Impedance-Based Stability Analysis of Different Current Control Categories of Grid-Connected Inverters

The impedance-based stability evaluated for different current control strategies of three-phase inverter equipped with $LCL$ filter is discussed in this paper. At first, the current control strategies including the single loop and the double loop current control categories, are presented. It is mentioned that the design procedure of the presented strategies is based on the sufficient and constant phase margin and the suitable open loop bandwidth. The open loop gain frequency response of each category validates the design process. Further on, the impedance-based stability criterion is applied to analyze and determine the stability margin of each category. It reveals that among the investigated strategies, the most stable case belongs to the single loop inverter-side current control category and the double loop grid-side current control category has the worst stability margin. This analysis is carried out in presence of a constant power load by maintaining the principle of linear analysis of the system, too. As expected, the single loop inverter-side current control category and the double loop grid-side current control category are still the best and worst cases, respectively. Finally, the effectiveness of frequency responses and controller design are validated by simulation studies.