A bidirectional three‐phase triple‐voltage LLC (T2‐LLC) resonant converter for DC/DC stage in solid state transformer and DC transformer applications

Abstract

Abstract This paper proposes a novel bidirectional three‐phase triple‐voltage LLC (T2‐LLC) resonant converter, which can be applied as the submodule of DC transformer (DCT) with fixed input and output terminal voltages. A novel series‐connected three‐phase circuit is adopted at medium voltage (MV) side to improve the voltage level of single submodule (SM) and decrease SM number. Therefore, the power density of DCT can be improved. Besides, the proposed converter is open‐loop controlled with fixed switching frequency, and it naturally realizes bidirectional power flow without changing controlling modes according to the power direction. However, the efficiency and voltage conversion ratio of proposed T2‐LLC converter are sensitive to resonant parameters, and consistency of three‐phase resonant parameters should be ensured to achieve three‐phase symmetrical operation. Therefore, design and optimization methods are proposed to realize ZVS turn‐on, reduce power losses and ensure fixed voltage conversion ratio. The operation principle and optimization method are presented and a 1000 V/333.3 V/5 kW platform is used to validate feasibility of the proposed converter.