Comprehensive Control of Bipolar DC Solid-State Transformers Based on ISOP-QAB Converters

This paper proposes a comprehensive control strategy for bipolar dc solid-state transformers (SSTs) based on input-series output-parallel (ISOP) quad active-bridge (QAB) converters. While existing works have addressed controls for SST in ISOP structure, or for QAB converters, individually, an integrated control scheme that harmonizes the entire system has remained unaddressed. The proposed strategy ensures both voltage and power balancing of the bipolar dc SST by leveraging a straightforward conversion of the power flows into phase-shift angles within each QAB module. The effectiveness of the proposed strategy is validated through experimental tests on an SST prototype with ±750 V / 36 kW ratings, composed of six QAB modules.