DC overvoltage suppression method of wind farm connected via MMC-HVDC system

Abstract

A new integration strategy of grid-forming-controlled wind farms connected to the bulk power systems through high-voltage direct current transmission based on the modular multi-level converter (MMC) is proposed to solve the problem of traditional uncontrollable DC overvoltage during the short circuit faults at the receiving end. However, a new DC overvoltage phenomenon appears after the fault is cleared, and the interaction between the wind farm and sending- and receiving-end MMCs makes the DC overvoltage mechanism more complex; further exploration shows that DC overvoltage during the sending-end fault recovery stage occurs under the new integration strategy. Therefore, the evolution process and mechanism of the new DC overvoltage are analysed. It is found that affected by the interaction between wind farms and MMCs, the alternate saturation of the integrators in the PI controller of MMCs is the main cause. Based on this understanding, additional controls are proposed to suppress this DC overvoltage during the fault recovery stage. Simulations are carried out on a test with MATLAB/Simulink, and the results verify the efficacy of the proposed methods in suppressing DC overvoltage.