Research On Low Voltage Ride Through Control Of Photovoltaic Grid Connected System

In recent years,with the large consumption of fossil fuels,environmental pollution problems become increasingly serious,in order to solve the problem of resource depletion and environmental pollution requires a lot of development and utilization of renewable energy.In many types of new energy in the photovoltaic power generation with low cost,high efficiency,no pollution,etc.,is one of the most important new energy.However,there are still many problems in the research on the development and utilization of solar energy,which restrict the large-scale application of photovoltaic grid-connected power generation.Low voltage crossing technology not only requires the normal operation of the system to ensure the quality of photovoltaic power output system,while the system appears a certain degree of failure to maintain the PV module continues to output to the grid,while making full use of photovoltaic inverter fast reactive power output capacity,In the fault during the system to provide a certain amount of reactive power to help the grid voltage back to normal.In this paper,the fault control strategy of PV system is studied.At present,the PV system will have some error in the process of fault traversing.In this paper,The first-order complex filter can overcome the second-order resonant link pair,and the first-order complex filter can overcome the second-order resonant link Positive and negative sequence components have no choice of shortcomings.In order to make the controller in the PV grid system software and hardware inherent error range to play a better effect,this paper introduces the cut-off frequency to increase the ROR controller bandwidth.And the Bode is used to analyze the cutoff frequency effect of the studied controller and the optimal performance of the system.The simulation model of PV grid-connected system is established to verify that the ROR regulator can improve the speed of positive and negative sequence separation of the fundamental system during the fault crossing process.Based on the analysis of the working principle and small signal model of the traditional SRF-PLL(synchronous reference coordinate system),the appropriate phase-locked loop parameters are designed.The problem of unbalanced voltage and in the condition of harmonic distortion,the negative sequence and harmonic components are detected by SRF-PLL in the fundamental normal component which make SRF-PLL failure.In this paper,we propose a three-phase phase locked based on the Unbalanced Harmonic Compensation Mechanism(UHCM)module,which contains the negative sequence and harmonic components in the fundamental normal component of the SRF-PLL.And the influence of low-order harmonics on the phase-locked loop is eliminated by filtering the structure in parallel.Based on the mathematical model of PV grid-connected system,reasonable mathematical analysis is given to the principle of grid-connected fault,especially the asymmetric fault,and the traditional fault crossing current control target is calculated.It is found that the traditional control strategy is complicated by the simulation,and the control target has the limited ability to balance the short circuit current and the short circuit power and the unreasonable voltage control of the DC side busbar.Aiming at the shortcomings of the traditional fault crossing control strategy,the maximum current amplitude control strategy is proposed.Combined with the positive and negative sequence component separation technology,the proposed control method can stabilize the AC system and the DC side during the voltage drop Of the voltage level,in the failure period for the power grid to provide a certain amount of reactive power support to help the system voltage gradually returned to normal.