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 paper 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 Pulse Width 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 400-480 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.