Research on the black start of DC transformers based on soft converter control

Abstract

The DC transformers with dual active bridge （DAB） topology is characterized by zero voltage switching （ZVS） and provides the basis for realizing lossless turn-off buffer by directly paralleling buffer capacitors. First， the structure and operation control of IGCT-HDCT （IGCT-based high-capacity DC transformer） are introduced. Then， a black startup strategy based on a soft converter is proposed， which divides the start process into the extended phase shift and single phase shift startup to realize the ZVS of all devices during the black startup of IGCT-HDCT and prevent the surge current on the buffer capacitor. By analyzing the resonance process of the device after shutdown， the relationship between the phase shift and the system parameters in the extended phase shift stage is introduced， and the ZVS characteristics of the device in the single phase shift stage are verified. The transient bias current of the system is eliminated by predictive control and duty ratio design. Finally， the feasibility of the proposed control is verified by a 1.5 kV/2 MW prototype.

Keywords

• dc transformer
• black start
• zero voltage switching