Trapezoidal Modulation for Three-Arm ZVS in Dual Active Bridge DC Transformer With Series-Connected IGCTs

This paper proposes an improved trapezoidal modulation (ITM) strategy to achieve zero voltage switching (ZVS) in three arms of a series-connected device-based DC transformer (SC-DCT) equipped with lossless buffer capacitors. To overcome the challenge of achieving ZVS in arms 1, 2, and 4 under light-load and no-load conditions, the ITM introduces an auxiliary phase-shift angle. By analyzing current stress under various power levels and voltage mismatch conditions, a precise calculation method for the auxiliary phase-shift angle is proposed to minimize reactive current. Due to the asymmetry of the topology, a natural power reversal method is further developed to prevent the failure of ZVS in arm 4 during power direction changes. Finally, a 10kV/1.5kV 3MW prototype has been built and experimentally validated using a back-to-back test platform. The results confirm that all buffer-capacitor-equipped arms maintain ZVS over the full power range, and that the voltage balancing across series-connected devices remains within ±5%. The proposed solution is designed for medium-voltage DC distribution networks and features a simple topology with a peak efficiency of 98.3%.