Transient Voltage Estimation of Bipolar VSC Station Connected to Islanded Renewable Power Plant

In a multi-polar voltage source converter based high voltage direct current transmission (VSC-HVDC) system integrating with islanded renewable energy, when one VSC station is blocked suddenly, all the active power through it will flow into other VSC stations and cause transient overvoltage on the ac bus. Based on the analysis and simplification of the typical islanded system composed of modular multilevel converter (MMC) and large-scale renewable power, a calculation method for the overvoltage of ac bus is proposed. This method divides the overvoltage into two parts: dc component and harmonic component. The former is calculated by analyzing the transient process of the control system when the power changes while the latter is derived by calculating the variation of the modulation signal and the dc voltage. The validity of proposed method is verified by comparing it with the simulation results in PSCAD/EMTDC. The comparison results show that the proposed method can correctly calculate the transient ac voltage and current of the system when the converter is blocked.