In this article, the stability robustness of repetitive control (RC) in the grid-tied inverter is discussed. Due to the limited control band, high-frequency characteristics of the control plant are seriously affected by the modeling and parameter uncertainties, which can lead to RC controller instability. Then, an infinite impulse response (IIR) low-pass filter is applied inside the internal model of RC to improve the stability robustness. To alleviate the effects of the RC gain reduction induced by the phase lag of the IIR filter, an approximated simple computational method of the phase compensation is proposed. The situation of the gain attenuation and frequency deviation of RC caused by the IIR filter is analyzed, and a method that modifies the phase compensator in real time for the IIR filter is applied to resist the grid frequency variation. Parameter design is provided in details, and comparison experiments are performed in different uncertainty situations to demonstrate the effectiveness of the proposed approach.

In this article, multibandwidth repetitive control (RC) is developed and applied to a grid-tied inverter to resist grid frequency variation. In the proposed RC, each of the resonant bandwidth is set individually by the gain of the internal model filter. A design procedure for the internal model filter is proposed, which splits the filter into several zero-phase finite impulse response (FIR) filters for reducing the design difficulties. The control structure and stability analysis show that the gain of the multibandwidth RC is increased. An example design procedure for the multibandwidth RC is presented in detail. Comparison experiments with different RCs are performed under deviated grid frequencies, and the results demonstrate the effectiveness of the proposed multibandwidth RC scheme.

Influence on injected current arising from grid voltage is a noticeable issue in the design of grid-tied inverter, especially under low control gain or low power command conditions. The grid voltage full feedforward strategy is a widely used strategy to solve this problem, whereas the physical meanings of the full feedforward items are unclear. In this brief, the full feedforward of the direct current control type LCL-filter inverter and indirect current control type LCCL-filter inverter was reviewed from the circuit and control perspectives. The full feedforward items were rephrased. The new interpretations indicate that the full feedforward strategy is physically rational and complete.

LCL filter is usually used as an interface between inverters and the grid. However, due to the characteristics of LCL filter and system uncertainties, it is complex to design a controller with proper parameters. In this paper, with LCCL filter, the order of the inverter control system can be reduced from third order to first order, and an uncertainty and disturbance estimator based control strategy for grid-tied inverters with LCCL filter is proposed. Specifically, the proposed control strategy consists of differential feedforward, proportional-integral controller, and grid voltage feedforward. Moreover, with one-sampling computation plus half-sampling pulse width modulation delays considered, a simple and clear tuning algorithm for the proposed control strategy is presented. Finally, the inverter system with the proposed control strategy is investigated, and the effectiveness is supported by the tuning and comparative experiments with a 2-kW inverter.