Parallel Operation of Distributed Voltage Balancers for Bipolar DC System with Improved Reliability and Efficiency

Bipolar DC system provides flexible connection and decreased line-to-ground voltage level at the expense of potential voltage unbalance between positive pole and negative pole. Voltage balancer works as "Dependent Voltage Source" to eliminate the unbalance voltage. It is a critical grid-forming component for bipolar system. Considering the need of system expansion and "N+1" redundancy design, bipolar DC system with decentralized voltage balancers is first proposed in this paper. For coordination of decentralized voltage balancers, classical hierarchical control structure is expanded in this paper. Three control layers for distributed voltage balancers are presented, including primary layer based on "Unbalanced voltage-I" droop control, secondary layer for elimination of voltage unbalance and tertiary layer for minimization of system power loss. Compared with existing solution, the proposed parallel operation of voltage balancers have shown higher system reliability and improved efficiency.