High Efficiency Single-phase T-type LCL-filtered Inverter with Multimode Control and Smooth Mode Transition

Multimode control is an effective technique for inverter efficiency improvement. By selecting the appropriate operation mode at every given operating point, the power losses can be optimized over the whole power range without additional hardware costs. However, frequent mode transitions may cause unexpected filter oscillations and output current distortion. In this paper, a multimode control scheme with smooth mode transition is proposed. For low-to-medium loads, the boundary conduction mode (BCM) and boundary quadrilateral current mode (BQCM) are adopted to reduce switching losses. This is achieved through carefully designed inductor reverse current to ensure complete zero-voltage-switching (ZVS). Under heavy loads, the inverter will operate in continuous conduction mode (CCM) to decrease conduction loss. To alleviate the distortion caused by mode transition, a transition cycle is inserted between two different operation modes. Simulations show that the transition cycle can well suppress the filter oscillations caused by the hard mode transition, thus smoothing the output current. A 1kW T-type hybrid-bridge prototype has been built. The prototype achieves a peak efficiency of 99% and maintains a total rated current distortion (TRD) below 2%, verifying the proposed scheme’s effectiveness in improving efficiency and output current quality.