VSC Transmission Tutorial

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18/09/2005 VSC Transmission Tutorial 1

CIGRE B4

HVDC and Power Electronics

HVDC Colloquium, Oslo, April 2006

VSC Transmissionpresented by

Dr Bjarne R Andersen,

Andersen Power Electronic Solutions Ltd

3rd April 2006 CIGRE B4 Meeting Oslo, April, 2006 2

Presentation Overview

- Basic Characteristics of VSC Transmission,

- Comparison of VSC Transmission and LCCHVDC technology,

- VSC Transmission Applications,

- Components of a VSC Transmission

Scheme,

- System Issues,- Overview of VSC Transmission schemes,

- Future Trends.


LCC HVDC Transmission

In use since 1954

- Long Distance transmission

- Asynchronous Interconnections

- >60GW in service, Voltage up to 600kVdc

Uses Thyristors,

- Line Commutated Converters

- Converter absorbs reactive power

- AC harmonic filters are used to achievesatisfactory waveshape and power factor


The Voltage Sourced Converter

- DC Voltage source

- Semi-conductors capable of turn-on AND

turn-off are used

- An ac voltage with controllable amplitude

and phase angle is produced.


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3/29


VSC Start Up - 1

Diodes act as an uncontrolled rectifier.


VSC Start Up - 2

Diodes act as an uncontrol led rectifier.


IGBT Conducting - Inverter Operation.

VSC Operation - 1

What happens when we turn-off the IGBT??


IGBT Turned off - Diode picks up current

VSC Operation - 2

There will be a delay before the ac cur rent changes polarity because of L


4/29


Inverter Operation - Blanking Period has

passed and current has reversed.

VSC Operation - 3


VSC Operation - 4

Rectifier Operation

How do we turn off the diode?


VSC Operation - 5

Turning Off Diode - IGBT Conduction

causing a temporary short circuit


The diode turns off because of the short

circuit current

VSC Operation - 6


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VSC Operation - 7

When the current in L reverses the

IGBT turns of f and the diode turns on


Full Wave Conversion

Advantages- Low Power Loss

Disadvantages

- AC and DC voltage relationship fixed - makes

it unsuitable for dc transmission

- High magnitude of low order harmonics - large

ac harmonic filters are required

0 1 2

dc

ac

dc


Pulse Width Modulation

Carrier at 9 times fundamental frequency


PWM Control

Disadvantage

- Power loss larger because of more frequent

switchingAdvantages

- Gives additional degrees of freedom - e.g.

independence of converters, and control of ac

voltage amplitude with fixed dc voltage

- Reduces lower order harmonics - smaller

filters- Higher speed of response


6/29


Simplified Representation

( )X

UUUQ

LL cosconv(1))1()1( =

sin.UL(1))1(

X

UP

conv=


Four Quadrant Control

The Reactive Power output depends on the

voltage ampli tude:



The Active Power output depends on the

converter voltage Phase Angle:

U = Uconv L

P = 0 P < 0

UL

Iconv

LUU

conv

Iconv

conv conv

Uconv

-

Rectifier

operation

Inverter

operation

P > 0

conv


Rectifier

Mode

Inverter

Mode

CapacitiveInductive

Pconv

Qconv

Desired

Activ e

Power

Desired

ReactivePower

Uac = Max

Uac = Nom

Uac = Min


Simplified PQ Diagram


7/29







Scheme,

- System Issues,

- Overview of VSC Transmission schemes,

- Future Trends.


Comparison - Experience

LCC HVDC VSC Transmission

In service since 1954 In service si nce 1997

Installed Capacity >60GW Installed Capacity 930MVA

Largest Scheme 6300MW Largest Scheme 346MVA

Highest Voltage +/-600kVdc Highest Voltage +/-150kVdc

Reliability/Availability proven No formal records available at

present


Comparison - Converter

Technology


Uses 8.5kV, 4kA Thyr isto rs, alatching device

Uses 2.5kV, 2kA IGBTs, atransistor type device

Turn on by control, turn offwhen current tries to reverse

Turn on by control and turn offby control, irrespective of thecurrent flow at the time

Requires AC network voltagefor commutation

Is self commutating Can bethe sole supply to a passivenetwork

Faults and switch operations inthe network can causecommutation failure

No Commutation Failures

Faults on the dc line cleared bythyristors through controlaction

The diodes feeds current intofault on dc side AC Circuitbreaker action needed to clear


Comparison - Harmonics


12-pulse harmon ics (12n1),plus non-characteristic (2, 3, 4,

5, 7 etc )harmonics

PWM moves characteristicharmonics to higher orders

Requires large filters to limitharmonics typically on HVbus or on tertiary winding

Smaller and higher frequencyfilters required typicallybetween converter reactor andinterface transformer

Possibility of magnification ofpre-existing harmonics

Risk of magnification of pre-existing harmonics may besmaller, but needs evaluating

Switchable AC Harmonic Filtersalso used for Q Control Filter not normally switchable


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Comparison - Reactive Power


Converters absorb reactivepower (~55% of Real Power)

Converters can op erate at anyleading or lagging power factor.

Switchable AC Harmonic Filtersfor Q Control

Reactive Power can becontrolled by the converter

Large overvoltages can occurduring lo ad rejection

Load rejection overvoltage issmall

Site area is relati vely largebecause of need for switchableharmonic filters and shunt

capacitors

Site area is compact because ofsmall unswitchable filters andsimple ac switchyard


Comparison - Other


Used with overhead lines,cables and mixtures thereof.

Al l co mmercial inst allat ionshave used cables so far.

Relatively low capital cost whenused at large power andrelatively strong ac networks

Competitive capital cost atpower


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Benefits of Interconnections

Interconnection Benefits:

- Better utilisation of installed generation.- Reduction in overall spinning reserve.

- Emergency power support

The benefits f rom the use of HVDC include:

- The power flow is fully controlled.

- Asynchronous networks can be connected.

- HVDC is more economic when distance is

large, e.g. >800km overland or >70km

submarine.3rd April 2006 CIGRE B4 Meeting Oslo, April, 2006 34

Benefits of the use of cable

connections

It may be more expensive than an overhead

line but:- It is less intrusive on the landscape.

- It does not produce electric fields or varying

magnetic fields,

- More acceptable to the public, resulting in

planning approval being granted more quickly.

- It is not subject to flashover due to pollutionproblems,

- More reliable than an overhead line,


Interconnection to Small Isolated

network - 1



network - 2

- No Electrical Power at present

- Small diesel generation at present Expensive MWhr cost,

Maintenance requirement,

Reliability/Availability,

Damaging for the environment

Reasons for Interconnection:


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network - 3

- No need for Synchronous Compensators- Converter can dynamically control the ac

system voltage

- Economic and environmental benefit by using

main network generation

- Less Maintenance

- Cable connection has low visual impact

Benefits from Using VSC Transmission:


Interconnection between weak

networks - 1

~

=~

=

HVDC Line


Interconnection between weak

networks - 2

- Weak areas are likely to be remote from main

network. Provide damping control - improved security of

supply (fewer trips of ac line to main network)

- Asynchronous connection may be more

acceptable politically -(HVDC as Firewall)

- Converter can dynamically control the ac

system voltage - improved system stability.

Reasons for Interconnection:


Re-inforcement of Weak AC tie

Lines - 1

~

=

~

= HVDC Line


11/29



Lines - 2

- More capacity needed.- As load grows, instabilities may cause frequent

trips.

- Power oscillations may reduce useable power

capacity.

- Unacceptable loop power flows.

- Controlability of HVDC may add substantialbenefits

Reasons for Re-inforcement:



Lines - 3

- VSC Transmission can significantly increasethe available capacity on the ac Tie Line.

- Powerful damping control, through control of

active and reactive power.

ABB have shown that the capacity of a weak ac Tie

line can be increased by more than the rating of the

parallel VSC Transmission scheme



Connection of Offshore loads - 1



- More capacity needed for extraction and

transport.- Reduction of maintenance compared with

GTs,

- Reduction of CO2 emissions

- Reduction of fire risks

Reasons for Connection:


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- Does not require synchronous compensators,- Much lighter and more compact than LCC

HVDC,

- Can operate as a variable speed drive, for

large motors.



Connection of Remote Wind

Farms - 1



Farms - 2

- Transmission distance is large,

- De-coupling required between ac grid andwind farm ac network,

faults

power quality

- Enabling variable frequency of wind farm

network, for greater efficiency,

Reasons for Connection with HVDC:



Farms - 3

- Connection can be made to weaker point in ac

network,- Improved stability of the wind farm ac network,

- Reduction of flicker on wind farm network

- Smaller site area required than for LCC HVDC

- Power can be transmitted to wind farm network

when the wind does not blow,

Auxiliary Power - Control & Protection, Tele-

communication, Navigation, Safety.



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Scheme,

- System Issues,


- Future Trends.


Components of a

VSC Transmission scheme


VSC - 3 phase implementation

3 phase 2-level Voltage Sourced Converter

d

d


PWM Control

Different PWM control methods:

- Triangular carrier, pure sinewave for control

- Triangular carrier, sinewave with 3rd harmonicfor control

- Optimised PWM - Selective harmonic

elimination


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PWM - Waveshape & Harmonics

- 2

AC Volt age

phase to -

Neutral

Fundamental

frequency

component

Fourier

Analysi s of

Phase to

Phase

voltage

Triangular carrier at 21st harmonic, sinewave with 3rd

harmonic as control


OPWM / SHEM

Eliminates specific harmonics, but switching

instants change with operating conditions.- Pre-calculated or determined as you go

Can arrange to minimise switching at

maximum current

Results in a reduction in power loss,

compared with simple triangular carrier wave.


PWM - Waveshape & Harmonics

- 3

AC Volt age

phase to -

Neutral

Fundamental

frequencycomponent

Fourier

Analysi s of

Phase to

Phase

voltage

Optimised PWM or Selective Harmonic Elimination


3-level (NPC), 3-phase VSC

Waveshape shown for fu ll wave switching


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Advantages of multi-level

converters

Fewer switch operations for similar harmonic

performance. Lower voltage per switch.

Ampl itude of fundamental frequency vol tage

can be adjusted even with ful l wave switching

- giving additional degree of freedom.

Lower power loss.


Pulse Width Modulation

0 90 180 270 360

1

0

1

Degree

Line-to-neutralvoltage(pu)

0 90 180 270 360

1

0

1

Degree

Line-to-neutralvo

ltage(pu)

2-Level VSC

3-Level VSC

PWM switching at 21 times fundamental frequency


Series connected 2-level

converters (HVDC Plus)

Converter

Transformer


Components of a


Converter Topology

VSC Valves

DC Capacitor

Controls

Converter Reactor

AC Fi lters

Interface Transformer

DC Cables


16/29


VSC Valves - Device choice

Currently the device of choice is the IGBT

- Can be turned off in short circuit conditions.- Active control of the voltage across the device,

- Low power control of the device (Voltage

control - MOSFET device).

- High switching speed capability.


IGBT Press Pack

- Valve operation with failed devices,

- Pack includes IGBTs and Diodes

- 6kV designs available, but 2.5kV, 2kA

devices generally used,

- Multi-chip design.

Illustration Courtesy of ABBIllustration Courtesy of ABB


VSC Valve design - 1

Coherency of switch on and off is essential.

Voltage distr ibut ion control led by IGBT

transistor action Stray inductance kept as low as possib le

Voltage divider and surveillance circuits for

device monitoring

Energy for gate electronics obtained from

main circui t via voltage divider

Fibre optic interface with ground level control


VSC Valve design -2

A valve for 150kVdc may contain >300

series levels.

IGBTs and diodes are water cooled Devices mounted with great pressure against

heatsinks

Valves housed in metallic enclosure

- contains EMF


17/29


VSC Valve for 150kVdc

Photo Courtesy of ABB


Components of a


Converter Topology

VSC Valves

DC Capacitor

Controls

Converter Reactor

AC Fi lters


DC Cables


DC Capacitor Design

Provides energy storage on the dc s ide,

acting as a dc voltage source.

Capacitor must have low inductance Capacitor placed close to VSC Valves to

minimise stray inductance in commutating

loop


DC Capacitor

Capacitance must be large enough to l imit

harmonic ripple to design limit

- Ripple depends on direct current amplitudeand on the switching strategy

long pulses of high current causes more ripple

Voltage variations during faults in ac

networks also need to be taken into account.


18/29


DC Capacitor Design

Dry type capacitors used

- minimises fire risks Self healing metalised

film design used

Plastic Housing

Compact



Components of a


Converter Topology

VSC Valves

DC Capacitor

Controls

Converter Reactor

AC Fi lters


DC Cables


Controls

Determines the instant at which each

individual VSC valve is switched on or off to

meet the operational requirements.

Typically:

- One station controls the direct voltage.

- One station controls the active power

- Both stations can also control ac voltage or

reactive power

Implemented as a dupl icated digital controlsystem.


Control System Block Diagram

Figure Courtesy of ABB


19/29


Components of a


Converter Topology

VSC Valves

DC Capacitor

Controls

Converter Reactor

AC Fi lters


DC Cables


Converter reactor

Provides constant fundamental frequency

impedance for the control of the VSC active

and reactive power output.

Provides a high frequency blocking filter

between the VSC and the ac network.

Limits rate of rise of short circuit currents.


Converter reactor for 65MVA,

80kVdc VSC Transmission scheme

Dry type air-insulated

air-cored reactor

Typical impedance of

15%

Low st ray capacitance

Metallic screen to

eliminate external

magnetic fields

Forced air cooling



Components of a


Converter Topology

VSC Valves

DC Capacitor

Controls

Converter Reactor

AC Fi lters


DC Cables


20/29


AC Harmonic Filter - 1

Characteristic harmonics tend to be at higher

orders.

Must check non-characteristic harmonics,

when the ac system can be unbalanced.

Design methods similar to those for LCC

HVDC scheme

Typical rating of the ac harmonic filter is 15%.

Typically includes tuned and high passbranches.


Components of a


Converter Topology

VSC Valves

DC Capacitor

Controls

Converter Reactor

AC Fi lters


DC Cables


Interface Transformer - 1

Enables the VSC to be designed

independently of the ac connection vol tage.

Blocks zero sequence current Provides additional series reactance

- beneficial for harmonics

- adds to converter reactor reactance



Typically does not have dc stress or

significant harmonic stress.

- Ordinary substation transformer can be used.A converter winding tapchanger can achieve:

- larger steady state rating

- lower power loss

A tert iary winding may be used for auxi liary

power supply

Typical impedance of 10-15%

C t f


21/29



Photo Courtesy of ABBIllustration Courtesy of ABB


Components of a


Converter Topology

VSC Valves

DC Capacitor

Controls

Converter Reactor

AC Fi lters


DC Cables


DC Cables

Extruded polymeric cables can be used, as

the dc voltage does not change polarity.

- Lighter- Smaller bending radii

- No significant environmental risks.

These features make them easier and quicker

to install.

This type of cable has been proven in service

at voltages up to 150kVdc.


Land Cables

Installed lengths of 1-

2km are joined in the

field.

Depending on theground conditions, cable

can be buried using

direct ploughing

methods.

Illustration Courtesy of ABB


22/29


Laying a Land Cable

Photo Courtesy of ABBIllustration Courtesy of ABB


Deep Sea Submarine Cables

Can be manufactured

and laid in continuos

lengths of > 100km,depending on rating.



Components of a


Converter Topology

VSC Valves

DC Capacitor

Controls

Converter Reactor

AC Fi lters


DC CablesOtherequipment/components


Other equipment in a VSC

Transmission substation

AC circuit breakers.

RFI and PLC fil ters.

Voltage and Current measuring transducers. Surge arresters.

Disconnectors and earth switches.

Auxi liary power suppl ies.

Fire Protection.

Civil works.

Ph i l L t 65 MVA


23/29


Physical Layout, 65 MVAHVDC LightTM Converter Station

Building

45 x 18 m

AC Yard &

Harmonic filters

Phase Reactors

Phase A, B and C valve enclosures

Auxiliary Power System

& Cooling Control

Cooling towers

DC Yard Equipment








Scheme,

- System Issues,


- Future Trends.


System Issues - 1

Protection in ac network.

- During ac faults the current delivered by a VSC

Transmission scheme is limited to rated.- Conventional over-current protection may not

work.

- However, low current may permit longer fault

detection

- Protection needs careful consideration

- VSC Transmission does not add inertia Frequency tripping limits may need to be reassessed


System Issues - 2

Feasibility Studies can use third party

commercially available models.

- However, these models may not reflect fullcapability and/or limitations of the VSC.

- When specifying a scheme make the

specification functional, and provide necessary

system data.

- Studies done during feasibility phase typically

has to be repeated by the manufacturer duringthe implementation phase


24/29


System Issues - 3

When comparing options during feasibility

studies all issues must be considered:

Capital cost

Power Loss

Maintenance

Reliability

Availabi li ty

Operation cost

Anci llary service Benefi ts :

Reactive Power control,

Black Start Capability

Environmental Impact

Visual Impact

EMF

Time to In-service Public Enquiry

Time to implement







Scheme,

- System Issues,


- Future Trends.


Gotland - the first commercial HVDCLightTM project

Technical Data

Commissioning year: 1999

Power rating: 50 MW

AC Voltage: 70 kV both endsDC Voltage: 80 kV

DC current 350 A

Length of DC cable: 2 x 70 km

Main reasons for choosing HVDC system: Wind power (voltage support ).

Easy to get permission for underground cables.



Gotland HVDC Light converter station at Ns, exterior view


Gotland HVDC Light


25/29


Converter station at

Ns blends in well with

surrounding farms.

Illustration Courtesy of ABBIllustration Courtesy of ABB

Gotland HVDC Light


Eagle Pass HVDC Light link

The HVDC Light i nstallation in Eagle Pass miti gates voltage instability , and

at the same time allows power exchange between the U.S. and Mexico.

Technical Data

Commissioning year: 2000

Power rating: 36 MW

AC Voltage: 132 kV (both sides)

DC Voltage: 15,9 kV

DC current 1,1 kA

Configuration: Back-to-back

Main reasons for choosing HVDC system: Controll ed asynchronous

connection for t rading. Voltage control



Eagle Pass HVDC Light,

simplified SLD


Eagle Pass converter st ation, AC yard.


Eagle Pass


26/29


Murraylink

Technical Data

Commiss ioning year: 2002

Power rating: 200 MW

AC Voltage: 132/220 kV

DC Voltage: 150 kV

DC current: 739 A


Main reasons for choosing HVDC system: Controlled

asynchronous connection for trading. Easy to get permission

for underground cables.



Murraylink - valve enclosures

At factory

Placed on foundation

Inside building

Illustrations Courtesy of ABB


Murraylink

Murraylink, the Berri converter station.



Technical Data

Commiss ioning year: 2002

Power rating: 330 MW AC

Voltage: 345 kV at New Haven138 kV at Shoreham

DC Voltage: 150 kV

DC cur rent 1175 A


Main reasons for choosing HVDC system: Controlled

connection for trading. Submarine cables without oil.


Cross Sound Cable

C S d C LDynamic Response to Network


27/29


Building

90 x 18 m

Cross Sound Converter Layout



Dynamic Response to Network

Faults

March 17, 2005 Cross arm fault on 353 Line (345kV)



Dynamic Response to Network

Faults

Cross arm fault on 353 Line (345kV)Illustration Courtesy of ABB


Troll A HVDC LightTM link

The HVDC LightTM install ation between the Norwegian main land and the Troll

A oil platform consists o f two ci rcui ts , each feeding a 40 MW compressor

motor

Technical Data

Commissioning year: Planned2004/2005

Power rating: 2 x 42 MW

AC Voltage: 132 kV/56 kV

DC Voltage: 60 kV

DC current 350 A


Main reasons for choosing HVDC LightTM system: Environmental improvement

by elimination of gas turbines on platform. Low weight and small space on

platform. Ability to f eed and black-start motors, without local generation.


T ll A HVDC Li htTM li k T ll A


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MotorFormer4-pole, 40MW0 - 65 Hz

56kV

Troll A

SM

70 km+/- 60kV

HVDC Light

138kV

Kollsnes

~

=

=

~

Troll A HVDC LightTM link

Illustrations Courtesy of ABB


Troll A





- Comparison of VSC Transmission and LCC

HVDC technology,- VSC Transmission Applications,


Scheme,

- System Issues,


- Future Trends.


VSC Transmission Outlook

VSC Transmission has many technical

advantages over LCC HVDC

WG-37 has not identified any technicalreason why VSC Transmission cannot be

developed for very high voltage and power,

say 500kVdc, 3000MW

Main drawbacks relative to LCC HVDC are its

presently limited power rating and higher

power losses.

VSC Transmission trends Recommended Further reading


29/29


VSC Transmission trends

Future Developments

- Increased dc voltage (higher power, longerdistances)

- Increased dc current (higher power)

- New topologies (lower losses)

- New Semi-conductor devices (lower losses

and cost)

Resulting in:- VSC Transmission competing head on with

LCC HVDC in more and more applications.3rd April 2006 CIGRE B4 Meeting Oslo, April, 2006 114

Recommended Further reading

VSC Transmission, Cigre Brochure 269, Working

Group B4.37, April 2005.

Its Time to Connect, Technical Description of

HVDC Light technology, ABB website,

www.ABB.com/hvdc

18/09/2005 VSC Transmission Tutorial 115

Thank you for your Attention!

Any Quest ions?

[email protected]

VSC Transmission Tutorial

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