Characteristic Analysis and Valve Control Strategy Optimization Method for Single-Phase-to-Ground Faults on the Converter-Transformer Valve Side of the CLCC

The controllable line-commutated converter (CLCC), which achieves forced commutation and effectively prevents commutation failures, has seen initial engineering applications in high-voltage direct current (HVDC) transmission systems. This paper analyzes fault characteristics and develops a valve control strategy for single-phase-to-ground faults on the valve side of the CLCC converter transformer. First, mathematical models of the DC-side circuit and its control system are established. Next, based on the fault location and changes in current paths during the fault, the transient characteristics of the valve current are analyzed, accounting for the effects of the control system. Furthermore, a relationship describing how faults affect the extrema of the valve current during commutation is derived. Finally, a method for optimizing the valve control strategy is proposed based on predicting the times at which the valve current reaches its extrema. This prediction is obtained using a current transfer time model derived from an equivalent circuit describing current transfer between the main and auxiliary branches of the CLCC. Simulation results show that the proposed method can effectively reduce the current stress imposed on the arrester by fault-induced abnormal commutation, improve the current waveform during commutation, and enhance the operational stability of the system during faults.