Key Techniques Of Grid-connected Inversion Under Unbalanced Grid Conditions

With the development of grid-connected technology of renewable energy such as solar energy and wind energy, PWM inverter is widely used considering its various advantages such as grid-side current sinusoidal, unit power factor, bidirectional energy flow and green transformation of electrical energy. However, under unbalanced grid conditions, the following problems existed in inverter should be solved: the accuracy of the tranditional phase locked method is low, and the dynamic response speed is slow; under unbalanced grid conditions, there are large numbers of harmonic currents in inverter operating with balanced control strategy, which cannot meet the demand of the allowable value of harmonic currents into the utility grid specified in GB/T 14549-1993 Quality of Electric Energy Supply-Harmonics in Public Supply Network. In order to avoid arm short-through, dead-time must be added between the contol signals of power switchings, which causes dead-time effect and the increasing of harmonic contents of grid-connected currents. This paper takes the three-phase voltage source grid-connected inverter as the research object, makes further study and offers better solutions of above related problems.The rapid and exact acquisition of grid signal is the key of grid-connected control. Under unbalanced and harmonic polluted grid conditions, tranditional phase locked method based on deleyed signal cancellation(DSC) technique has shortcomings of low accuracy and slow dynamic response speed, so a fast positive and negative sequence decomposition(FPNSD)-based phase locked strategy is proposed, and its operational principle is analyzed. The simulation and experimental results show that the proposed phase locked method can eliminate the impact of the grid harmonics, and improve the phase locked speed and accuracy.Both tranditional DSC-based and FPNSD-based phase locked methods demand that the grid frequency is time-unvaried, so these two methods are invalid when the grid frequency changes. Based on the theory analysis of adaptive notch filter(ANF), a novel adaptive phase locked method based on FPNSD-ANF is proposed. It uses the frequency adaptive unit of ANF update the frequency of FPNSD, which realizes the tracking of the grid frequency under unbalanced grid conditions. The simulation and experimental results show that the proposed algorithm with high dynamic performance can track the grid frequency, and the proposed method does not need voltage-controlled oscillator, which makes its structure much simpler.Power compensation control strategy under unbalanced grid conditions needs the positive current component to compute the compensating power, so the accuracy of the extracted positive current component impacts the accuracy of the power compensation. The widely used harmonic current extraction method based on instantaneous reactive power theory, is unable to realize a selective order harmonic extraction, and its dynamic response speed is slow. Based on the theory analysis of the harmonic current generation of inverter under unbalanced grid conditions in detain, a parallel ANF-based harmonic current extraction method is proposed. The main function of this harmonic current extraction method is to provide accurate reference signal for power compensation of inverter under unbalanced grid conditions and elimination of certain harmonics applications. Finally, the validity of theoretical analysis is proved by experimental results.To improve the grid-connnected inverter performance under unbalanced grid conditions, power ripple and control effect of power compensation are analyzed using the theory of instantaneous power. To reduce double frequency power ripple pulsating into grid, a modified power compensation control stratrgy is proposed based on direct power control, the negative voltage component is prohibited in the inner power loop. The simulation and experimental results show that the modified power compensation control strategy realizes three targets, namely obtaining the grid-connected current without negative component, removing reactive double frequency power ripple and cancelling the active double frequency power ripple. At the same time, the new control strategy improves the precision of compensation power, reduces the grid-connected current THD, and greatly improves the performance of inverter.Dead-time added between the control signal of power switching causes errors in the control voltage, which leads to the grid-connected current distortion. To weaken the impact of dead-time, based on the analysis of dead-time effect of inverter, the dead-time is adjusted on line, and dead-time compensation of inverter based on an exponent approaching law with fast terminal sliding mode controller is proposed. Then fast terminal sliding mode theory is introduced and sliding mode controller is designed; in the controller, the differential operation of the sliding mode controller is replaced by Levant differentiator, which enhances the sliding mode controller’s ability of noise resistance. The novel dead-time compensation method without current polarity detecting and current PARK transfomation simplifies the system structure, and improves the robustness of the system. Finally, the correctness of theoretical analysis is verified by experimental results.