Research On Low Voltage Ride Through Strategy Of Photovoltaic DC/AC Converter Based On Constant Power Tracking Control

With the continuous increase of photovoltaic installed capacity,if the photovoltaic system is cut off when the grid voltage fails,it will not only lead to the oscillation of the grid,but also cause the imbalance of frequency and voltage,affect the safe operation of the whole grid system,and have serious consequences.Therefore,for the sake of stable operation and power quality of power grid,China has put forward a series of grid connected requirements specifications.Among these specifications,low voltage ride through(LVRT)capability is one of the most important indexes to measure grid connected system.In this thesis,the key problems of LVRT in photovoltaic system are studied as follows.Firstly,the LVRT standards of various countries are discussed,and the LVRT standards of Germany and the United States,which are relatively advanced in the photovoltaic industry,are mainly studied,and the LVRT standards of China are quantified.After that,two methods of implementing LVRT are summarized,that is,increasing hardware facilities and improving strategy methods.The causes of failure in phaselocking and drop depth detection under voltage imbalance are studied,and the separation methods of positive and negative sequence are proposed.The delay method,double second-order generalized integral method and notch filter method are analyzed and discussed,and the simulation model is built to verify the effectiveness of these three methods,and the advantages and disadvantages of each method are compared and analyzed.The simulation results show that the notch filter method can give consideration to both the speed and accuracy of positive and negative sequence separation.Secondly,the mathematical models of the front-end photovoltaic array,boost circuit and the back-end DC / AC converter are established.The algorithm technology of maximum power point tracking(MPPT)is analyzed,and the advantages and disadvantages of several common methods are discussed.When the grid voltage is balanced,the DC/AC converter is controlled by the double closed-loop strategy,so that the voltage and current are in the same phase to meet the grid-connection conditions.On this basis,the main differences between symmetrical voltage sags and asymmetrical voltage sags and their effects on the whole system are discussed,and the mathematical models of DC / AC converters under these two sags are established.The positive and negative sequence separation method is used to complete the phase locking of the grid voltage,and the implementation method of constant power tracking control strategy is proposed,that is,the dichotomy method is used to gradually approach,so that the photovoltaic array can emit the specified power.Finally,under the condition of power network voltage sag,the causes of photovoltaic system disassembly are analyzed,that is,DC bus overvoltage and AC side overcurrent.The front boost circuit is controlled separately,and the constant power tracking control strategy is adopted to suppress the DC bus voltage rise.The reactive power compensation strategy is used to control the post-stage DC/AC converter,and the reactive power of the AC / DC converter itself is used to transfer reactive power to the grid.In order to suppress the influence of negative sequence components under asymmetric sags,Positive and negative sequence voltage and current components are separated by a positive and negative sequence separation process,and they are controlled independently,so as to suppress the grid-connected current distortion and the double frequency fluctuation of DC side voltage.The simulation model of LVRT control strategy is built to simulate the system under symmetrical and asymmetrical drop conditions,and the control effect is compared with the traditional double closed-loop strategy.The simulation results show that the proposed LVRT strategy can solve the problems of over-voltage on DC side and over-current on AC side during voltage sag,and verify the correctness of the proposed strategy.