Digital Control Design Scheme for an LCL-Type Inverter Based on QPR Controller With GVWF Under Weak Grid Conditions

Abstract

This paper introduces a digital control strategy for an inverter with an LCL filter that can be used in weak power grids. Due to the effects of digital control delays and the influence of grid voltage and impedance in a weak power grid (WPG) on the inverter system, it is easy for the system to oscillate, distort the output current, or even become unstable. By introducing grid voltage weighted feedforward (GVWF) based on the feedback of the filter capacitor current and using a Quasi-Proportional Resonance (QPR) current controller along with proper design, the system can achieve stable performance and high-quality, high-precision current output under various conditions, including WPG. The effectiveness of the proposed strategy was validated through hardware design and experimental tests. A comparative analysis was conducted under different control schemes with grid impedances of 0.5 mH, 6 mH, and 12 mH. The results demonstrated that the proposed strategy is a more suitable digital control method for LCL-type grid-connected inverter.

Keywords

• Grid-connected inverter
• LCL filter
• QPR current controller
• weak power grid