation (Rectifier and Inverter)

Detailed Analysis Graetz Bridge Converter (With and Without Overlap Angle)

Control Aspects and Control Characteristics

De-energization and Energization of DC LInks

• Provide an overview of the various types of overvoltages

Ferroresonance

Switching

Lightning

• Deliberate on the Methods of protection of HVAC and HVDC transmission systems

2

COURSE CONTENT

• UNIT- I: Introduction (10 Periods)

EHVAC and HVDC transmission - Comparison between HVAC and HVDC overhead and underground transmission scheme - Standard transmission voltages - Factors concerning choice of HVAC and HVDC transmission - Block diagram of HVAC and HVDC transmission schemes – Modern trends in EHVAC and HVDC transmission systems.

• UNIT- II: EHVAC Transmission and Corona (18 Periods)

Problems of EHVAC transmission at power frequency - Generalized constants - Power circle diagram and its use - Voltage control using compensators - Properties of bundled conductors - Inductance and capacitance of EHV line - Surface voltage gradient on single, double and more than three conductor bundles - Design aspects of EHV Lines.

Corona effects - Power loss - Increase in radius of conductors - Qualitative study of corona pulses - Corona pulse generation and properties.

• UNIT- III: HVDC Converters and control (15 Periods)

Converter configurations for HVDC system - Three-phase fully controlled Graetz Bridge converters for HVDC system - Operation as rectifiers and line commutated inverters - Analysis of Bridge Converters (Without and With Overlap – Two & Three Valve Conduction mode)- Converter equivalent circuits.

Basic means of control - Desired features of control - Control characteristics - Power reversal - Constant current control - Constant extinction angle control- Energization and de-energization of DC links.

• UNIT- IV: Overvoltage in EHV Systems (12 Periods)

Origin and types - Ferroresonance overvoltage - Switching surges, reduction of switching surges on EHV systems - Introduction to EHV cable transmission, electrical characteristics of EHV cables, properties of cable insulation materials - EHV insulators - Characteristics and pollution performance - Protection of HVAC and HVDC systems.

3

REFERENCES • Rakosh Das Begamudre, “EHV AC Transmission Engineering”, Wiley

Eastern Limited, 2006.

• E.W.Kimbark, “Direct Current Transmission, Volume- I”, Wiley Interscience , 1971

• Prabha Kundur, “Power System Stability and Control”, 2nd Reprint Edition, Tata McGraw Hill (P) Limited, New Delhi, 2006.

• K. R. Padiyar, “HVDC Power Transmission Systems: Technology and System Interactions”, New Age International Pvt. Ltd, First Edition 1990, Reprint 2005.

• J. Arillaga, “High Voltage Direct Current Transmission”, Peter Pregrinus, London, 1983.

• Dr. S. N. Singh, “High Voltage DC Transmission”, National Programme on Technology Enhanced Learning (NPTEL) Web Course Series

• IEC 60038: 2002-07, “IEC Standard Voltages”, Edition 6.2, 2002 • www.sari-energy.org/PageFiles/What_We_Do/activities/HVDC_Training

/Presentations/Day_1/1_HVDC_SYSTEMS_IN_INDIA.pdf, “HVDC Systems in India”, Powergrid Report

4

http://www.sari-energy.org/PageFiles/What_We_Do/activities/HVDC_Training/Presentations/









EVOLUTION OF EHVAC AND HVDC TRANSMISSION SYSTEM

• The first public power station

Holborn , London – 1882

Produced direct current (DC) at low-voltage

Service limited to localized areas and used mainly for electric lighting

• The first major a.c. power station

• Deptford, London - 1890

• Supplying power to central London

• Distance - 28 miles

• Operating Voltage- 10 kV

• EHVAC transmission has seen its development since the end of the Second World War (1945) Installation of 345 kV in North America

– 400 kV in Europe

• In 70 years, the highest commercial voltage has increased globally substantially due to the raising demand

1200 kV – Ultra High Voltage AC Transmission (UHVAC)

800kV – Ultra High Voltage DC Transmission (UHVDC)

• India has embarked on setting up 1200kV a.c. transmission system

The first step towards this is the implementation of 1200kV D/C/ Bina Test Station)

• India is also implementing power transmission projects to transmit HVDC power at 800kV transmission Including the first 800kV Multi Terminal HVDC Project in the world: Biswanath- Agra

5

MAJOR HVAC TRANSMISSION SYSTEMS IN CRONOLOGICAL ORDER

Year Power Transmission

Voltage Level (kV)

Location and Country

1890 10kV Deptford, London

1907 50kV Munich, Germany

1912 110kV Lauchhammer, Germany

1926 220kV North Pennsylvania, U.S.

1936 287kV Boulder Dam, Arizon- Nevada, U.S.

1952 380kV Harspränget − Hallsberg, Sweden

1959 525kV Russia, Formerly USSR

1965 735kV Manicouagan- Montreal, Canada

1967

1969

765kV Russia, Formerly USSR

Ohio-Kentucky, AEP, U.S.

1985 1150kV Russia, Formerly USSR

1999 1000kV Kita- Iwaki Powerline, Japan

2009 1000kV China

2013 1200kV Bina Test Station, Madha Pradesh, India (Results will be

utilized for Wardha- Aurangabad 1200kV Project-

Ongoing)

6

MAJOR HVDC TRANSMISSION SYSTEMS IN CRONOLOGICAL ORDER

Year Power Transmission

Voltage Level (kV)

Location and Country

1954 100kV, 96km, 20MW Gottland, Sweden

1961 100kV, 64km, 160MW England- France

1970 400kV, 1362km, 1440MW Pacific Intertie, U.S.

1972 450kV, 892km, 1620MW Nelson River 1, Canada

1978 533kV, 1414km, 1920MW Cabora Bassa, South Africa

1978

1985

250kV, 930km, 900MW

500kV, 892km, 1800MW

Nelson River 2, Canada

1984

1986

300kV, 785km, 200MW

600kV, 892km, 2383MW

Itaipu 1, Brazil

1986 500kV, 784km, 1920MW Intermountain, U.S.

1987 600kV, 805km, 3150MW Itaipu 2, Brazil

1990 500kV, 1000km, 1200MW Gezhouba– Shanghai, China

1991 500kV, 910km, 1500MW Rihand-Delhi, India

2002 500kV, 960km, 3000MW Three Gorges, China

2003 500kV, 1369km, 2000MW Talcher- Kolar, India

Ongoing 800kV, 1728km, 6000MW Biswanath- Agra, India (First Multi-

terminal HVDC Project)

7

NEED FOR UHV/ EHV AC & DC TRANSMISSION FOR BULK POWER TRANSFER

• High Power Transfer/ Evacuation Requirements:

Electric power (P) transmitted on an overhead a.c. line increases approximately with the surge

impedance loading or the square of the system’s operating voltage ( P= V2/ Zs with Zs 250Ω)

V (kV) 400 700 1000 1200 1500

P (MW) 640 2000 4000 5800 9000

• Aspects related to Right of Way (RoW): • Erecting power transmission lines involves obtaining the extremely elusive RoW • The difference in RoW requirement for a 400kV line and a 1,200kV is not extremely

significant.

8 Source: Technical Report, “HVDC Systems in India”, Powergrid, India

NEED FOR UHV/ EHV AC & DC TRANSMISSION FOR BULK POWER TRANSFER

• Power Demand-Supply Situation: (In the Indian Context) Power generation centres are typically in eastern and north-east regions while

consumption centres are spread across rest of India – North, West and South

Generation hubs are limited consumers, warranting the need for carrying power across long distances

Provides possibility to import power from hydropower-rich neighbours (Bhutan

9

Source: Technical Report, “HVDC Systems in India”, Powergrid, India

INCREASE IN TRANSMISSION VOLTAGE IN INDIA

10 Source: Technical Report, “HVDC Systems in India”, Powergrid, India

MAJOR MILESTONES IN HVDC TRANSMISSION SYSTEMS IN INDIA

Power Transmission

Voltage Level (kV)

Location

2 x 250MW, 70kV, Back-to-Back Link Vindhyachal

500kV, 814km, 1500MW, Bipolar Link Rihand- Dadri (Northern Region)

2 x 250MW, 140kV, Back-to-Back Link Chandrapur (Southern & Western Region)

500kV, 752km, 1500MW, Bipolar Link Chandrapur-Padghe (Western Region)

500MW, 140kV, Back-to-Back Link Vishakapatnam (Southern- Eastern Region)

500MW, 140kV, Back-to-Back Link Sasaram (Northern- Eastern Region)

2 x 500MW, 140kV, back-to-Back Link Gazuwaka (Eastern- Northeastern)

500kV, 1376km, 2000MW, Bipolar Link Talcher- Kolar (Eastern- Southern Region)

500kV, 780km, 2500MW, Bipolar Link Ballia- Bhiwadi

800kV, 1728km, 3000MW/ 6000MW,

Multi-terminal HVDC Link

Biswanath- Agra (North Eastern i.e. Assam-

West Bengal- Bihar- Uttar Pradesh)- Ongoing

1 x 500MW Interconnector Project India- Bangladesh Grid – Under Consideration

± 400 kV, 334km, 4 x 250 MW, Bipolar Link

with Submarine Cable ( app 90 Km)

Indo-Sri Lanka Inteconnector Link (Madurai- Sri Anuradhapura)- Under Consideration

11

SOME IMPORTANT ISSUES RELATED TO UTILITY OF EHV/ UHV

• Complexities related to bundled conductors Need for detailed studies related to spacers, spacer span calculation, Phase Pull

Force Calculation etc

• High surface voltage gradient on conductors: Inhomogeneous/ Non-Uniform Electric Field

• Effect of Corona Discharges Audible Noise, Radio Interference, Corona Energy Loss, Carrier Interference and TV

Interference.

• High electrostatic field under the line- environmental challenges: – Effect of Electrostatic field on human, animal, plants

• Switching Surge Overvoltages which cause more devastation to air-gap insulation (than lightning or power frequency voltages)

• Increased Short-Circuit currents and possibility of ferro-resonance • Use of gapless metal-oxide arresters replacing the conventional gap-type

Silicon Carbide arresters-lightning and switching-surge duty – Limitations related to Energy Capability – Complexities related to overlapping requirements of Lightning and Switching

overvoltages

• Insulation coordination based on switching impulse levels

12

STANDARD TRANSMISSION VOLTAGES

• Voltages adopted for transmission of bulk power have to conform to standard specifications formulated internationally. (IEC 60038)

A.C. three-phase systems having a nominal voltage above 35 kV and not exceeding 1200 kV

13 Source: IEC 60038: 2002-07, “IEC Standard Voltages”, Edition 6.2, 2002

COMPARISON OF HVAC AND HVDC TRANSMISSION SYSTEMS

• Major factors considered by a system planner of Power System for the choice

Choice of Power Transmission (HVAC/ HVDC)

Economics of Transmission

Investment Cost

ROW

Transmission Towers

Conductors

Insulators

Terminal Equipment

Operational Cost

Line Losses

Dielectric Power Losses

Corona Losses

Skin Effect

Technical Performance

Aspects related to Power Electronics Devices and

Converters

Stability Limits

Voltage Control

Line Charging Current

Line Compensation

Problems of Interconnections

Reliability

Energy Availability

Transient Reliability

MTTF

MTTR

14

ECONOMICS OF POWER TRANSMISSION- INVESTMENT COST

• Investment Cost:

Right of Way (RoW)

Lesser in HVDC than HVAC Systems

15 Source: IS 5613: 1989 (Part 3/ Sec 2), “Code of Practice for Design, Installation and Maintenance of Overhead Power Lines- Part 3: 400kV Lines”, Reaffirmed 2004

• Transmission Towers Simpler in Construction and Cheaper for HVDC

Associated number of conductors are reduced (2 as compared to 3 for a S/C Transmission Line)

Number of Insulator Strings are reduced (2 sets as compared to 3 for a S/C Transmission Line)


16 Source: Technical Report, “High Voltage Direct Current Transmission- Proven Technology for Power Exchange”, Siemens, Germany

CONFIGURATION OF TYPICAL TRANSMISSION TOWERS

17 Source: Robert D Castro, “Overview of the Transmission Line Design Process”, Electrical Power Systems Research , 35, pp. 109-118, 1995


• Number of Conductors: Reduced in the case of HVDC ( 2 instead of 3 for S/C Configuration)

• Power handling Capability: (assuming similar Insulator Characteristics- Insulation Level )

Can carry as much power as AC does ( 2 instead of 3 for S/C Configuration)

• Insulators/ Insulation: Reduced requirements in the case of HVDC

• Terminal Equipment: Cost increases in HVDC transmission system due to the following:

Metal Oxide Surge Arresters for DC Applications

Conversion Equipment- Increased Ratings of Thyristor Valves

Filters (DC and AC) for suppression of Harmonics

Non- availability of Voltage transformation equipment (transformers) in DC

Operational Cost: Line Losses: [2I2R= 3I2R]

About 67% that of AC system (assuming same current carrying capacity)

18

ECONOMICS OF POWER TRANSMISSION- OPERATIONAL COST

• Dielectric Power Losses: Reduced in the case of HVDC ( more-so with DC power cables)

P= (V2/ R)

From Phasor Diagram: tan δ = (V/R)/VωC;

V/R= V ωC tan δ;

Therefore P = V2 ωC tan δ = V2 2f C tan δ

19

ECONOMICS OF POWER TRANSMISSION- OPERATIONAL COST

• Corona Losses: Peek’s Empirical Formula:

20

phkmkWd

rVV

fxP

op//

225510214

where ‘f’ is the supply frequency

Vp is the operating voltage in kV

V ois the critical disruptive discharge voltage in kV

δ is the air density correction factor

‘r’ is the radius of the conductor

‘d’ is the spacing between conductors

Corona Losses due to DC are far lesser than AC (due to the term f in the equation above

• Skin Effect:

Absence of skin effect in DC reduces marginally the power losses

ECONOMICS OF POWER TRANSMISSION- COMPARISON OF COST

21

Source: Technical Report, “High Voltage Direct Current Transmission- Proven Technology for Power Exchange”, Siemens, Germany

TYPES OF HVDC LINKS

22

Monopolar Link Bipolar Link

Homopolar Link Back-to- Back Link

TECHNICAL PERFORMANCE- ASPECTS RELATED TO POWER ELECTRONICS DEVICES AND CONVERTERS

• Fast Controllability of Power Transmitted HVDC transmission system provides better avenues due to the advent of developments in

fast switching high power rated power electronic devices (SCR, GTO, MCT)

• Full Control over the of Power Transmitted HVDC transmission system offers facility to control the firing angle (α) and extinction angle

(γ) of the converters which regulates the power flow magnitude and direction

23

TECHNICAL PERFORMANCE- ASPECTS RELATED TO POWER ELECTRONICS DEVICES AND CONVERTERS

24

ciLcr

doidor

dRRR

CosVCosVI

TECHNICAL PERFORMANCE- STABILITY LIMITS

• Stability Limit: Ability of an ac system to operate with all synchronous machines in synchronism

• If a long line is loaded to a certain values (steady state stability limit) the synchronous machine accelerates and goes out of synchronism with those in the receiving end.

• This slipping out of the electro-dynamic system results in failing to transmit power

leads to objectionable fluctuation in the voltage

• Even if a line is operated below steady-state limit, the machine at sending end and receiving end may lose synchronism after a large disturbance (short circuit) unless the line is operated below its transient stability limit (lower than the steady state stability limit)

• Practically, for small disturbances the transient stability limit becomes the measure of the steady state stability limit

25

SinX

VVP

Rs

• In DC transmission link there is no direct influence of stability problems due to the following aspects: Two separate as systems interconnected by a dc link need not necessarily

operate in synchronism (even if their nominal frequencies are equal)

No influence of ‘X’

Each of the separate as systems may have its own internal stability issues

Sustained interruptions of the power on the dc line constitutes a mild threat to stability (caused by loss of a large load in the sending end system/ loss of a generator in the receiving end system)

26

TECHNICAL PERFORMANCE- STABILITY LIMITS

• Assuming that transmission line is lossless, Reactive power absorbed by the line Q L = I2ωL Reactive power supplied by the line Q C = V2ωC

• When reactive power supplied and absorbed by the line are equal the resultant leads to the concept of SURGE IMPEDANCE LOADING (Zs) V2ωC = I2ωL

27

TECHNICAL PERFORMANCE- VOLTAGE CONTROL

sZ

CL

I

V

At SIL: voltage throughout the length of the line is the same transmission line is terminated by a load corresponding to its surge impedance

with the voltage at both ends being constant

• In practice the load given to a overhead line is larger than SIL • The net reactive power absorbed by the line must be provided from one/ both

ends of the line and from intermediate series capacitors When I2ωL > V2ωC – VOLTAGE SAG When I2ωL < V2ωC – VOLTAGE RISE

• Maintenance of constant voltage requires REACTIVE POWER CONTROL IN AC SYSTEMS

• HVDC transmission system does not require reactive power control • However, converters at both ends of the DC line require reactive power from the

ac systems

28

TECHNICAL PERFORMANCE- VOLTAGE CONTROL

TECHNICAL PERFORMANCE- LINE CHARGING CURRENTS

29

• Line charging current in AC more over poses serious problems in cables

As length increases (for a lightly loaded system) the capacitance increases

Receiving end voltage becomes more than sending end voltage (Ferranti Effect in the case of Overhead lightly loaded/ no load transmission line)

• Line Compensation

AC lines require reactive power compensation systems to overcome issues related to line charging currents and stability limitations

• Series Capacitors and Shunt Inductors

• Issues related to AC Interconnection

Two power systems connected with an AC tie (synchronous interconnection), Automatic Generation Control of both systems have to be coordinated

• Using Tie- line power and frequency signals

Problems arise due to:

Presence of large power oscillations

Increase in fault level

disturbance from one system to the other

Two separate as systems interconnected by a dc link NEED NOT necessarily operate in synchronism (even if their nominal frequencies are equal)

30

TECHNICAL PERFORMANCE- AC INTERCONNECTION

• Role of Ground Impedance in AC transmission

Presence of zero-sequence currents cannot be permitted due to high ground impedance

• Leads to poor efficiency of power transfer

• Increase in RI

Role of Ground Impedance in DC transmission

Negligible in DC currents

DC links can operate using one conductor with ground return (Monopolar Mode)

In Monopolar Mode:

AC network feeding the DC converter station operates with balanced voltages and current (single pole operation of DC is possible for extended periods)

Objectionable only when buried metallic structures (pipes) are present

Leading to corrosion due to DC current flow

31

TECHNICAL PERFORMANCE- GROUND RETURN

• Applications

Long distance bulk power transmission

Underground or underwater cables

Asynchronous interconnection of AC systems operating at different frequencies

Control and stabilization of power flows in AC ties

Limitations

Difficulty in breaking DC currents

Inability to use transformers to transform voltage levels (transformer)

High cost of conversion equipment

Generation of harmonics which necessitate AC and DC filters are costly

Complexities of control

32

APPLICATIONS & LIMITATIONS OF HVDC TRANSMISSION

RELIABILITY

• Energy Availability

33

%1100

timetotal

timeoutageequivalenttyAvailabiliEnergy

Equivalent time is product of outage time fraction of system capacity lost due to outage

• Transient Reliability

faultsACrecordableofNo

designedasperformedsystemHVDCtimesofNoliabilityTransient

.

.100Re

Recordable AC faults cause one or more AC bus phase voltage to drop below 90% of voltage prior to fault

• Energy Availability and Transient Reliability of existing HVDC systems • > 95%

• Developments in technology have ensured improved reliability • Control and Protection • LTT has lead to elimination of high voltage pulse transformer and auxiliary supplies

FACTORS INFLUENCING THE CHOICE OF TRANSMISSION

• CHOICE BASED ON VOLTAGE LEVEL

• CHOICE BASED ON INSULATION RATIO

• CHOICE BASED ON POWER TRANSFER CAPABILITY

• Choice Based on Voltage Level:

• Objective: Choice made such that cost for transmission of power (P) is minimized

• The following are the costs:

Cost due to Investment (C1)

Cost due to Losses (C2)

Cost due to Investment (C1):

Proportional to Voltage Level (V)

Proportional to number of conductors (n) and area of cross-section (q)

Overhead charges (A0)

34

C1= A0 + A1 nV + A2 nq

FACTORS INFLUENCING THE CHOICE OF TRANSMISSION- VOLTAGE LEVEL

Cost due to Losses (C2):

Loss/ length of transmission

Time of operation of the transmission system (T)

Load Loss Factor (L)

Cost/ Energy (p)

35

Losses = n I2R

nVP

V

n

P

condI

/

nqV

P

qnV

PnR

nV

PnlengthLoss

222

/

C2 = Loss x Time of Operation x Loss Load Factor x Cost/ energy


Total Cost (C) = C1 + C2

To optimize the transmission system cost the material cost (nq) should be minimized

36

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37

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FACTORS INFLUENCING THE CHOICE OF TRANSMISSION- INSULATION RATIO

38

Factors that affect insulation of overhead transmission lines: Operating Voltage Switching Surge Overvoltage Lightning Overvoltage Operating Voltage influences the Leakage Distance Switching and lightning over-voltages influence the required insulator chain length and

striking distance


39

• Withstand Voltage Factor (K Factor):

Ratio of d.c. voltage and a.c. voltage with respect to ground

Typical values of K:

K=√2 – Indoor Porcelain

K=1 – Outdoor Porcelain (implies poor wet flashover performance)

K- 2 to 6- Power Cables

O/H lines are insulated for over-voltages expected during faults, switching operations etc

AC transmission lines are normally insulated against over-voltages > 4 times rated voltage

AC insulation Factor (K1):

K = d.c. withstand voltage level/ a.c. (rms) withstand voltage level

K 1= a.c. insulation level/ rated a.c. voltage (rms) level

K 1= a.c. insulation level/ Ep 2.5


40

For d.c. lines,

DC Insulation Factor (K2):

Insulation Ratio:

K 2= d.c. insulation level/ rated d.c. voltage

K 2= d.c. insulation level/ Vd 1.7

Insulation Ratio = Insulation required by 1 AC phase/ Insulation required by 1 DC pole

voltagewithstadcdlevelinsulationcd

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41

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42

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• Assuming same Power Transmitted (P) equal losses in both transmission

systems

Power Loss in a.c. system (Lac):

Power Loss in d.c system (Ldc):

Assuming equal losses:

RILpac

23

RILddc

22

RIRIpd

2232

pdII

2

3


43

Power Transmitted by a.c. (Pac):

Power Transmitted by d.c . (Pdc):

Insulation Ratio:

• If K=1, K1=2.5 and K2=1.7

• Insulation Ratio = 1.2

• INSULATON REQUIRED FOR AC IS MORE THAN THAT FOR DC SYSTEM

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FACTORS INFLUENCING CHOICE OF TRANSMISSION- -POWER HANDLING CAPABILITY

44

Conversion of Double Circuit 3 Phase AC Line to 3 DC Circuits of 2 Conductors:

Power Transmitted by a.c . (Pac):

• Power Transmitted by a.c . (Pdc):

Considering that equal current and insulation are considered:

IL = Id

Power Ratio:

If =1, K1=2.5 and K2=1.7

Power transmitted with DC can be increased to 147% (47% > AC)

LpacIVP 6

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BLOCK DIAGRAM OF HVAC AND HVDC TRANSMISSION SYSTEMS

45

COMPONENTS OF HVDC LINK

46 Source: S. Kamakshaiah, V. Kamaraju, 'HVDC Transmission', 1st Edition, Tata McGraw Hill, 2011


47

COMPONENTS OF HVDC LINK • Converter (Bridge) Unit:

Located in “VALVE HALL” Single (or) Multi- Bridge (Series/ Parallel) Valve Configuration- Single/ Double/ Quadri Cooling Arrangement- Air/ Oil/ Water/ Freon/ De-ionized Water Valve Firing Strategy- Light guide system using Optic fiber Protection- Snubber Circuits, Surge Arresters

Converter Transformer: Winding- 3φ, 2 winding/ 1φ, 3 winding/ 1φ, 2 winding Vector Group- Star- Star and Star- Delta

Valve side of the transformer- Neutral point is ungrounded AC side of transformer- grounded

Leakage reactance of transformer chosen to limit S/C current DC magnetization & Core Saturation Designed to withstand DC voltage stresses K factor based Transformer- accommodate harmonic currents generated

by non-linear loads K factor is a weighting factor of the harmonic currents in the load according to

their effects on transformer heating (ANSI- IEEE C57.110)

48


49 Source: Valve Hall in Chandrapur, India, ABB constructed Chandrapur- Padghe HVDC Link

• Smoothing Reactor

Reduce incidence of commutation failure in inverter (due to dip in AC voltage)

Smooth ripples in DC current (during lightly loaded condition)

– Limit the value of peak current in rectifier (due to S/C on DC line)

Limit current in valves during bypass pair operation (due to discharge of shunt capacitance of DC line)

Prevent commutation failures in inverter (reducing rate of rise of DC in bridges when direct voltage of another series connected bride collapses)

50


Source: Songo- Mozambique Converter Station, Hidroeléctrica De Cahora Bassa (HCB) in Mozambique and Eskom in South Africa, 2013

• FILTERS

Provide a path of low impedance to AC harmonics

Tuned filters- Single & Double Tuned

Damped Filters

Connected between converter transformer and AC station

Suppress HF currents

Filtering oscillations and ripples in DC

REACTIVE POWER SOURCE

Huge reactive power requirements at the converter terminal

Reactive power requirement of about 50- 60% of active power

Provided by AC Filters (partly)

Shunt Capacitors (switched)

Synchronous Condensers

Static VAR Systems 51


• DC CIRCUIT BREAKER

No natural current zero in the case of DC circuit

Can be brought to zero ONLY by applying a counter voltage higher than system voltage

Need for dissipation of large energy stored in the inductance of the DC circuit

52


RECENT TRENDS IN HVDC TRANSMISSION SYSTEM

• Developments in major aspects: Power Semiconductor Devices Digital Electronics & Control Protection Equipment

Power Semiconductor Devices: New devices- GTO: 8kV, 4kA; IGBT: 6.5kV, 1kA; Thyristor: 6kV Size of device- >100mm (diameter) leading to reduced no. of

parallel connections Increase in current rating (higher overload capacity) Increase in voltage rating Manufacturing Process- Silicion cost reduced by 15-20% using

magnetic CZ (czochralski) method instead of conventional method Development of LTT (improved reliability of converter operation) ZnO arresters Cooling Methods (Deionized water cooling- Two phase flow using

forced vaporization)

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• Development of HVDC- VSC Stations: Interconnecting weak AC systems

Connecting large-scale wind power to the grid

HVDC interconnections to be expanded to become Multi-terminal link 54


• Suspended Valves: Quadri-valve structure leads to compact

assembly BUT they are tall (about 16m) In regions where seismic effects are observed

appropriate to suspend the valves from ceiling Comprises spring and damper arrangement for

mechanical isolation of valve (from building due to vibrations)

Connection to wall bushings and between walls Flexible Bus

• Static Induction Thyristor: Thyristor with a buried gate structure (gate

electrodes are placed in n-base region) Normally ON-state Gate electrodes must be negatively biased to

hold off-state Rating: 4000V, 400A (developed in Japan as an

alternative to GTO)

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• Assymetrical Thyristor: Blocking voltage for symmetrical thyristors < 4 to

5kV

Series Connection of Asymmetrical Thyristors (ASCR) and a diode

Possibility of adjusting the turn –OFF time

Not used in HVDC (under experimentation)

• Light Triggered Thyristor: Infinite gate isolation

Total noise immunity (control circuit)

Fast Turn-On time

Elimination of HV Pulse Transformer and Power Auxiliaries

Light Sources- Gallium Arsanide LED, Laser Diodes

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COMPARISON OF DEVELOPMENTS IN RATINGS OF POWER ELECTRONIC DEVICES

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• Converter Control: Development of Micro-computer based Converter Control Equipment

Redundant Converter Control

Possibility to perform scheduled preventive maintenance on stand-by system (due to reduced outage rate)

Possibility for adaptive control algorithms/ expert systems for fault diagnosis and protection

Conversion of Existing AC Lines

Constraints of RoW in AC

Issues related to Electromagnetic Induction in AC Lines

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RECENT TRENDS IN HVAC TRANSMISSION SYSTEM- FACTS

• Issues related to Reactive Power Flow in AC Transmission Systems: Consumer loads require reactive power (continuously varying and

increases the transmission losses and affect voltage in the tranmission network)

Slow mechanically switched components which are used for reactive power compensation leads to less precise and less efficient control of transmission characteristics (use of passive elements such as reactors and capacitors)

Leads to limitations in power transfer, steady state and dynamic stability limits

• Aspects related to Long Distance High Voltage large Power Transfer Capacity • Offshore wind farms also have very long transmission lines (can be “tens

to hundreds of miles”) 59

RECENT TRENDS IN HVAC TRANSMISSION SYSTEM- FACTS

• FACTS (Flexible Alternationg Current Transmission System) Developed in 1986 by EPRI, USA

Slatt Substation in Northern region. 500 kV, 3-phase 60 Hz substation, developed by EPRI (Bonneville Power Administration and General Electric Company)

• "A power electronic based system and other static equipment that provide control of one or more AC tranmission system parameters to enhance controllability and increase in power transfer capability“

• Merits of FACTS: Reactive Power Control

Power Oscillation Damping

Improved Power Transfer Capability

Enhanced Steady State, Transient and Dynamic Stability

Improved Voltage Quality due to enhanced Voltage Control

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RECENT TRENDS IN HVAC TRANSMISSION- IMPROVEMENTS IN POWER TRANSFER CAPABILITY

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In the case of a no-loss line, voltage magnitude at the receiving end is the same as voltage magnitude at the sending end: Vs = Vr=V. Transmission results in a phase lag δ that depends on line reactance X.

Active power P is the same at any point of line

Reactive power at sending end is the opposite of reactive power at receiving end

Active power mainly depends on δ Reactive power mainly depends on voltage magnitude


(SERIES COMPENSATION)

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FACTS for series compensation modify line impedance: • X is decreased so as to increase the transmittable active power. However, more reactive power must be provided.

http://en.wikipedia.org/wiki/File:FACTS_theory_series_compensation.PNG


(SHUNT COMPENSATION)

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• Reactive current is injected into the line to maintain voltage magnitude. • Transmittable active power is increased but more reactive power is to be provided.

http://en.wikipedia.org/wiki/File:FACTS_theory_shunt_compensation1.PNG

TYPES OF FACTS CONTROLLERS

• Basic Types of FACTS Controllers

Series Controllers

Shunt Controllers

Combined Series- Series Controllers

Combined Series- Shunt Controllers

Series Controllers

Series controller could be a variable impedance or a variable source both are power electronics based.

In principle, all series controllers inject voltage in series with the line.

Shunt Controllers

Shunt controllers may be variable impedance connected to the line voltage causes a variable current flow

Represents injection of current into the line

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TYPES OF SERIES CONTROLLERS

• Types of Series Controllers Static Synchronous

Series Compensator (SSSC)

Thyristor Controlled Series Capacitor (TCSC)

Thyristor Switched Series Capacitor (TSSC)

Thyristor Controlled Series Reactor (TCSR)

Thyristor Switched Series Reactor (TSSR)

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Series capacitor bank is shunted by a thyristor controlled reactor

Series reactor bank is shunted by a thyristor controlled reactor

TYPES OF SHUNT CONTROLLERS • Types of Shunt Controllers

Static synchronous compensator (STATCOM) Static VAR compensator (SVC)

Thyristor‐controlled reactor (TCR) • Reactor is connected in series with

a bidirectional thyristor valve. Thyristor valve is phase-controlled. Equivalent reactance is varied continuously

Thyristor‐switched reactor (TSR) • Thyristor is either in zero- or full-

conduction. Equivalent reactance is varied in stepwise manner.

Thyristor‐switched capacitor (TSC) • Capacitor is connected in series

with a bidirectional thyristor valve. Thyristor is either in zero- or full- conduction. Equivalent reactance is varied in stepwise manner.

Mechanically‐switched capacitor (MSC)

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http://en.wikipedia.org/wiki/File:FACTS_shunt_compensation1.PNG

ENHANCEMENT OF STABILITY IN POWER SYSTEM

• Transient stability analysis based on “Equal Area Criterion”

• If A2 >=A1, system is stable; otherwise, system is unstable

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High Voltage Transmission System

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