Transient Analysis and Overcurrent Limited Strategy for Supply Restoration-Oriented Hybrid Soft Open Point

The hybrid soft open point (HSOP), formed by the parallel combination of the normally open point (NOP) and the soft open point (SOP), combines the advantages of flexibility and high cost-efficiency. At the power outage moment, the seamless switching process of HSOP contains a co-operation state with NOP and SOP parallel working together. This state involves current transitions between NOP and SOP, which may cause overcurrent and trigger the protection shutdown. To address this issue, a wide-band mathematical model is established to analyze and evaluate the transiting current, including high-frequency transiting current caused by the phase difference of pulsewidth modulation, and low-frequency overcurrent caused by a grid voltage step change. Based on the analysis, a high-frequency overcurrent limitation strategy is introduced, relying on the synchronization modulation strategy, and a low-frequency overcurrent limitation strategy is proposed, based on virtual impedance. The effectiveness of the proposed strategy is validated by operating the whole switching process with the simulation and a downscaled experimental platform. According to the experimental results, the SOP can support the load voltage within 20 ms after the power outage. Moreover, the high-frequency currents can be suppressed to around 10% compared to extreme cases, and the low-frequency overcurrent can be completely inhibited.