Single-Stage Isolated Voltage Balancer for Bipolar DC Distribution System Using Three-level Dual Active Bridge Converter

Abstract

In the bipolar dc distribution system, an isolated voltage balancer is an essential power stage to construct bipolar dc poles and offer galvanic isolation between ac grid and dc bus replacing a bulky line-frequency transformer. However, in conventional manner, the isolated voltage balancer is generally implemented by two power conversion stages: two isolated dc-dc converters or a single isolated dc-dc converter and additional non-isolated voltage balancer pair. In this paper, a single-stage isolated voltage balancer is proposed to improve its power density and power conversion efficiency in the bipolar dc distribution system. The proposed voltage balancer is implemented by only three-level dual active bridge converter. Furthermore, improved switching modulation method is developed for the proposed voltage balancer to regulate the bipolar voltage level under unbalanced power flow conditions without additional power stages. Finally, the effectiveness of the proposed voltage balancer is verified by experiments using a 3-kW laboratory prototype converter.