A Multilevel Three-Phase Integrated AC-DC Bidirectional Resonant Converter for BESS

Two-stage isolated bidirectional ac-dc resonant converter consists of a front-end hard-switched ac-dc power factor correction (PFC) rectifier/inverter followed by a soft-switched bidirectional dc-dc resonant converter. The switching frequency of the ac-dc stage is limited due to hard switching. This article presents a novel low device count isolated bidirectional multilevel bridge tapped resonant (MBTR) ac-dc converter for battery-to-grid integration. It is a unique integration of an ac-dc three-level diode-clamped converter followed by an isolated dc-dc bidirectional converter in which the two stages operate in a decoupled fashion so that the ac-dc stage operates with continuous conduction mode without imposing any variation of the duty cycle of the dc-dc stage. Resonant currents realize natural soft switching. A novel space vector pulsewidth modulation (SVPWM) technique is proposed to control dc-link voltage and bidirectional power flow. The proposed converter, modulation scheme, and control are validated with a 12-kW prototype. The ac voltage varies from 400 to 480 V, and the output battery voltage variation is 550-800 V. A peak efficiency of 95.8% is achieved while operating at 85-95-kHz switching frequency.