Research On Low Voltage Ride-Through Technology Of Grid-Connected Photovoltaic Inverter

Economic development and social progress have caused a large amount of traditional energy to be consumed,which followed by environmental pollution.At present,countries around the world have begun to establish a microgrid system composed of renewable energy such as wind and photovoltaic to solve the problem of resource depletion and environmental pollution.However,the access of large-capacity new energy(photovoltaic,wind power)grid-connected power generation systems poses a huge challenge to the stability of the grid.During the grid fault state,the instantaneous power cut caused by the off-grid operation of the grid-connected system will have a huge impact on the grid.Therefore,the new energy grid-connected power generation system must have low voltage ride through capability.That is,the grid operation must be maintained during the grid fault.Firstly,this thesis introduces the development status of photovoltaic around the word and the technology about low voltage ride through.The two-stage photovoltaic grid-connected power generation system is selected as the main model.In addition,the working principle and mathematical models of photovoltaic cells and energy storage systems are established.Their output characteristic curve is obtained by building a simulation model with Matlab.Secondly,The problem of power balance between photovoltaic side and grid-connected side during grid fault is analyzed.For the stability problem of DC bus voltage during fault,the MPPT control strategy based on adaptive duty cycle disturbance and the trajectory tracking control method of PV system are proposed,Which can achieve fast and accurate tracking of the maximum power point of the photovoltaic system and stability of the DC bus voltage.For the control problem of grid-connected inverters,power decoupling is realized based on grid voltage oriented vector control strategy.According to the difference of the grid voltage drop depth,the inverter is controlled to generate reactive power of a corresponding magnitude to support the recovery of the grid voltage.A grid signal synchronization method based on cascaded filter phase-locked loop is proposed,which can eliminate the harmonic pollution of the power grid and the influence of DC offset.The posposed method ensure the accuracy of the current command under the fault condition and the synchronization of the grid signal,which meet the stable control requirements of the photovoltaic grid-connected power generation system.In the case of unbalanced fault,the suppression of negative sequence current is selected to be the main control target.Aimed at the target of achieving fast synchronization of grid-connected current commands,a grid’s positive sequence signal synchronization method based on improved structure of SOGI-PLL is proposed.In addition,considering the influence of grid frequency offset caused by unbalanced faults,a Quasi-Proportional resonance control strategy based on full feedforward of grid voltage is proposed.Finally,the Matlab/Simulink software is used to establish the control strategy model and verify the accurateness of the proposed theory.