Sliding Mode Control of a Grid-Connected Distributed Generation Unit under Unbalanced Voltage Conditions

The increasing presence of inverter-based distributed generation (DG) units in distribution networks (DNs) requires control methods that achieve high performance not only during normal operating conditions, but also under unbalanced conditions. With a high probability, a type of voltage unbalance in DNs is unequal three-phase voltage magnitudes at the fundamental system frequency. This can occur temporarily due to faults or permanently due to uneven distribution of unbalanced loads, on the three-phases of the DN. This paper proposes a sliding mode (SM) based controller for grid-connected DG units, under unbalanced grid voltage condition. The proposed control strategy employs a nonlinear control scheme to directly cancel out the negative-sequence (NS) components of DG output current under unbalanced voltage condition; and directly regulate the positive-sequence (PS) active and reactive power injected by DG units to main-grid. The control method proposed in this paper is shown to be robust and stable system parameter uncertainties. The validity and effectiveness of the proposed controller is verified by using time-domain simulation studies, under the MATLAB/Simulink software environment.