VSC HVDC C t D iVSC HVDC Converter Design with Fault Blocking Capability

for OHL Applications

N M MacLeod, C D Barker,

for OHL Applications

R S Whitehouse, W Liang

GRID

LCC vs VSC Comparison

LCC HVDC

• Good overload capability

VSC HVDC

• Weak overload capability

• Requires strong AC systems, SCR > 3

• No “black start” capability

• Operates into weak AC systems, SCR not critical

•“Black” start capabilityp y

• Generates harmonic distortion, AC & DC harmonic filters required

p y

• No harmonic generation, hence no filters required

G• Weak reactive power control

• Large site area, dominated by harmonic filters

• Good reactive power control

• Compact site area, 50 – 60% of LCC site area

• Mature technology • Emerging technology, MMC version

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LCC vs VSC Comparison

LCC HVDC

• Needs converter transformers,, built by specialist facilities

VSC HVDC

• Uses conventional transformers, built by any facilityby specialist facilities

• Significant system interaction studies required

Si ifi t li ti i i

by any facility

• Minimal system interaction studies required

R d d li ti i i• Significant application engineering required

•Low station losses, 0.75%

• Reduced application engineering required

•Higher station losses, 1.1%

•Lowest cost

•High power capability, up to 7200MW at ±800kV

•Higher cost by 10 – 15%

• Limited power capability, up to 1000MW at ±320kV

• High reliability • Lower reliability, due to high power electronic component count

EPRIHVDC/FACTS Conference Aug 2011

LCC vs VSC Comparison

LCC HVDC

• Power is reversed by changing

VSC HVDC

• Power is reversed by changing y g gpolarity of the converters

• Requires use of MI cables

y g gdirection of current flow

• Ideal for use with XLPE cables

• Multi-terminal schemes are difficult to engineer

• DC grids are not considered

• Multi-terminal schemes are easier to engineer

• DC grids become possible usingDC grids are not considered possible

• Able to suppress DC side fault currents

DC grids become possible using stations from multiple vendors

• Not able to suppress DC side fault currentscurrents fault currents

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VSC-HVDC 2 – Level Converter

Series-Connected IGBTs +V +V

Conceptually simple circuit

Requires Pulse Width Modulation (PWM) controlModulation (PWM) control

High switching losses - V-V -V

Harmonic filters are required to create an adequate waveform

U

+½Udc

U

-½Udc

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VSC Single Phase, 2-level

Alternating Voltage Output

Steady DC Voltage Input

+ VSC

Neutral

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VSC-HVDC Modular Multi-level Converter

Multi-level circuit + V+ V

Low switching losses

Easily “scaleable” to high voltagesvoltages

Virtually no harmonics generated

- V- V

= chain link module

g

More complex control algorithms

+½Udc

U

dc

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-½Udc

VSC Multi-Level Converter

Output Voltage

Multi-level VSC

+Steady DC Voltage Input

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Modular Multi-level Converter : Half-bridge (HB)

Module Output voltageT1

UT2

+ V+ V

• Lowest component count

• Only one possibility of output voltage polarity

- V- V

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• Only one possibility of output voltage polarity

Modular Multi-level Converter : Half-bridge (HB)

Module Output voltageT1

UT2

+ V+ V

•No capability of suppressing DC-side faults•AC circuit breakers must be tripped

- V- V

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•AC circuit breakers must be tripped

Modular Multi-level Converter : Full-bridge (FB)

Module Output voltage

U

+ V+ V

• Same circuit as ALSTOM STATCOM chain circuitO t t DC lt b ith l it

- V- V

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• Output DC voltage can be either polarity

DC fault current suppression – Full-bridge (FB)

+ +

++

++

Va Vb Vc

+ +

+ +

+ +

C C f

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• Can suppress DC side faults• No need to trip AC side circuit breakers

Half-bridge/Full-bridge IGBT mounting

Half-bridge requires a protective thyristor Full bridge IGBTs

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Half-bridge requires a protective thyristor Full bridge IGBTs

Assembled sub-module (HB or FB)

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VSC Valve Hall

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FB Converter – Issues/Solutions

Issues

• Higher station losses than FB topology• Higher station losses than FB topology• 1.3 – 1.4%

• Higher capital cost due to additional IGBT devicesg p• Protective thyristor is not required

Solutions

• Hybrid topology using FB and series connected IGBTs• Lower semi-conductor losses• Lower component costsp

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Series Hybrid Circuit

F ll h i li kFull chain links

Series IGBT valve

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Series Hybrid Circuit

Integrated chain link + Series Switch

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Conclusions

• VSC HVDC provides additional functionality to operators compared with LCC technology

• To date most VSC schemes have been cable systems so the need to ride through DC faults is not a key issuea key issue

• Current generation of VSC converters can not suppress DC side faults

• The Full Bridge topology can suppress DC fault currents thus avoiding the need to trip AC circuit breakers but with higher loss/costbreakers, but with higher loss/cost

• Second generation VSC converters using hybrid topologies can overcome these limitationstopologies can overcome these limitations

EPRIHVDC/FACTS Conference Aug 2011

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