Research On Control Technology Of Grid-connected Photovoltaic Inverter Under Non-ideal Conditions

Photovoltaic grid-connected inverter is the core link of the power conversion between the photovoltaic array and the grid.It's duty is to convert the DC electric inverter into AC power into the power grid,and require the AC electricity to conform to the related standard.With the large-scale integration of photovoltaic power generation system into power grids,it is necessary to study how grid connected inverters achieve safe and efficient grid connection under non ideal power grid conditions.Based on the background of non ideal power grid conditions such as voltage fluctuation,harmonic pollution and frequency offset,this paper mainly discusses the phase-locked synchronization technology of photovoltaic grid connected inverter,the damping control method of filter and the control strategy of grid connected current.Firstly,the grid synchronization technology of grid connected inverter is studied.The working principle of the traditional phase locked method single synchronous reference frame phase-locked loop(SSRF-PLL)and the widely used double second-order generalized integrator phase-locked loop(DSOGI-PLL)are introduced respectively.After theoretical analysis and simulation verification,it is pointed out that under the ideal power grid condition,both of the two can achieve phase locking,while the dynamic response of DSOGI-PLL is slower.In the case of non ideal power grid,SSRF-PLL loses the phase locking function.DSOGI-PLL can still lock the phase successfully,but the response time of the system is still long.In order to make up for the shortage of DSOGI-PLL,this paper studies a kind of first-order generalized integrator frequency-locked loop with a moving average filter.This frequency locked loop can track the voltage quickly and accurately in the power grid with voltage drop,frequency mutation and phase jump.Moreover,it can filter out the influence of harmonics on the phase locked loop.The simulation results verify the validity and feasibility of the phase-locked synchronization technology.Secondly,this paper selects the LCL filter as the grid connected filter.After deriving its mathematical model,the filter characteristics of L and LCL filters are compared.It is pointed out that the filtering effect of the LCL filter is better,but the low impedance at the resonant frequency will cause the problem of the resonant peak.In order to increase the damping of the system at the resonant frequency,the passive damping method and the active damping method are compared and analyzed.Finally,the active damping method using the capacitance current feedback inner loop is selected to solve the resonance problem of the LCL filter.Finally,based on the double closed loop control principle of capacitive current inner loop grid current outer loop,the performance of the outer loop controller using PI and PR controller is compared,and the shortcomings of the PR controller are also pointed out.In order to overcome its shortcomings,a quasi proportional resonant current outer loop controller is proposed,and the proposed control strategy is simulated on the Simulink platform.The simulation results show that the current double closed loop control based on QPR controller has better effect on harmonic suppression,stronger resistance to power grid interference and can guarantee good quality of grid connected current.