Research And Implementation Of Low Voltage Ride-through Technology For Photovoltaic Grid-connected Inverter

Solar photovoltaic power generation has the advantages of cleanness, regeneration and sustainable development, and the permeability of the power system is increasing. Photovoltaic grid-connected inverter as an important carrier for photovoltaic energy connected to the grid, the mutual influence between its operation state and the grid also is increasing. The coordination between the two is achieved by low voltage ridethrough technology, and it has an important significance for the large scale photovoltaic energy connected to the grid. The key to realize low voltage ride-through is to solve the problem of detecting the grid dropping fault fast and accurately, phase locked, and the control of grid current.The failure of the three-phase phase locked loop(PLL) based on the synchronous reference frame under the unbalanced fault of the grid is analyzed, a three-phase PLL technology based on a complex-coefficient filter(CCF) with positive sequence and negative sequence separation is proposed in this thesis. Based on depth analysis about the working principle of this phase locked technique, a frequency domain small signal model of the phase locked system is established. The best parameters of the system adjustment are achieved by using the frequency domain analysis method to analyze the performance of steady state and using the time domain analysis method to analyze the performance of dynamic state. Through analyzing the influence of the sampling signal mixed with DC bias on the phase locked technique on obtaining grid synchronization information, a method is proposed by adding a first-order low-pass filter to eliminate the influence of DC bias on the phase locked system, so as to obtain accurate grid synchronization information. By analyzing the fault types of the grid, the judgment of computing drop depth of the grid based on the positive sequence voltage and negative sequence voltage of the grid is proposed. Analyzing the mathematical model and power flow of photovoltaic grid-connected inverter based on unbalanced grid, the control method of low voltage ride-through which balances on the positive sequence current under double d/q rotating coordinate system is proposed. To prevent the inverteroccurring over current caused by drop depth of the grid, improvement scheme considering the grid drop depth as current compensation coefficient is proposed. To reduce the unbalance of the fault grid, strengthen the ability of supporting grid to the photovoltaic grid-connected inverter, a scheme of low voltage ride-through control that peak current is limited and reactive power is compensated is proposed. The correctness of the theoretical analysis is verified based on Matlab/Simulink simulation platform.An experimental platform of the 500 kW photovoltaic grid-connected inverter is established. The soft start of the grid inverter and the technical index of the unit power factor under the normal operation are verified. Furthermore, it is verified which exists blind area and error by detecting the grid drop based on positive sequence voltage. The conclusion that detecting the full range drop depth of the grid based on the positive and negative sequence voltage is achieved. Under the asymmetrical drop fault of the grid, the feasibility of the low voltage ride-through which balanced on the positive sequence current under double d/q rotating coordinate system is verified.