Transient switching performance of VSC and the DC fault partitions

Abstract

Two-level voltage source converters (VSC) have been widely used in energy conversion and transmission but they are vulnerable to the high discharge current from the DC-linked capacitor when the DC faults occurs. Previous studies have divided the DC fault process into three stages. The capacitor discharge stage (first stage) and the fault steady-state (final stage) are relatively simple and have been analyzed in detail. In contrast, the transition between these stages, i.e., the uncontrolled rectification stage, exhibits an extremely complex transient current feeding process when a DC fault occurs due to the unilateral conductivity of the internal power electronic devices and charge/discharge characteristics of the internal energy storage components. However, the switching characteristics of the converter in this stage have not been investigated to date. This study focuses on the analysis of the transient current feeding process of the uncontrolled rectification stage. Two time-boundaries of the commutation are defined based on the calculation of the limit of the current feeding rate and the effects of different fault distances on the transient switching performance of the converter are investigated. It is demonstrated that it is relatively easy to divide the DC line into three partitions by using the converter’s switching information instead of measuring the line’s electrical quantities; this provides a basis for developing DC protection. In the simulation analysis of the proposed method, a range of fault distances and initial feeding angles are considered.