Post-fault load flow control through grid forming wind turbines connected to bipolar HVDC system with offshore AC-side pole coupling

HVDC transmission has become prominent for offshore wind integration. While today’s systems are built as monopolar point-to-point links, several network development plans foresee to transition to 2 GW bipolar multi-terminal systems, radially connected on the offshore DC-side. To increase immediate post-fault redundancy, it is discussed to couple the P and N pole of the bipolar system on the offshore AC-side, to enable an active power transfer between the poles. This power transfer can be achieved via a droop control of the offshore converters. Alternatively, the grid forming control (GFM) can be shifted to the wind turbine generators (WTGs) in the offshore wind power plant. This paper therefore compares different operational modes for immediate DC post-fault power transfer by means of EMT simulation. The focus lies on the load flow control via the AC coupling, while maintaining stable system operation. It is shown that GFM WTGs are capable of providing a faster load flow redistribution than the droop control. Furthermore, the post-fault load flow redistribution even works when the HVDC system is connected to a block load, with minor impact on the WTGs, requiring a DC-chopper activation for a few milliseconds for high and low-wind situations.