Operation and Control of Bipolar-Type Modular Solid State Transformer With Active Circulating Current Injection and Arm Voltage Adjusting Method

Abstract

Modular multilevel converter (MMC) based solid state transformer (M-SST) shows promise in hybrid ac/dc distribution grids. To further meet low voltage dc (LVdc) users’ requirements, a bipolar-type M-SST (BM-SST) is developed in this article, which not only has medium voltage ac and dc ports but also directly provides a bipolar low voltage dc (BLVdc) port for various LVdc sources and loads such that an extra SST or voltage balancer for achieving similar outcomes could be saved. Featuring various ac and dc ports, easy voltage conversion, and flexible power control capability, BM-SST is most suitable for multiple ac and dc grids interconnection as an energy hub. The most crucial issue of BM-SST is how to deal with the unbalanced power caused by the bipolar LVdc port which leads to submodule capacitor voltage deviation and uncontrollable circulating current between upper and lower arms of MMC. An arm voltage regulation (AVR) combing with active circulating current injection (ACCI) method is proposed in this article to eliminate the unbalanced power and its negative effects. Based on detailed circuit analysis of the BM-SST, the coordinated control of the BM-SST with optimum AVR&ACCI is designed. Moreover, the effects of AVR&ACCI method on current stress, voltage stress, and power loss of BM-SST are evaluated. The feasibility of the proposed BM-SST has been verified by experimental results.