A multi-loop controller for LCL-filtered grid-connected converters integrated with a hybrid harmonic compensation and a novel virtual impedance

The LCL-filtered converter is widely adapted to interface renewable energy sources and energy storage devices to the grid. While the LCL filter is capable of eliminating current harmonics caused by the high-frequency PWM of the converter, low-order current harmonics are reduced selectively by the stationary-frame resonant harmonic compensators (HCs). There are two types of the harmonic compensator, namely, harmonic current compensator (HCC) and harmonic voltage compensator (HVC). HCCs are in general embedded in parallel with conventional grid current controllers by which the active and reactive power are regulated. If autonomous islanding operation is required, the active and reactive power are controlled by a V-f droop method and a filter capacitor voltage controller with which HCCs or HVCs are added in parallel. In this paper, a multi-loop controller is proposed as one possible alternative to the conventional grid current controller to improve the harmonic compensation performance by using both HCCs and HVCs, which is a hybrid harmonic compensation (HHC). In addition, a novel virtual impedance implementation technique which fits to the multi-loop frame is presented to maintain stability of the controller in case of large grid impedance.