Efficiency Improvement of Solid-State Transformers With MMC Front-End AC-DC Converters Through Adjusting DC Bus Voltage and Active Cells

The inefficiency of solid-state transformers (SSTs) under medium (or light) loads poses a significant challenge for wide power range applications, which leads to reduced energy utilization, increased electricity costs, and potential reliability issues. To solve this problem, this article proposes an SST efficiency improvement strategy considering full power range. The SST utilizes a full-bridge modular multilevel converter (MMC) as the front-end and an input-series-output-parallel dc-dc stage as the back-end. By dynamically adjusting the number of back-end dc-dc cells according to the load power, this strategy significantly reduces the fixed loss of SST at medium and light loads. Meanwhile, a variable intermediate dc bus voltage control is introduced by the front-end MMC. It guarantees the smooth adjustment without inrush current and high system efficiency at heavy loads. Based on the loss model of MMC-SST, a 10 kV/1 MW case validates that the proposed strategy can improve the SST efficiency by 3.3% at light loads while rarely impacting the efficiency at rated loads. Simulations and experiments confirmed the feasibility.