Single-pole tripped emergency control and multistep power recovery strategy for a symmetric bipolar VSC-HVDC system

In a symmetric bipolar voltage sourced converter-high voltage direct current system (VSC-HVDC) with a conventionally independent power controller, a single-pole converter unit malfunction or conversion capacity limitation results in a power imbalance for this converter station. To address this, a centralized power control strategy with single-pole tripped emergency control and a multistep power regulation strategy for parallel double converter units is proposed. Firstly, considering the power conversion limitation or power asymmetrical operation caused by single-unit malfunction, the tripped emergency coordination between two converter units can be accomplished to realize a constant transmission power capacity of this converter station. Secondly, for the recovery process of the faulty converter unit, the multistep power switching strategy is adopted. When the fault has been removed and the faulty converter unit is deblocked to reconnect to the DC grid, the proposed strategy is used to achieve self-adaptive power regulation. By adopting this strategy, the large power fluctuations in the process of reconnection can be effectively alleviated. Finally, a simulator based on PSCAD/EMTDC is indicated. The results show that the developed control strategy can realize self-adaptive imbalanced power adjustment between two parallel converter units and single-pole tripped unit smoothing reconnection.