Comparative Analysis of the Operational Effects of AC/DC and DC/DC SOPs in an AC/DC Hybrid Distribution Network

This paper compares the operational effects of DC/DC and AC/DC soft open points (SOPs), focusing on the difference between internal DC power redistribution and external AC power support, in an AC/DC hybrid distribution network based on a modified IEEE 33-bus unipolar DC system. The performance is evaluated under normal operation, increased and decreased EV charging demand, and an NREL-based time-varying charging profile using voltage profiles, line losses, voltage deviation index (VDI), load balancing index (LBI), and EV charging curtailment. The results show that the AC/DC SOP provides the strongest DC voltage support and the largest reduction in DC feeder line losses under normal and increased charging conditions by exchanging active power with the external AC grid, while also eliminating charging curtailment under increased charging demand. However, under reduced charging demand, SOP operation can increase line losses and line loading depending on the power-flow direction. These results indicate that SOP operating strategies should account for system loading conditions and power-transfer direction.