Coordinated Control Strategy for Improving Frequency Stability of MMC-HVDC System at Sending End

In order to resolve the issues of damping and frequency support capability degradation of the sender system caused by large-scale clean energy via MMC-HVDC, a multi-timescale frequency control strategy based on the frequency modulation deadband is proposed to achieve the synergy of MMC-HVDC system and synchronous generator, so as to realise the coordinated complementarity of multiple types of frequency modulation resources in the system. The frequency regulation requirements of the sending end system under different scenarios are analysed in accordance with the frequency reserve capacity of the synchronous unit at the sending end and the imbalanced power generated by power interferences, and an adaptive control strategy for the frequency support of the MMC at the transmission end that takes into account the deadband of frequency adjustment is proposed, so as to make the MMC-HVDC System have the capability of damping regulation and primary frequency regulation at the same time. Moreover, the dead band value of frequency adjustment is formulated in combination with the maximum power regulation margin of the sending end frequency regulation resources, in order that it can adaptively switch between damping regulation and primary frequency regulation according to the disturbing conditions, which ensures the reliability of the supporting send-side frequency and reduces the influence range of disturbance at the same time, and realises the coordinated control of the supporting frequency of the sending end of the flexible interconnected grid under different disturbing scenarios. Finally, an MMC-HVDC interconnected grid simulation model is constructed based on the RTLAB real-time digital simulation platform to verify the effectiveness of the proposed control strategy.