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HVDC Training Course- Steady State -

DIgSILENT GmbH

HVDC Training Course 2

Introduction

• „HVDC“ - general definition:

High Voltage Direct Current Transmission

Application in Long-Distance and Cable Transmission Systems

• Part of FACTS:Flexible AC Transmission Systems

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HVDC Training Course 3

AC vs. DC Transmission

HVDC Training Course 4

AC vs. DC Transmission

AC Transmission• Easy, robust and reliable

• Rather cheap• Only applicable to systems with

the same nominal frequency

• Cable capacitance limits the distance of submarine cables (or increases the cost because of additional compensation)

• Contribution to short-circuit currents

• Dynamic/Transient stability limits

DC Transmission• More complex, power electronics,

including controls are required• Expensive technology• Can connect systems of different

nominal frequency/asynchronous systems

• No limitation by cable capacitance

• No contribution to short-circuit current in interconnected systems

• No dynamic or transient stability limits

3

HVDC Training Course 5

AC vs. DC Transmission

HVDC Advantages• Possibility to connect two networks with different frequency or

different power-frequency control strategies.

• Transmitted power can be controlled and can be held constant independent of network situation within power range.

• Control is flexible and different control strategies can be used.

• The control is fast acting, so the transmitted power can be changed rapidly.

• HVDC systems can also be used in parallel to AC lines for stabilizing the network.

HVDC Training Course 6

AC vs. DC Transmission

AC vs. DC Transmission• Break-even-distance with

overhead lines at about 600-800km

• Break-even-distance is much smaller for submarine cables (about 50 km)

• Distance depends on several factors (both for lines and cables) and an analysis is required.

DC transmission can only be justified, if AC-transmission is impossibleor extremely expensive because of additional compensation

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HVDC Training Course 7

Circuits and Components

HVDC Training Course 8

Diode Turn-On Turn-On & Turn-Off

Thyristor GTO IGBT

Valves/Semiconductor Devices

5

HVDC Training Course 9

Diode Thyristor GTO IGBT

Valves/Semiconductor Devices

Classification of valves into three groups according to their controllability:

Ideal Characteristic:

HVDC Training Course 10

Valve Characteristic Parameters

• Current carrying capability– e.g.: 1000A...4000A (Thyristor, GTO)

• Forward blocking voltage– e.g. 8-10kV (Thyristor)– e.g. 5-8kV (GTO)– e.g. 3-5kV (IGBT)

• dv/dt capability • di/dt capability• Turn-on time and turn-off time• On-resistance (and associated losses)• Switching losses

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HVDC Training Course 11

HVDC Valves

Thyristor element with Thyristor Control Unit (TCU)

Thyristor Module

HVDC Training Course 12

HVDC Valve Halls

Chandrapur - PadgheHVDC Transmission1500MW, ±500kV800km

New Zealand Inter-Island HVDC Link1240MW, ±300kV600km

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HVDC Training Course 13

Semiconductor Capabilities

• Capability and usability of valve devices are depending on:

– Rated Voltage

– Rated Current

– Switching Speed

HVDC Training Course 14

Snubber Circuits

• Snubber circuits are used to change the current and voltage waveform of the valve to reduce the electrical stresses on the switching devices to safe levels.

• RC – Snubbers:– Limit the maximum voltage– Limit dv/dt during turn-off or recovery

• LR – Snubbers:– Limit di/dt during turn-on

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HVDC Training Course 15

Line-Commutated Converter

Vdc

Idc

Vac

Vdc Vac

Idc

HVDC Training Course 16

Self-Commutated Converter

Udc Uac

ACUDC

U

9

HVDC Training Course 17

Self/Line-Commutated Converters

Self-Commutated:• Very good P and Q controllability• Low Harmonic contents (high

switching frequency)• Q can be controlled/provided by

the converters• Independent from the strength of

AC network• High no load losses• New technology (long term testing

required)• Only possible up to 200..330MW

Line-Commutated• Only P controllability, Q resulting• High Harmonic contents, large

filters required• High Q consumption of both

rectifier and inverter• Short-Circuit capacity of network is

important for operation• No load losses can be neglected• Well established, robust technology

• Efficient for high power transfers

HVDC Training Course 18

Self/Line Commutated Converters

Self-Commutated• Modular concept with

standardized sizes possible. • DC circuit is by ‘nature’ a bipolar

technology. Two conductors are required.

• Using turn on/turn off IGBT valves• Very fast and flexible

controllability possible,frequency control possible

• No need of communication between stations

Line-Commutated• always tailor made to suit a specific

application• Can be designed as a monopolar

or bipolar system.• Well established, robust technology• Using turn on GTO valves• Good controllability,

No frequency control

For high power transfers (>200MW), the line commutated converteris still the only possibility

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HVDC Training Course 19

Applications

Self-Commutated

• HVDC light (<330MW)• FACTS (UPFC, STATCOM)• Variable Speed drives

(machine side)• Doubly-fed induction machines

Line-Commutated

• HVDC (High Power)• Back-to-Back HVDC• Synchronous machine drives• DC-machines

HVDC Training Course 20

Line-Commutated Converter

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HVDC Training Course 21

Analysis of the Line-Commutated Converter

HVDC Training Course 22

0.015 0.012 0.010 0.007 0.005 0.002 ..

200.00

100.00

0.000

-100.00

-200.00

Rectifier: Phase Voltage/Terminal DC in kVRectifier: Phase Voltage/Terminal DC in kVInverter: Phase Voltage/Terminal DC in kVRectifier: Line-Line Phase Voltage B/Terminal AC in kV

DIg

SILE

NT

DC-Voltage Wave-Forms

α

α

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HVDC Training Course 23

DC-Voltage

lllllllld UududuVππ

θθπ

θθπ

π

π

π

π

23ˆ3)cos(ˆ3)(3 6

6

6

6

0 ==== ∫∫−−

Diode Rectifier:

Thyristor Rectifier:

ααππ

απαππ

θθπ

θθπ

ααπ

απ

απ

απ

coscos6

sin2ˆ3

6sin

6sinˆ3)cos(ˆ3)(3)(

0

6

6

6

6

dll

lllllld

Vu

ududuV

=⎟⎠⎞

⎜⎝⎛

=⎟⎟⎠

⎞⎜⎜⎝

⎛⎟⎠⎞

⎜⎝⎛ +−−⎟

⎠⎞

⎜⎝⎛ +=== ∫∫

+

+−

+

+−

HVDC Training Course 24

DC-Voltage

lld Uq

qsV ⋅⋅⎟⎟⎠

⎞⎜⎜⎝

⎛⋅=

32sin0

0π

π

n-pulse Bridge:

12-pulse Thyristor Rectifier:

Ideal no-load dc voltage

s0 = sum of valves in seriesq = number of branches in parallel

)cos(232

)cos(32

3sin34)cos(0

απ

αππ

α

⋅⋅⋅

=

⋅⋅⋅⎟⎠⎞

⎜⎝⎛⋅

==

ll

lldd

U

UVV

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HVDC Training Course 25

AC-Current Wave-Forms

0.030 0.020 0.010 0.000 [s]

0.150

0.100

0.050

0.000

-0.0500

-0.1000

-0.1500

REC 1: Phase Current A/Terminal AC in kA

DIgSILENT Rectifier AC-Current

Date: 2/14/2003

Annex: 1 /7

DIg

SILE

NT

3π

3π

−DI

HVDC Training Course 26

AC-Current Fund. Frequency

DDDAC IIdIIπ

πππ

θθπ

π

π 232

3sin

3sin2cos

22 3

3

=⎟⎟⎠

⎞⎜⎜⎝

⎛⎟⎠⎞

⎜⎝⎛−−⎟

⎠⎞

⎜⎝⎛== ∫

−

RMS-value of fundamental frequency component:

Power Factor:

DDDCACllAC IVPIUP === ϕcos3

απ

ϕπ

cos23cos26

DllDllAC IUIUP ==

αϕ coscos =

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HVDC Training Course 27

Commutation

HVDC Training Course 28

Commutation

Id

v1(t)

v2(t)

i1(t)

i2(t)

02112 =−+−

dtdiL

dtdiLvv

dIii =+ 21

dtdiLvv 2

12 2=−

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HVDC Training Course 29

Commutation

( )dtdiLtUvv c

212 2sin2 =+=− αω

( ) ∫∫ =+2

02

0

2sin2it

c diLdttU αω

( )( )αωαω

+−= tL

Uti c coscos22)(2

( )( )αµαω

+−= coscos22

LUI c

d( )llc UU =

HVDC Training Course 30

0.015 0.012 0.010 0.007 0.005 0.002 ..

200.00

100.00

0.000

-100.00

-200.00

Rectifier: Phase Voltage/Terminal DC in kVRectifier: Phase Voltage/Terminal DC in kVInverter: Phase Voltage/Terminal DC in kVRectifier: Line-Line Phase Voltage B/Terminal AC in kV

DIg

SILE

NT

DC-Voltage with Overlap

µ

α µ

α µ

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HVDC Training Course 31

DC-Voltage with Overlap

ddd VVV ∆−= αcos0

dcdd IZLIV ==∆ ωπ3

αcos0d

V

dI

dV

cZ

( )2

coscos0

µαα ++= dd VV

HVDC Training Course 32

AC-Current with Overlap

0.030 0.026 0.022 0.018 0.014 0.010 [s]

0.150

0.100

0.050

0.000

-0.0500

-0.1000

-0.1500

REC 1: Phase Current A/Terminal AC in kA

Constant x= 0.018 s

-0.000 kA

Constant(1) x= 0.019 s

0.100 kA

DIgSILENT Rectifier AC-Current

Date: 2/14/2003

Annex: 1 /7

DIg

SILE

NT

17

HVDC Training Course 33

AC Current with Overlap

( )2

coscoscos3 0

µααϕ ++= ddACll IVIU

dAC IIπ6

≈Approximation:

( )2

coscoscos µααϕ ++≈

In PowerFactory:Precise expression for AC-current from Fourier analysis used

HVDC Training Course 34

HVDC Configurations

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HVDC Training Course 35

12-Pulse Configurations

• Monopolar

• Homopolar

• Bipolar

• MTDC (Multi-Terminal HVDC)

- Short-distance connection- Sea cable connection

- Short-distance connection- Sea cable connection

- Long-distance transmission- Sea cable connection

- Long-distance transmissionwith several connections

HVDC Training Course 36

Detailed 12-Pulse Bipolar HVDC System

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HVDC Training Course 37

12-Pulse Bipolar System in Power Factory

V~ V ~

DIg

SILE

NT

HVDC Training Course 38

HVDC Layout

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HVDC Training Course 39

HVDC Components

Converter bridges

Converter Transformers: three- or single-phase transformertwo- or three-winding transformernot grounded at valve side

Smoothing reactors: large inductance (<1H)reduces harmonics in DC current and voltageprevent commutation failures and discont. Currentslimit extensive currents at DC short-circuit

Harmonic filters: reduce harmonics at AC and DC sideprovide reactive power for converter operation

Electrodes: use earth or metallic return conductor as neutral

DC Line

HVDC Training Course 40

Large Scale HVDC Projects

1500km2 x 1125MW2 x ±600kVCanadaQuebec-New England

940km2000MW±500kVNelson River 2

890km1854MW±500kVCanada

Nelson River 1

1361km3100MW±500kVUSAPacific Intertie

71km2 x 1000MW2 x ±500kVGB, FranceCross Channel 1+2

1440km2 x 960MW2 x ±270kVMozambiqueCabora-Bassa

800km2 x 3150MW2 x ±600kVBrazilItaipu

890km2 x 3000MW2 x ±500kVChinaThree Gorges

DistancePowerVoltageCountryProject