Improved VDCOL Control Method of Multiple HVDC Systems for Commutation Failure Mitigation

The voltage-dependent current order limiter (VDCOL) is an important link in the current control of HVDC systems. However, due to the fixed startup voltage threshold and the fixed slope of the characteristic curve, its control performance in complex multiple HVDC systems needs to be improved. This paper first analyzes the characteristics of VDCOL and the limitations of existing improved strategies in different post-fault transient stages, and then proposes a unified improved VDCOL control method covering the whole post-fault process to reduce the commutation failure (CF) risk of multiple HVDC systems. At the instant of fault occurrence, a dc current startup control strategy is constructed considering asymmetrical faults and voltage harmonics, improving the fault response sensitivity and engineering applicability. During the post-fault transient process, the dc current order is adjusted according to the VDCOL characteristic curve optimized to minimize the overall active power loss of multiple HVDC systems under typical faults, achieving a balance between CF mitigation and active power transmission. During the voltage recovery stage, the VDCOL input signal of each HVDC system is coordinately determined based on the virtual commutation voltage that considers the real-time commutation voltage states of adjacent HVDC systems, reducing the risk of cascading CF. PSCAD/EMTDC simulations confirm its effectiveness and superiority over existing strategies.