vsc hvdc c t d ivsc hvdc converter design with fault...
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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
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