IEEE Power & Energy Magazine - January/February 2014 - 49

VSC HVdc Black-Start Field Test:
The Estlink Experience

january/february 2014

350
300
250
MW

200

CSC Interconnector
Energized

Blackout Hits
150 Long Island Federal Order Permits
Use of CSC Interconnector
100
50
17 August, 24:00

16 August, 24:00

15 August, 24:00

14 August, 24:00

0
13 August, 01:00

estlink is a 350-mW hvdc cable interconnection between
finland and estonia crossing the gulf of finland. The networks are asynchronous. The hvdc stations contain vscs.
The northern terminal is located in southern finland, near
helsinki. The southern terminal is located in estonia,
near Tallinn. The system includes black-start functionality. full-scale system tests of the black-start feature were
conducted on the estonian network during the summer of
2007. The network configuration with double buses, extra
transmission capacity, and reserve generation enabled system testing of the black-start function without disrupting
consumers.
The black-start tests consisted of starting up the converter
from the dc side to energize the ac bus, energizing a large
transformer bank, energizing a long ac transmission line,
energizing a generator start-up transformer and unit auxiliary
loads at a thermal power station, and, finally, synchronizing
the unit to the islanded grid here to fore fed solely by the
converter station.
figure 4(a) shows results for the controlled energization
of the ac bus by the dc converter. The converter station in
estonia is fed from the dc side, which is energized over the
dc cable from the converter station in finland. The top trace
shows the dc voltage being held at its rated value of 300 kv
(±150 kv) by the remote station operating in dc voltage control. The second trace shows the frequency being maintained
at its 50-hz reference. The third trace shows the three ac
phase voltages at the point of interconnection being ramped
from zero up to their rated values by the black-start converter
control. The gradual buildup of ac voltage minimizes the
converter transformer inrush current.
figure 4(b) shows results for a local 250-mva transformer being energized by closing a circuit breaker to the
energized bus. The transient effects of the inrush current
on transformer energization can be seen in the traces of ac
voltage, dc voltage, and converter reactive power output.
figure 4(c) shows results for energizing a 20v0-km-long,
330-kv ac line from the converter bus, including the dc voltage, ac voltage, frequency, real power, and reactive power
traces. Because the capacitive line charging was about 100
mvar, excess reactive power was absorbed by the converter
operating in ac voltage control. The damped line energization transients appear in the ac and dc voltages.
a 25-mva generator start-up transformer for a 250mva thermal power plant was connected at the remote
end of the 200-km-long, 330-kv ac line. The temporary
increase in reactive power demand due to transformer
inrush was supplied by converter reactive power output.
once the transformer was energized, auxiliary load was
connected, which allowed the power plant to be started up
and then synchronized to the islanded grid heretofore fed
only by the dc link. synchronization of the power plant is
shown in figure 4(d).

Time

figure 5. The operation of the CSC following the 2003
Northeast blackout.

System Restoration Assistance:
The Cross-Sound Experience
The cross-sound cable (csc) is a 330-mW, vsc-based
hvdc interconnection across long island sound between
New haven, connecticut, and shoreham, New york. it
links iso New england (iso-Ne) with New york independent system operator (Nyiso). csc was commissioned
during the summer of 2002. commercial operation was
delayed, however, due to permit compliance issues with the
cable burial depth at a few locations. Until this issue was
resolved, the csc remained off-line for about a year after
commissioning.
following the Northeast blackout of 14 august 2003,
the csc was started up under an emergency order from the
U.s. department of energy. The blackout had left most of
upstate New york, New york city, and long island without
power, but most of New england was unaffected. The hvdc
links with the asynchronous Quebec network continued to
deliver power to New england and the maritimes. The csc
was used to help restore service to long island, and it was
the first long island interconnection in service following
the blackout. despite its not being furnished with blackstart control, the csc was able to operate with a minimal
system intact on long island in the vicinity of shoreham.
in addition to providing power for system restoration, converter dynamic reactive power capability was instrumental
in stabilizing system voltage during the restoration process
as lines and transformers were energized, generators were
restarted, and cold loads were picked up. dynamic voltage
support from the converters operating with ac voltage control helped the system ride through three major disturbance
events on long island during restoration.
figure 5 illustrates the power profile for csc during system restoration on long island following the blackout. figure 6(a) shows the areas restored on long island about seven
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