Voltage Support and Imbalance Mitigation during Voltage Sags by Renewable Energy Fed Grid Connected Inverters

In this study, symmetrical component-based current injection is used to analyse a single-stage grid-connected inverter with low voltage ride-through (LVRT) capability. During voltage sags, the renewable source-fed voltage source inverter (VSI) may disconnect from the main grid at the point of common coupling. This condition arises due to the Under-voltage/over-current protection in the renewable inverters. This sudden dis-connecting of the inverters may cause voltage collapse, frequency instability, and synchronous angle instability in utility networks with significant penetration of power from renewable inverters, which can result in blackouts. Utility grid rules require low-voltage ride-through capabilities in grid-connected inverters to mitigate this effect. In order to support voltage and reduce volt-age imbalance at the point of common coupling, a symmetrical component-based real and reactive power injection approach has been developed in this study. This technique is implemented by considering the constraint of not exceeding the inverter's rated current capability. The proposed technique effectively boosted the positive sequence voltage component to a reference value by using a PI controller. The negative sequence reactive power was drawn using the remaining reactive power capacity in order to suppress the voltage's negative sequence component. Results are presented in the paper for various voltage sag conditions. The proposed techniques are validated by simulating the model in a Matlab simulink environment.