Virtual circuit design of grid-connected half-bridge converters with higher-order filters

This paper presents a structured design method for half-bridge converters with higher-order lossless output filters, focusing on the popular LCL filters. The method builds on the theory of terminated ladder networks and on a control algorithm actively emulating the ladder network's termination. Given a desired transfer function, a normalized ladder network is accordingly synthesized and scaled to meet the physical requirements. The component sizing and the active damping of filter resonances are simultaneously tackled. The associated digital-control architecture for LCL filters is explained. A Kalman observer is employed to deal with the inherent sample delay. An LCL filter is designed based on the proposed method and is experimentally validated with the half-bridge converters operating as active front end. The grid reactive currents are stepped and the results show properly damped responses. The system has good disturbance rejection and is passive according to the simulated input admittance. The robustness of the system is also demonstrated by numerical sensitivity analysis. These excellent characteristics can facilitate the design of the upper-layer or incorporated outer-loop controller, and can enhance the performance of the complete system.