Research On The Control Strategy Of Voltage Unbalance And Harmonic Voltage Compensation In AC Micro-grid

With the increasing penetration of distributed generators(DGs) in the power system, the negative effects on reliability and power quality of power system brought by DGs are becoming more and more serious, micro-grid(MG) as an effective way to solve this problem has been the focus of attention of scholars and research institutions at home and abroad in recent years. When MG capacity is increasing, the types of load are becoming more complicated, especially the widespread use of single-phase high power load and nonlinear load, and intermittent distributed power generation characteristics, which can result in DGs’ output voltage and MG’s voltage at point of common coupling(PCC)dropping in quality. Therefore, in order to solve out three-phase voltage unbalance and harmonic problems, in the two-phase stationary alpha beta coordinate frame, this dissertation proposes a new voltage unbalance and selective harmonic compensation control strategy based on hierarchical control method to improve the MG’s voltage quality at PCC.The system controller designed by this dissestation includes DG local controllers and MG central secondary controller. The MG central secondary controller is designed to mainly manage the total compensation reference signals. When the voltage unbalance and harmonic distortion occur, this dissertation adopt a novel selective harmonic voltage extraction method based on cascade delayed signal cancellation(CDSC), which can precisely extract the positive and negative components of fundamental and main order harmonic voltage, and then the individual harmonic distortion and voltage unbalance factor can be calculated, which can help to get compensation signals and finally integrate them into the total compensation signals which can be sent to the DG local controller by commuication lines.The DG local controllers mainly consist of power droop controller, virtual impedance loop, voltage and current controller and compensation controller. Power droop controller adopts improved droop control strategy to provide voltage and frequency supportion for islanded MG. In order to effectively inhibit the MG circulated current, this dissetation designs an improved virtual impedance loop, improving the accuracy of the fundamental current and harmonic current sharing and also improving MG’s reactive power sharing precision and power decoupling accuracy. The compensation effort controller according to the each DG’s rated capacity and output current harmonic distortion, calculates each DG’s voltage compensation reference signal to correct the voltage reference value generated by power droop controller, and then voltage and current controllers generate a voltage command value, the space vector pulse width modulation(SVPWM) technology is adopted to control interface inverter to achievevoltage unbalance and harmonic compensation.Finally, this dissertation builds compensation system simulated model in Matlab/Simulink to verify the proposed DG local control strategy, harmonic detection method and MG voltage unbalance and harmonic hierarchical compensation control strategy, the simulation results show that the proposed control strategy is correct and effective.