Analysis and Prevention of Subsequent Commutation Failures Caused by Improper Inverter Control Interactions in HVDC Systems

This paper presents the essential reason for subsequent commutation failures (SCFs) in high voltage direct current (HVDC) systems, and accordingly proposes a novel adaptive extinction angle recovery (AEAR) control strategy to prevent SCFs. Firstly, based on the changes of the inverter control mode after the first CF, the interaction process of inverter control loops is divided into 3 stages and analyzed in detail. It is found that the control aim of DC current is ambiguous in stage 2, causing the output of current error control (CEC) to persist above zero. In stage 3, however, the CEC output drops rapidly, leading to an uncontrolled status of extinction angle and bringing the risk of SCFs. Furthermore, the precondition for the control mode switching is derived. On the basis of that, an AEAR control strategy is proposed. By adopting an adaptive extinction angle reference, the proposed scheme maintains the inverter at the constant extinction angle (CEA) mode after the first CF. Thus, the uncontrolled status of extinction angle can be avoided. The problem of SCFs due to improper inverter control interactions is also able to be fixed at the root. Simulation results show the validity of the analysis and the superiority of the proposed control strategy in preventing SCFs.