Repetitive Controller of Capacitor-Less Current-Fed Dual-Half-Bridge Converter for Grid-Connected Fuel Cell System

This paper proposes a repetitive controller (RC) for a capacitor-less current-fed dual-half-bridge (CF-DHB) converter for a grid-connected fuel cell system. The pulsating power caused by the ac grid makes the dc-link voltage and input current fluctuate with twice the grid frequency. To achieve zero-voltage-switching (ZVS) for the capacitor-less CF-DHB converter and to increase the lifetime of the fuel cell, we need to suppress the low-frequency dc-link voltage ripple and the low-frequency input current ripple. We first make use of the duty cycle to regulate the dc-link voltage, and of the phase-shift angle to regulate the input current. Because the transfer function from the duty cycle to the dc-link voltage has a right-half plane (RHP) zero, the conventional proportional-integral controller cannot achieve satisfactory performance at the dc-link voltage. To compensate for the phase lag due to the existence of the RHP zero, we propose to use an RC with phase-lead compensation for dc-link voltage control. Because the transfer function from the phase-shift angle to the input current has one left-half plane zero, the conventional RC is used to control the input current. In developing the proposed controller, we first derive the dynamic model of the CF-DHB converter in the grid-connected environment, and then use the model to design an RC. We also provide a detailed and practical design guideline to select the control parameters of the capacitor-less CF-DHB converter that can meet the desired performance. The proposed RC can reduce the input current ripple significantly and suppress the dc-link voltage ripple within the predetermined range and thereby achieves ZVS. Experimental results demonstrate that the proposed control scheme achieves desirable performance.