Interconnection of Offshore Wind Farms using Variable Frequency in Offshore AC Grids

Over the recent years, the large-scale development of offshore wind power in accordance with HVDC transmission has received extended attention. For the exploitation of such systems protection during onshore AC faults is of large importance. This paper examines an offshore transmission system consisting of two large wind parks, and two point-to-point HVDC links. A frequency droop control method applied to the converters of the offshore AC network is proposed. Based on this control strategy, power flow among HVDC links can be achieved, while power reallocation in case of AC faults is also feasible. A DC overvoltage control method is presented, in case of AC onshore grid faults' occurrence. Once a fault in onshore network is detected, it causes voltage rise of the affected HVDC link and hence DC overvoltage control mode is enabled. This control mode leads the current HVDC link to absorb the minimum possible active power and simultaneously triggers AC circuit breakers at the terminals of the link, isolating the branch that carries the fault. Furthermore, active power dispatch is rescheduled through the droop control strategy and delivered to the onshore network via the other HVDC link. Simulation results of several operation conditions, conducted with PSIM software using PMSMs as generators, verify the effectiveness of the DC Overvoltage control method.