An nonlinear control strategy for single-phase Quasi-Z-source grid-connected inverter
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The Quasi-Z-Source inverter (qZSI) applied to the photovoltaic (PV) system has nonlinear and unstable zero dynamic characteristics, which leads to the problem of narrow system bandwidth and slow dynamic response. A nonlinear control strategy is proposed based on the input-output feedback linearization method. The nonlinear affine model of qZSI is presented and transformed to a linearized and decoupled form with the help of the input-output feedback linearization technique. Based on the local linearized model, simple linear controllers are employed to regulate the input voltage, which significantly reduces the difficulty in controller design. The method of indirectly control of the converter input voltage with a inner inductor current feedback is used to solve the problem of unstable zero dynamics. The Quasi-Proportional-Resonant controller is used to eliminate the steady state error of grid-connected current. The proposed control method realizes decoupling control for DC side (boost control) and ac side (grid-connected current control of PV system). Simulation results of proposed method and conventional control method are compared. The results show that the proposed control strategy can track the maximum power point voltage reference quickly and accurately. The effect of unstable zero-dynamic on DC-link voltage is reduced, and the transferring of disturbance from DC side to the AC side of the qZSI is mitigated effectively. The system has higher stability and dynamic performance.