Design of VSC Connected Low Frequency AC Offshore Transmission with Long HVAC Cables

Low frequency ac transmission (LFAC) has been proposed as an alternative to high voltage dc transmission for medium distance (80-150 km) offshore wind farms. Long HVAC cables and their associated low frequency resonance, connected to voltage source converters (VSC), provide technical challenges for the control of the offshore voltage. This paper provides the design of the offshore voltage 'grid forming control' to maintain a stable offshore voltage accounting for the connection of a long HVAC cable connected to the VSC. Simulations are performed on an LFAC test system to examine the influence of controller parameters and the associated design tradeoffs between the selection of dq controller time constants and voltage control bandwidth. The LFAC system design and control is then validated in a hardware experiment where the proposed controller operates in a real-Time hardware-in-The-loop experiment.