A transformer-flux-balance controller for a high-frequency-link inverter with applications for solid-state transformer, renewable/alternative energy sources, energy storage, and electric vehicles

This paper outlines a controller that balances the flux of the isolation transformer in a multi-stage high-frequency-link (HFL) bi-directional inverter. It comprises a primary-side high-frequency (HF) dc/ac converter (referred to as Bridge I) followed by cascaded ac/pulsating-dc and pulsating-dc/ac converters (referred to as Bridge II and Bridge III) on the secondary side of the transformer. The overall inverter is preceded by a capacitive filter and is followed by an output LC filter. The issue with the flux balance arises when energy is fed back from the ac output of the inverter to the dc energy source. The problem arises in the HFL inverter in this bi-directional mode since without an intermediate dc-link capacitor, the output of Bridge III, which is pulsating, has an additional problem with the variation in the maximum amplitude of each pulse due to an underlying beat frequency. Without addressing the flux imbalance, the HF transformer may saturate and have an impact on its size and thermal sustenance. The simulation results of the flux-balance controller shows satisfactory performance.