An enhanced commutation failure prevention control in LCC based HVDC systems

Abstract

Most commutation failures in high-voltage direct current (HVDC) systems are caused by AC faults on the inverter side. Conventional control methods for commutation failure prevention are on the premise of fault detection. However, a time period between the occurrence and detection of faults is inevitable. There are no effective control methods yet dedicated to this time period to mitigate commutation failures. In this paper, an enhanced commutation failure prevention (ECFPREV) control method is proposed. During the stage that a fault has taken place but not been detected, the proposed scheme still contributes to regulate the firing angle slightly for better commutation failure mitigation performance. To achieve this, a novel concept of pseudo commutation process is presented first, so that commutation processes with time delay can be performed in near real-time. Based on the constructed pseudo commutation voltage waveform, pseudo commutation parameters can be calculated at each sampling instant. Then, the ECFPREV, which is equipped with a pseudo firing angle prediction module, is proposed. An appropriate firing angle is calculated precisely for each pseudo commutation process regardless the fault detection signal, thus keeping a sufficient commutation margin for the upcoming real commutation process. The feasibility of the proposed strategy is validated through the CIGRE Benchmark HVDC model in PSCAD/EMTDC platform. Simulation results demonstrate that the proposed ECFPREV control is able to effectively improve the commutation failure immunity of HVDC systems.