Optimal-droop voltage-restorer for zonal dc distribution system with simultaneous consideration of dynamic current balance and power loss reduction

A dc voltage restorer (dc-VR) with optimal droop control is developed to reduce the power loss and solve the dynamic current imbalance simultaneously of the zonal dc distribution system integrating multi-parallel energy storage. Firstly, the power loss model composed of line loss and converter loss is built and it is proved to be a strictly convex function with respect to droop coefficients, which indicates the power loss can be mitigated by optimizing the current distribution. Thus, an image-based droop optimization method is proposed to search for the optimal droop coefficients. Then, the economy of voltage compensation on power loss reduction is investigated and an ESS-combined dc-VR is designed, including an interleaved parallel architecture and a capacitor-integrated electric spring (C-ES). Unified virtual inertia is proposed for interleaved parallel bridge arms to make their dynamic performance consistent. And the newly introduced C-ES can keep the bus voltage at the rated level to reduce the system cost. Therefore, the problem of dynamic currents imbalance is addressed from the points of converter structure (dc-VR) and control system (unified inertia), and the power loss can be reduced by voltage recovery (C-ES) and optimizing the current reallocation which is achieved by updating sharing coefficients from the image-based droop optimization method. Finally, the simulation cases validate the effectiveness of the proposed optimal-droop dc-VR on dynamic current balance and power loss reduction, and hardware in the loop experiment results prove the consistent dynamic characteristics of the dc-VR's arms.