Capacitor Balance Control of a Modular Multilevel Converter Based on Parallel Connected Branches for a MVAC/LVDC Solid State Transformer

Abstract

To accomodate the growing share of large scale distribution consumers such as data centers, electric cars charging stations and distributed generation systems; Low-Voltage (LV) DC interconnection points, have been proposed in recent years, as more efficient, reliant and more power dense alternative to the current AC distribution grid. In this paper, the capacitor balance of a modular multilevel cascade converter (MMCC) with parallel connected branches is proposed for a novel modular Solid State Transformer (SST). The balance is achieved sharing power between the parallel branches using circulating currents. The primary side of the SST consists of an MMCC connected to the Medium-Voltage Low-Current (MV-LC) AC grid and the secondary side MMCC to the Low-Voltage High-current (LV-HC) DC grid. Simulation results validate the proposed topology and the capacitor balance control of the MMCC.