Research On Stability And Control Optimization Of Digitally Controlled Grid-connected Inverter In Weak Grid

Distributed generation units are generally relatively remote,long transmission lines,transformer leakage inductance and a large number of non-linear loads cause grid connected inverters to operate on weak grid which affecting the stable operation of the power generation unit.Improving the adaptability of grid connected inverter to weak grid has become an urgent issue.In this paper,the effects of digital control delays,grid impedance fluctuations and phase locked loop on grid connected inverter in weak grid are studied,and corresponding improved control methods are proposed.First,the study found that the inherent digital control delay in the three phase LCL grid connected inverter system reduces the effective range of capacitive current feedback active damping and limits the selection range of active damping coefficient and reduce the phase at the open loop cutoff frequency.In order to reduce the impact of digital delay,the nature of the first order lead used for delay compensation is analyzed in detail,and its infinite gain will bring the hidden danger of noise pollution to the inverter system.The method of cascading a digital filter with first order lead link to suppress the infinite gain,so as to construct a delay compensation link that is more suitable for digital control inverter system.Secondly,it is found that as the grid impedance increase,the resonance frequency of the filter quickly approaches or even fall below the open loop cut off frequency is the main reason for the insufficient or even unstable steady state margin of the inverter system.A control parameter design method that can adapt to the grid impedance fluctuation is proposed.This parameter design method gives clear conditional restrictions on the selection of controller parameters and active damping coefficient,which ensure the stability of the inverter system in weak grid and always has a sufficient steady state margin.Then,a small signal model of the phase locked loop of the synchronous reference coordinate system is established,and it is concluded that the all pass nature of the background harmonics of the grid under weak grid is the main reason that it will bring instability to the inverter system.In this paper,a second order generalized integrator(SOGI)cascading with positive complex coefficient filter is used to construct a new filtering structure.The shortcomings of the performance of the dual second order generalized integrator phase locked loop(DSOGI PLL)are pointed out.The performance comparison of dual second order generalized integrator phase locked loop(DSOGI PLL)with phase locked loop with new filtering structure is performed in terms of output accuracy and response speed.Finally,the simulation platform built by MATLAB/Simulink and the 10 kW three phase LCL grid connected inverter experimental platform verify the correctness of the theoretical analysis and the feasibility and effectiveness of the proposed method.