Design and Implementation of Repetitive Control for Medium-Frequency Inverter Based on Double-Zero Approximation

The conventional implementation of the repetitive control (RC) in the standard 50/60 Hz inverter features fast roll-off of the gain-frequency response (GFR) at the frequency beyond the control bandwidth. Contributed by the high output frequencyf0of the medium frequency (MF) applications, the fine reshape of the RC plant at the frequency up to several kilohertz is nontrivial for enhancing the low-order harmonic rejection capability. Therefore, the implementation of RC is revisited from the perspective of structure selection, parameter tuning, and the trade-off between tracking performance and stability. Orienting at double-zero approximation over a wide frequency band, the design criteria and the optimizing principle of RC are elaborated for MF inverter. The plugged-in RC is employed, where the proportional-integral parameters are tuned by constraining the gain of the RC plant lower than 1. A biquad filter in series with the fractional-order zero-phase-shift (ZPS) filter is further used for the 0 dB approximation of the GFR of the RC plant, and the Lagrange interpolation-based phase-lead filter is adopted for 0-degree correction. Moreover, the damped ZPS filter-based internal model effectively avoids the center frequency shift and guarantees a high gain (>30 dB) at selectednf0. Finally, experiments are conducted on a 600 VA prototype, confirming that the dynamic and steady-state performance of the MF inverter are satisfactory with the implementation of the proposed RC.