Finite Control Set Model Predictive Control for an LCL-Filtered Grid-Tied Inverter with Full Status Estimations under Unbalanced Grid Voltage

Abstract

This paper proposes a novel finite control set model predictive control (FCS-MPC) strategy with merely grid-injected current sensors for an inductance-capacitance-inductance (LCL)-filtered grid-tied inverter, which can obtain a sinusoidal grid-injected current whether three-phase grid voltages are balanced or not. Compared with the conventional FCS-MPC method, four compositions are added in the proposed FCS-MPC algorithm, where the grid voltage observer (GVO) and Luenberger observer are combined together to achieve full status estimations (including grid voltage, capacitor voltage, inverter-side current, and grid-injected current), while the sequence extractor and the reference generator are applied to eliminate the double frequency ripples of the active or reactive power, or the negative sequence component (NSC) of the grid-injected current caused by the unbalanced grid voltage. Simulation model and experimental platform are established to verify the effectiveness of the proposed FCS-MPC strategy, with full status estimations under both balanced and unbalanced grid voltage conditions. However, these control schemes are usually designed under the ideal grid voltage condition, and the negative effects caused by the negative sequence component (NSC) are not considered. Therefore, under the unbalanced grid voltage condition, the feasibilities of current sensorless control schemes need be further verified [17].To alleviate the adverse effects of the unbalanced grid voltage, the positive sequence component (PSC) and the NSC of the grid voltages are required to be separated first, and then applied in the control algorithm [18][19][20][21]. In [18], for the distributed generation inverter, a flexible reference generator based on positive and negative active and reactive powers, was proposed to keep feeding the grid and support the grid voltage under the unbalanced grid voltage condition. In [19], to overcome the distortion of grid-injected currents caused by the unbalanced grid voltages, Zheng et al. proposed an improved virtual synchronous generator control method with the additional positive-sequence current adjuster, allowing the reference currents to track the positive sequence currents and inhibiting the negative-sequence components. In [20], Suul et al. proposed a virtual-flux-based method for the voltage-sensorless grid synchronization under variable grid frequency and unbalanced voltage conditions, integrating the functions of frequency-adaptive bandpass filtering, the virtual flux estimation, and the sequence separation into one operation. In [21], Yang et al. proposed a sliding-mode grid voltage observer for the voltage-sensorless operation under an unbalanced network, separating the PSC and NSC inherently. The variables of methods proposed in [18,19] are all based on the full state measurements, while only one kind of sensor is reduced in [20,21]. Note that it is of great challenge for the LCL-filtered grid-tied inverter to further reduce the number of sensors, especially under the unbalanced grid voltage con...

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