Cascaded droop control for dc overvoltage suppression in a multi-terminal HVDC system under onshore side AC faults

In a multi-terminal HVDC (MTDC) system connecting windfarms, the widely used dc voltage margin and droop control cannot be directly applied to offshore converters due to the lack of existing ac grid. Therefore, the dc voltage regulation only relies on the onshore converters, and its variation will not affect wind power generation. This paper introduces a cascaded droop control scheme for offshore converters and windfarms, which enables autonomous wind power adjustment during onshore station side ac faults, using offshore side ac voltage magnitude as an intermediate variable. Different from the traditional dc voltage droop control (νdc - pdc droop), the embedded dead-band and offshore station transformer leakage impedance will greatly impact the operation point of the MTDC system if the proposed cascade droop control is used, thus requiring special consideration during droop parameters design. Simulation results from a four-terminal HVDC system generated with Matlab/Simulink and experimental results from a scaled down prototype are provided to support the theoretical analysis and proposed control scheme.