Sliding mode control of four-leg inverters in a stand-alone microgrid for unbalance, neutral to ground voltage, and harmonics compensation

In a stand-alone microgrid, voltage control is an important issue to keep the grid balanced under unbalanced load conditions. The other important challenges in microgrids include handling the neutral to the ground voltage (NGV) as well as total harmonic distortion (THD) under its limits due to the IEEE standards. A four-leg inverter is the best choice for a three-phase transformerless inverter employed in a stand-alone microgrid. To control the inverter, sliding mode control (SMC) is a well-known nonlinear control system to handle unbalanced and nonlinear load conditions as it can provide high sinusoidal load voltage with high performance and fast dynamic response. This paper proposes an enhanced SMC control method for minimizing the NGV and THD in addition to compensating for unbalanced load voltage conditions simultaneously in a three-phase four-leg inverter. In order to verify the results of the proposed method, simulation results with MATLAB/Simulink are presented to confirm the effectiveness of the proposed SMC under various load conditions. The proposed method provided fast dynamic response and fixed switching frequency. Further, a significant reduction occurred in NGV and THD. The simulation results of this method compared with the MPC method indicated the better performance of the proposed method against the unbalanced and nonlinear various load conditions.