Hybrid Multiple Chopper Cells of PWM and Square-wave Operation for Solid-state Transformer

Abstract

This paper proposes a modulation method combining PWM and square-wave drive for solid-state transformers (SSTs). Modular multilevel configurations based on input series and output parallel (ISOP) have been widely used. The advantage of ISOP configuration is that low on-resister and low-switching-loss devices are available because the applied voltage on each cell is divided by the number of cells. For this reason, SST based on the ISOP configuration is widely used in a medium-voltage system. However, this configuration has a problem that a high number of medium frequency devices, such as SiC-MOSFET, increases the cost of SST. In order to solve above problem, a reduction method of medium frequency devices are proposed. In the proposed method, one cell is driven with the PWM operation in order to compensate for the harmonic component. The other cells are driven by square-wave operation. Thus, the power factor correction (PFC) is held by different switching frequencies in each cell converter operated. Owing to the proposed operation, the medium frequency driving devices can be replaced with low-frequency devices, such as Si-IGBT, for cost-saving. As the drawback of the proposed control, the conduction time is unbalanced in the square wave cells. Thus, this paper also proposes the sorting operation to balance the output power of cells operated with square-wave drive. From the experimental results, THD of the input current is 2.91 %, the input power factor is 0.99, the maximum efficiency is 94.2% at 0.3p.u.(1.p.u.=1.0 k W).