Variable Structure Control for Three-Phase LCL-Filtered Inverters Using a Reduced Converter Model

This paper presents a new concept in active damping techniques using a reduced model of an LCL-filtered grid-connected inverter. The presence of the LCL filter complicates the design of the inverter control scheme, particularly when uncertainties in the system parameters, especially in the grid inductance, are considered. The proposed control algorithm is addressed to overcome such difficulties using a reduced model of the inverter in a state observer. In this proposal, two of the three state variables of the system are obviated from the physical inverter model, and only the inverter-side current is considered. Therefore, the inverter-side current can be estimated emulating the case of an inverter with only one inductor, thus eliminating the resonance problem produced by the LCL filter. Besides, in the case of a distorted grid, the method allows us to estimate the voltages at the point of common coupling free of noise and distortion without using any phase-locked loop based synchronization algorithm. This proposal provides the following features to the closed-loop system: first, robust and simple active damping control under system parameters deviation; second, robustness against grid voltage unbalance and distortion; and third, an important reduction in the computational load of the control algorithm, which allows us to increase the switching frequency. To complete the control scheme, a theoretical stability analysis is developed considering the effect of the observer, the system discretization, and the system parameters deviation. Experimental and comparative evaluation results are presented to validate the effectiveness of the proposed control scheme.