Damping contribution from wind farms (WFs) is
likely to become a mandatory requirement as part of the grid
codes. For remote offshore WFs, connected through a voltage
source converter (VSC)-based direct current (DC) link, the most
convenient option for the onshore transmission system operator
(TSO) is to modulate the reactive power at the onshore VSC
within their own jurisdiction. In this paper, we show that
supplementary control through the onshore VSC alone, although
attractive for TSOs, could result in unacceptable voltage variations
in the onshore power grid. On the other hand, modulation of
active power output of the wind turbine generators (WTG) alone
turns out to be inadequate due to the limited overload capability
of the WTGs and the on- and offshore VSCs. Coordinated control
over both onshore VSC and aggregated WF output overcomes the
above limitations and is shown to be effective for power oscillation
damping. A homotopy approach is used to design the coordinated
controller, which can be implemented locally (at offshore WF and
onshore converter site) using a decentralized architecture. Case
studies on two test systems show that the proposed controller
yields similar system dynamic response as supplementary control
through the WF alone.