Research On Control Technology Of The Grid-connected Inverter With Power Quality Compensation Function

Due to the wide application of distributed generations,grid-connected inverters are commonly adopted as the DC/AC conversion interfaces and become one of the major causes of power quality problems.To satisfy the high requirement of the power supply,the grid-connected inverter not only should inhibit the harmonic current output,but also is required to improve the power quality condition by compensating the harmonic,unbalanced and the reactive components of the local load currents.To meet these requirements,the improved current control and active compensation technologies are proposed in the paper.Moreover,the method to analyze the harmonic stabiligy of the gird-connected inverter in weak grid condition is also presented in the paper,considering the unbalanced local load and the inverter based active power quality compensation.The main contents of the paper are summarized as follow:To achieve better harmonic suppression performance with low algorithm complexity,the paper adopts the H_?repetitive controller to control the ouput current of an LCL-type grid-connnected inverter.Based on the modeling and analysis of the current control algorithm and the active damping mechanism,the paper proposes the general design and stability analysis method for the current control loop.The simulation and experimental results verify the effectiveness of the proposed current control algorithm and stability analysis method.To improve the local power quality condition,the compensation current extraction method is introduced to detect the harmonic,unbalanced and reactive components of the local load currents.Based on the basic principle of the Conservative Power Theory(CPT),the paper proposes an improved compensation current extraction method for the three-phase three-wire system,which can provide accurate extraction result and simplify the algorithm.Moreover,the H_?repetitive controller is adopted to track the harmonic compensation current reference.The experimental results verify that the proposed compensation current extraction method and the repetitive current controller can realize the comprehensive compensation of the local power quality problems.When the gird-connected inverter is used to compensate the power quality problem,the ouput current of the inverter and the disturbance voltage of the PCC are coupled because of the load current and the compensation current extraction method.Considering the interactions between the positive and negative components introduced by the local unbalanced load,the coupling mechanism is more complicated.To analyze the phenomenon,the complex space vector based admittance model for the unbalanced local load is proposed,which can reveal the relationship between the distortation components of the load current and the disturbance voltage.Then,the distortation current extraction methods are modeled to illustrate the coupling between the compensation current reference and the voltage disturbance under different compensation targets.The proposed mechanism is critical to analyze the harmonic stabiligy of the grid-connected system,which includes the gird-connected inverter and the local unbalanced load.Based on the proposed coupling mechanism between the disturbance voltage,the unbalanced load and the compensation current,the paper proposes the admittance model of the equivalent compensated load to analyze the harmonic response of the gird-connected system considering the inverter based power quality compensation.Moreover,the admittance model based stabiligy analysis method is proposed for the grid-connected system considering the postitive sequence and negative sequence coupling introduced by the load imbalance and gird-impedance.Thus,the stability of the grid-connected system is analyzed considering different gird-impedances and compensation current extraction algorithms.The experimental results verify the effectiveness of the modeling and stability analysis methods.