Abstract
The growing integration of grid-connected inverters (GCIs) with utility grids due to the dynamic nature of the power system brings about advanced control for GCI systems for efficient operation. Conventional proportional-integral repetitive controller (PIRC) for the GCI control, can eliminate periodic disturbance under steady-state conditions. However, when the disturbance is varying the PIRC struggles with stability, leading to entire system instability. This disturbance happens because of the impedance in the long transmission line connecting the grid with the inverter. The system strength is measured by the short circuit ratio (SCR). If the SCR is greater than 3, the grid is considered strong. If the SCR is less than 3, the grid is weak; if it falls below 2, it is very weak. In other words, higher impedance means a lower SCR and lower impedance means a higher SCR. To address this issue, an enhanced repetitive controller (ERC) with an external compensator (EC) is proposed in this study. MATLAB/Simulink is used to simulate and compare the performance of both the conventional PIRC and the proposed ERC-EC controllers, with key metrics including voltage total harmonic distortion (THD), settling time, tracking accuracy, and overall system stability. Simulation results show that the ERC-EC handles voltage variations and frequency fluctuations under strong, weak, and very weak grid conditions. Stability analysis via pole placement of characteristic polynomials shows that the 3 poles of the system polynomial for the proposed ERC-EC are all within the unit circle, showing a higher stability margin compared to the conventional PIRC, where 1 of the 3 poles is outside the unit circle. The proposed ERC-EC also shows a quicker response, with settling times of 0.0012s compared to 0.0018s for the conventional PIRC during transitions. Furthermore, the ERC-EC significantly reduces voltage harmonics from 827.34V with the PIRC under distorted grid conditions to 466.57V with the ERC-EC, achieving a THD of 1.98%, compared to 12.60% for the conventional PIRC.

Key words: Grid Connected Inverters, Frequency and Voltage control, Enhanced Repetitive Controller (ERC).