The Research On LCL Three Level Grid Connected Inverter Control Strategies Under Non-ideal Power Grid

Due to the shortage of traditional energy and the aggravation of pollution,the demand for clean renewable energy and environmental protection is increasing.Distributed generation has become a research hotspot in recent years.The power electronic converter is indispensable for distributed generation.The inverter plays the role of power quality conversion.The stability of inverter is related to whether the device can access the power grid.Due to the nonlinear load and other reasons,the grid connected by the distributed inverter is often time-varying weak network.There are a large number of low order harmonics.Meanwhile,the three-phase unbalanced phenomenon is caused by short-circuit fault,which affects the safety and stability of the distributed generation system.Therefore,it is necessary to study the current control link when distributed inverters are connected to non-ideal power grid,which is an essential step to ensure efficient and stable operation of grid connected inverters.The research status of distributed generation in the paper firstly with LCL type three level grid connected inverter as the research object of topology,establishes a mathematical model of ideal and non ideal power grid abc three-phase static coordinate system,two-phase static coordinate system and dq synchronous rotating coordinate system,and deduces no decoupling control model of decoupling control dq synchronous rotating coordinate axis and two-phase static coordinate axis under.At the same time,the basic principle of three level SVPWM modulation is adopted and deduced,and the utilization ratio of the DC side is improved.In order to finish the grid voltage positive and negative sequence in non-ideal grid and get accurate phase information,research and analysis of several typical phase locking method,analysis of dynamic performance and steady state characteristics of them,the results of the simulation analysis when the unbalanced SSRF-PLL can not accurately output lock phase angle,and DSOGI-PLL’s dynamic response time.When the power grid contains harmonics,the DDSRF-PLL fluctuates at the fundamental frequency.Finally,a phase-locked loop method based on sine amplitude integrator is selected for non-ideal power grid.Considering the voltage asymmetry or the high harmonic content under the condition of weak electric network,the current distortion and imbalance of grid connection will be caused.The traditional double closed loop current control strategy has been unable to suppress the voltage disturbance.This paper is obtained after the analysis of grid connected inverter grid voltage feedforward function,to meet the stable operation of the three-phase inverter non-ideal grid,this paper presents a specific component of the grid voltage feedforward control strategy through sinusoidal amplitude integrators extraction voltage specific components and offset the impact of adding SVPWM modulation waves with grid voltages generated.Based on the dual loop current control strategy of alpha beta stationary coordinate system,the PR current controller with voltage feed-forward component specific strategy,realize the non ideal grid under unbalanced load component and harmonic components,thereby reducing the steady-state error of the grid current.In order to verify the correctness of the control strategy,we simulate PSIM under different conditions and analyze the THD content of grid current to satisfy the requirements of network access.Finally,the experimental platform is built,and the phase-locked loop based on sine amplitude integrator is adopted in the experiment.The experimental results are consistent with the simulation results,which proves the theoretical feasibility of the strategy.