Transient-Energy-Based Coordinated DC-Voltage Control for Dual-DC-Bus Soft Open Point

Dual-DC-bus soft open points (SOPs) provide an effective means of enhancing the operational flexibility and resilience of distribution systems. However, transient power imbalances caused by source/load variations and feeder operating-state changes may result in considerable DC-bus voltage deviations. To address this issue, this paper proposes a transient-energy-based coordinated control strategy for a dual-DC-bus flexible interconnection system. Based on the instantaneous power-balance relationships, the transient power coupling between the medium-voltage DC (MVDC) and low-voltage DC (LVDC) buses is first analyzed. The proposed control uses the normalized transient-energy deviations to coordinate the transient power regulation of the input-parallel output-series dual-active-bridge (IPOS-DAB) and battery-side dual-active-bridge (Battery-DAB) converters. In particular, the Battery-DAB compensates for the LVDC-side power variation introduced by the MVDC-support action of the IPOS-DAB while simultaneously regulating the LVDC-side transient energy. These auxiliary power commands are implemented through the existing phase-shift control channels without modifying the original local DC-voltage control structures. The effectiveness of the proposed strategy is verified by simulations under grid-connected source/load disturbances, a single-feeder outage, and a double-feeder outage. For the three cases, the maximum LVDC voltage deviations are reduced from 2.19%, 1.96%, and 2.80% to 0.77%, 0.68%, and 0.97%, respectively, while the corresponding maximum MVDC voltage deviations are reduced from 1.85%, 1.37%, and 2.34% to 1.06%, 0.91%, and 1.35%, respectively. The results demonstrate that the proposed strategy effectively suppresses the maximum voltage deviations of both DC buses under different operating conditions while maintaining stable post-disturbance recovery. The small-signal and time-domain analyses further indicate that the coordination gains should be selected by balancing voltage-deviation suppression against modal damping and recovery performance.