IEEE Power & Energy Magazine - January/February 2014 - 47

LCC HVdc Black Start with Synchronous
Compensators: The Gotland Experience

1,000
900
800

95% Power
Factor Reference

700
Pac (MW)

1,200-mva vsc converter is illustrated in the P-Q diagram given in figure 2. There is some variation in reactive power capability with ac voltage level. The black
"v-shaped" lines in figure 2 simply represent a 95%
power factor reference. The P-Q capability of the converter is similar to the P-Q capability curve or "d-curve"
for a synchronous generator.

600
500
400
300

january/february 2014

0
10
0
20
0
30
0
40
0

-1

-2

-3

-4

-5

as stated earlier, the first commercial hvdc system, commis200
sioned in 1954, linked the island of gotland in the Baltic
100
sea with the swedish mainland. at that time, the 20-mW
0
capacity of the hvdc link exceeded the total load on the
island. economic dispatch often dictated the shutdown of
Qs (Mvar)
local diesel generation. This resulted in the dc link serving as
Uac = 0.95
Uac = 1.0
the island's sole power supply. since the dc converters were
Uac = 1.05
Uac = 1.1
line-commutated, synchronous compensators were required
for start-up and for operation without local generation being
online. during operation, the hvdc link matched the got- figure 2. A VSC P-Q curve showing reactive power capaland demand by means of frequency control. The synchro- bility, 1,200 MVA/1,000 MW.
nous compensators supported converter commutation and
controlled the ac voltage. since that time, the load on gotland has increased, and the hvdc capacity has expanded to
✔ once the synchronous compensators provide ade2 × 160 mW for redundancy. additional synchronous comquate commutation voltage, the dc converters can be
pensators have been installed. The possibility of running the
deblocked (started) in frequency control, the compensystem without local conventional generation remains, howsators brought up to synchronous speed, and the startever. in the meantime, significant wind generation has been
ing motors disengaged.
added on gotland. The control of the dc link effectively acts
✔ The hvdc link is then able to supply the running
to compensate for the variation in wind power with regulatpower to the synchronous compensators and its own
ing power from the mainland.
The gotland network, with its
hvdc interconnection to the mainland, is shown in figure 3. The
process of starting up the gotland
HVdc to Mainland
hvdc link without any local generation follows the sequence summarized below. The simplified circuit
Visby
Slite
dc Link to Mainland
arrangement for black start is also
shown in figure 3.
Bäcks
✔ start-up a small diesel genYgne
erator to supply auxiliary
Roma
Ygne
power to the converter station.
This ensures enough start-up
To Substations
power for station auxiliaries
SC ×3
Aux G
and keeps the station batteries
D
charged to provide essential
M
Hemse
Stenbro
power to the control and protection equipment.
70 kV
Näs
✔ With adequate auxiliary power
30 kV
established, the starting motors
Burgsvik
HVdc Light
for the synchronous compenHVdc Classic
sators can be energized, the
machines brought up in speed,
figure 3. The Gotland HVdc, LCC black start with a synchronous compensator.
and excitation applied.
ieee power & energy magazine

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