A dual-current control loop for Balanced and Unbalanced Current control for LCL filter interfaced Grid connected VSIs

Grid connected voltage source inverters (GC-VSIs) are gaining importance due to the penetration of renewables. Typically these are interfaced to the grid through third order LCL (inductor-capacitor-inductor) filters. In the grid connected mode, the objective is to control the current absorbed from and/or supplied to the grid. Conventionally the control is done in the synchronously rotating frame through proportional integral (PI) controllers in a multi-loop structure. This paper discusses a dual inductor current loop current control technique for regulating the outer inductor current which is an alternative to the conventional outer inductor current - capacitor voltage - inner inductor current control scheme. The LCL filter oscillations are damped actively using the derived capacitor current. The proposed technique has been validated for its stable operation and performance through off-line simulations on Matlab-Simulink and on an experimental setup in the laboratory for both balanced and unbalanced current references.