Active Thermal Control of High-Frequency Transformers in Cascaded H-Bridge and Dual Active Bridge Based Solid State Transformer

Solid state transformers (SST) with cascaded H-bridge (CHB) based active front end (AFE) and dual active bridge (DAB) based isolated dc-dc converter has become an increasingly popular technology in hyper-scale power consumption scenarios. However, the inherent double-line frequency fluctuation caused by AFE H-bridge results in unexpected power losses and high tem-perature increase of high frequency transformers (HFT), which severely threatens the operation reliability and service life of HFTs and SST. In this paper, characteristics of HFT power losses are deduced analytically. A 3rd order harmonic injection based active thermal control method is proposed to re-organize the HFT cur-rent frequency spectrum and lower its winding power losses. The analytical expression of power losses after injection is derived, and the optimized amplitude and phase angle of the injected harmonic is analyzed. The proposed control strategy is validated in both sim-ulated and experimental 2MW/10kVac input/800Vdc output SST prototypes. By injecting the optimized 3rd order harmonic, 17% winding power losses can be reduced, in which a 34°C temperature reduction of potted winding can be achieved.