Primary Frequency Support Strategy for MTDC System With Enhanced DC Voltage Response

This paper presents a mutual frequency support strategy for ac grids interconnected by multi-terminal high voltage dc transmission system. The proposed scheme has different strategies for ac side disturbance and converter outage. During ac side disturbance, the proposed scheme makes sure that the actual aggregated droop gains of the converters in the same ac grid is equal to the designed aggregated droop gains by effectively utilizing the available headroom (AvHR) of the converters. Moreover, the paper proposes a threshold compensation strategy to eliminate the effect of the frequency deviation threshold that is intentionally imposed on the frequency droop control of converters. Therefore, the proposed scheme can accurately achieve the operator pre-specified ratios among the frequency deviation of the ac grids. The aggregated droop gain of the converters in each ac grid is designed to achieve the pre-specified ratios amongst frequency deviation of the ac grids. Furthermore, during an ac side disturbance, converters in the disturbed ac grid operates in $P-f$ droop control mode and the converters in the supporting ac grids work in $P-V_{dc}$ droop control mode. Therefore, the impact of the frequency deviation on the dc voltage profile can be reduced. During converter outage, the proposed scheme minimizes the frequency deviation of the ac grids by trying to restore the power flow between the ac grids and their converters to a level before the disturbance. The effectiveness of the proposed scheme is verified through rigorous simulation tests.