Control loop design of a two-stage bidirectional AC/DC converter for renewable energy systems

It is well known that single-phase ac/dc converter requires big output capacitor to filter the input ripple power from the grid. To interface a dc nanogrid to a single-phase utility, bidirectional power flow and small dc side output voltage ripple are also necessary. By cascading a full-bridge ac/dc converter with a bidirectional buck converter, a two-stage ac/dc topology is adopted to reduce the dc-link capacitor. The power stage is modeled and a bidirectional control system is designed. Besides PI compensator, feedforward and resonant controller are used to improve the ac current waveform and reduce the dc bus output voltage ripple. The designed controller can work for both rectifier and regenerative mode. A 10 kW prototype is used to verify the performance. The dc side output voltage ripple is small and the transition between rectifier and regenerative mode is smooth.