Research On Control Strategy Of Grid-Connected Converters Based On Inductance Nonlinear Characteristics And Delay Compensation

In recent years,voltage source grid-connected converters have been widely used in renewable energy generation and power quality management.In the main circuit,in order to reduce the high harmonics in the output current due to the switching action of the converter,it is necessary to introduce a filter at the output of the converter.At present,ferrite cores have become the mainstream material for filter inductance because of their advantages in highfrequency performance,cost,and volume.However,when using this material,the filter inductance value is greatly affected by the output current,which may lead to system instability when the current amplitude varies over a wide range.In addition,the delay caused by digital control in grid-connected converter control systems can also reduce the stability of the system.Therefore,this paper focuses on the above two issues.In order to investigate the effect of filter inductance on the stability of grid-connected converters,this paper firstly establishes a mathematical model of the system,and then analyzes the stability of the system under different current amplitudes by using Nyquist criterion combined with the characteristics of inductance variation with current.It is shown that the stability of the converter is negatively correlated with the current amplitude,that is,when the current is large,the stability of the system will be reduced,and the output current may even oscillate.Therefore,a grid-connected converter control strategy based on inductance nonlinear characteristic compensation is proposed according to inverse system theory,which avoids the influence of the current amplitude on the loop gain and ensures the stability of the system.To address the second problem mentioned above,this paper first rationalizes the delay link and further analyzes the effect of digital control delay on the stability of the gridconnected converter,and finds that the delay introduced by lagging one-beat loading is prone to induce system instability.To this end,a delay compensation method based on the pole-zero cancellation principle is proposed and its effectiveness is analyzed.The result shows that this method can overcompensate the phase of the system,thus improving the stability of the converter.Based on the theoretical analysis,a system simulation model is developed and an experimental prototype of a 220V/100A single-phase grid-connected converter is constructed,where a single-coil magnetic retention relay driving circuit is designed for the problem that the state of the magnetic retention relay cannot self-reset after a power failure.The simulation and experimental results verify the correctness of the theoretical analysis and the effectiveness of the proposed control strategy.