Sliding Mode Fixed Frequency Current Controller Design for Grid-Connected NPC Inverter

In this paper, a fixed-frequency pulsewidth modulation (PWM) based on sliding-mode current controller is designed and applied to a utility interface three-phase/wire/level neutral-point-clamped inverter. The proposed design methodology of the sliding-mode control is based on a constant switching frequency operation and on Gao's reaching law that allows chattering compensation. The aim of the controller is to inject a controlled active power from renewable energy sources into the grid while controlling the power factor and minimizing supply current harmonics. Moreover, the dc-link voltages across the split capacitors are controlled with a simple proportional-integral (PI) regulator. Experimental results show the advantages of the proposed control algorithm in terms of fast dynamic response, low voltage ripple on the dc bus, low current Total Harmonic Distortion, and robustness toward external perturbations from the dc and ac sides; moreover, a comparison with a PWM-PI current controller is presented.