Modified repetitive control based on comb filters for harmonics control in grid-connected applications

Abstract

Power quality plays an important role in the operation of grid-connected inverters. The injected current should have a minimum harmonic content in order to comply with the local grid codes. This can be challenging due to the fact that the grid voltage can also be distorted. This paper presents the modified repetitive control method for three-phase grid-connected inverters by means of a digital comb filter application. The proposed method provides multiple harmonics compensation, simple implementation and design. It consists in complementing the Proportional-Resonant (PR) control with an appropriately tuned comb filter, which results in mitigation of the output current harmonics and reduced THD. Two types of comb filters have been proposed as a harmonic compensator, namely feedback and feedforward structures. The compensator included an additional FIR low-pass filter for increased robustness. Regardless of the structure chosen the solution is targeted at simplicity of analysis, design and implementation. Furthermore, the compensator based on the feedforward comb filter is casual, hence its implementation on the digital microprocessors is greatly facilitated. The proposed two-stage design of the control loop ensures the stability even in the case of additional poles being introduced by the feedback comb filter. Investigation on frequency variations impact is also included. The research was conducted by means of the following IT solutions: data workflow management environment, code design and generation (Matlab), and user interface design (ControlDesk). The experimental results have demonstrated the effectiveness of the proposed solution for a grid-connected converter operating in a network with distorted supply voltage.