Analysis of submodule overvoltage mechanism and suppression strategy for DC chopper

In flexible DC transmission systems, when a fault occurs in the receiving-end AC system, it results in a power surplus on the receiving-end DC side. This necessitates a dynamic active power balancing device, leading to the emergence of the DC chopper in this context. However, it is important to note that overvoltage can occur during the operation of the DC chopper, requiring overvoltage suppression measures to ensure equipment safety. In this paper, aiming at a typical two-terminal flexible DC transmission system, the composition of the system is first introduced. The generation mechanism of overvoltage in the sub-modules of the DC chopper valve is analysed. Two major categories of suppression strategies, namely system-level and equipment-level strategies, are proposed. The effectiveness and applicability of each strategy are verified and evaluated through PSCAD simulations.