When connecting a wind park (WP) to the main grid (MG) by way
of an HVDC Light transmission system, the WP becomes decoupled from the
MG, which results in several technical and economic benefits, on a variety of
aspects, for transmission system operators (TSOs), WP developers and WTG
manufacturers. Perhaps most important for TSOs is that an HVDC-Light
connected WP becomes comparable to a normal power plant; the MG-side
HVDC Light converter can be directly connected to a control or power
dispatch center. A desirable consequence is also that AC faults appearing in
the WP or MG grid will not be propagated by the HVDC Light transmission
system, which has several desirable consequences such as possibly reduced
mechanical stresses on the WTGs. This paper presents operation and control
strategies of multi-terminal HVDC transmission system (MTDC) using
voltage source converters (VSCs) for integrating large offshore wind farms.
The framework and operation principles of the proposed system are described
and control strategies for coordinating various VSCs are proposed. DC
voltage control based on the DC voltage-current (V-I) droop characteristic of
grid side converters is implemented, to ensure stable system operation and
flexible power dispatch between various onshore AC grids. To validate the
performance of the proposed control strategies, a typical four terminal MTDC
networks, which connecting two offshore wind farms with two onshore AC
grids, is established in PSCAD/EMTEC. Simulation results under normal and
abnormal operation conditions verify the satisfactory performance of the
proposed control strategy and accuracy of the theoretical analysis.
Keywords: Control, HVDC, multi-terminal, voltage source converter, wind
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