Average value model of modular multilevel converters for switching overvoltage simulation in VSC‐HVDC grids

Abstract

The first ±500 kV voltage source converter based high voltage direct current (VSC‐HVDC) grid in the world is being built in China. The half‐bridge submodule‐based modular multilevel converter (MMC), the direct current (DC) circuit breaker (DCCB), and the overhead line are used in the project. Switching overvoltage will occur after the DCCB turns off the short circuit current (SCC) in the VSC‐HVDC grid. The existing average value models (AVMs) of MMC still have some limitations in simulating such a kind of overvoltage. The main reason is that these AVMs cannot accurately simulate the working states of MMC in the VSC‐HVDC grid with DCCB. This article summarizes the possible cases of the action strategies between the DCCB and MMC in the VSC‐HVDC grid. Then, the working states of MMC in the different cases are analyzed. Corresponding to different working states, the limitations of existing AVMs are analyzed. The existing AVMs cannot simulate the alternating current or DC components in the arm current before being blocked and the charging process of the submodule capacitor after being blocked. An enhanced AVM is proposed in this article, which can fully simulate the MMC's working states. Simulation results show that the accuracy of the proposed AVM is highly improved over the existing AVMs. Meanwhile, the simulation efficiency of the proposed model is improved over the detailed equivalent model that is highly accurate but relatively low efficient for VSC‐HVDC grid simulations. © 2019 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.