ACROSS&BORDERS&*&HVDC&SYSTEMS&AND&MARKET&INTEGRATION&
ACROSS&BORDERS&*&HVDC&SYSTEMS&AND&MARKET&INTEGRATION&
PAPER 113
1.1 New HVDC Projects Implementation
A Controlled DC Fault Clearance Mechanism for Full-Bridge
MMC VSC Converters
C. Karawita, D.H.R. Suriyaarachchi, M. Mohaddes
Canada
Lund%Symposium%27/28%May,%2015%
CLEARING DC FAULTS IN VSC SYSTEMS
1.  Half bridge converters - AC breakers + Mechanical DC
breakers
2.  Half bridge converters - Fast DC breakers
3.  Full bridge converters – Blocking
4.  Full bridge converters – Active DC Fault Current Control
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2%
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ACTIVE DC FAULT CURRENT CONTROL
•  Utilizing extra degree of freedom in full bridge converters
Vb%
Full/bridge%
converter%arm%
Va%
/  Vb%can%be%controlled%independent%of%Va%
/  Vb%can%be%changed%in%between%+Vdc,rated%
and%–Vdc,rated%almost%instantaneously%
%
%
•  Done through low level controls to get a quick response
High%Level%Controls%
Lund%Symposium%27/28%May,%2015%
3%
ACTIVE DC FAULT CURRENT CONTROL
•  When a DC fault is detected;
–  Vdc reference for the low level controls is controlled to bring the
DC fault current to zero
–  The operation of high level controls are moved to “STACOM
Mode”
•  d-axis: Average module voltage control
•  q-axis: AC voltage/reactive power control
–  If required, a high-speed switch or fast disconnect can be used
to isolate faulted dc line after restoring attempts
Lund%Symposium%27/28%May,%2015%
4%
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COMPARISON AGAINST BLOCKING
/  Fault%current%is%brought%to%a%safe%
value%almost%instantaneously%
%
/  Fault%current%is%kept%below%50%A%
within%30%ms%aLer%fault%occurrence%
Lund%Symposium%27/28%May,%2015%
5%
TEST SYSTEM FOR DEMONSTRATIONS
•  2000 MW, +/- 500 kV Bipole Full Bridge VSC System
–  400 km over-head DC transmission with dedicated metallic
return
–  AC system equivalents with SCR of 2.0 (weak systems)
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6%
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TEMPORARY DC FAULTS
DC&voltage&
DC%voltage%is%ramped%up%
aLer%deionizaPon%Pme%
DC&current&
AcGve&power&
ReacGve&power&
Impact%on%reacPve%power%
support%is%negligible%
AC&voltage&
Blue:&+ve&Pole&(faulted)&
Green:&*ve&pole&
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PERMANENT DC FAULTS
•  Two soft restarting attempts
–  No breaker operations within restarting attempts
7%
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PERMANENT DC FAULTS
DC&voltage&
DC%voltage%is%ramped%up%at%
each%restarPng%aSempt%
DC&current&
DC%current%is%monitored%
AcGve&power&
ReacGve&power&
No%impact%on%reacPve%
power%support%
AC&voltage&
Blue:&+ve&Pole&(faulted)&
Green:&*ve&pole&
HSS%Opened%
Lund%Symposium%27/28%May,%2015%
9%
ADVANTAGES OF ACTIVE DC FAULT CURRENT
CONTROL
•  Compared to full-bridge converters with blocking
–  The converters are under full control (no blocking)
–  Minimal disturbances on the reactive power support of converter
–  Extra reactive power support is also available in STATCOM
mode
•  Compared to half-bridge converters with DC breakers
–  Possibility of soft restarting attempts
–  Independent converter control – no communication required
–  Less breaker operations >> less maintenance
Lund%Symposium%27/28%May,%2015%
10%