Research On Power Quality Improvement Method Based On UPQC Hybrid AC/DC Microgrid

Hybrid AC/DC microgrid brings the advantages of AC and DC microgrid together,which is a direction of intelligent microgrid.There are many differences of power quality between microgrid and power grid.First,the output of distributed generation units is mostly intermittent and volatility,so the power quality in the microgrid will be reduced if it cannot be controlled real-time precisely.Second,there are various types of converters in the loads,a lot of harmonics has been brought by these nonlinear loads,which will increase the current harmonic content in the grid and threaten the normal work of other sensitive loads.In view of the basic structure characteristics of hybrid AC/DC microgrid,the topological structure of unified power quality conditioner(UPQC)is built,the working principle and power flow of UPQC are analyzed in depth,and the mathematical models are established for three main parts:the voltage compensation unit,parallel current compensation unit and DC energy storage unit.Whether real-time and accurately of compensation current and voltage signal in UPQC system being generated is the foundation of UPQC system implementation.Firstly,detective model of instruction voltage and instruction current is built based on instantaneous reactive power theory,on this basis,the Park transformation instruction voltage detective method,the p-q method and i_p–i_q methods are analyzed deeply.We also build the simulation platform on MATLAB to analysis the detection methods of various voltage and current on both time and frequency domain.Secondly,in view of the problems in UPQC system,such as the distortion voltage of the power grid will affect detection accuracy of the current signal,and algorithm of current signal distortion is complex and time-consuming,adaptive cancellation method is used to calculate the instruction current.The simulation results show that the adaptive cancellation method is simple and accurate,but it is still sensitive to the distortion signal of grid voltage.Finally,to the problem,a network-based fundamental voltage calculate circuit is added based on the adaptive cancellation method and the simulation results show this improved method has a good adaptive ability to the voltage distortion by filtering the distortion components in the voltage signal.The UPQC system is combined with the series and parallel inverter which be controlled accurately.In order to improve the response speed,the control signal of the series inverter is generated by triangle wave comparison method,and the control signal of the parallel inverter is generates by the hysteresis loop comparison method.Because the working point of inverter is fluctuate widely on variable working condition and serious nonlinear,the traditional PID control of the energy storage side voltage can hardly guarantee the stability and speed of UPQC.In order to solve this problem,the variable universe fuzzy control method is introduced into the control of energy storage side voltage.This method corrects the contraction-expansion factor online according to the size of the deviation,and changes the domain of the fuzzy control.The simulation results show this method can improve the response speed of UPQC,and this method also can adapt the nonlinear of system.Finally,the active power factor correction(APFC)technology is applied to control the voltage of DC bus,which improves power factor of power grid and reduces the current distortion rate.