Adaptive Decoupling Control Strategy for Improving Inertia Support Capability of MMC-HVDC System

Aiming at the problems of gradually reduced inertia and more and more prominently frequency stability of receiving-end AC grid caused by large-scale wind power connected to the grid through MMC-HVDC transmission, an active support control strategy based on adaptive decoupling of DC voltage and submodule capacitor voltage is proposed for improving inertia support ability of MMC-HVDC. Established a coupling relationship between receiving-end AC grid frequency and DC voltage, which makes MMC-HVDC can actively restrain the frequency change of receiving-end AC grid, and reveals the key factors affecting its inertia support ability; based on the above analysis, an adaptive decoupling modulation strategy based on local measurement information is designed respectively for receiving-end and sending-end MMC, which adaptively adjusts inserted number of submodules without communication to realize the effective decoupling of DC voltage and submodule capacitor voltage, fully utilize the energy margin of capacitors and improve inertia support ability for MMC-HVDC. Considering the safety constraints of frequency, DC voltage and capacitor voltage, a control parameter design method is proposed, and an adaptive recovery strategy is designed to maintain the safe and stable operation of MMC-HVDC while improving the inertia level of receiving-end AC grid. Finally, based on the RTLAB OP5600 real-time digital simulation platform, the simulation model of offshore wind farms connected to the grid through MMC-HVDC transmission is built to verify the effectiveness of the proposed control strategy.