Research On Grid Current Waveform Control Method For Grid-connected Inverters In Complex Grid Environments

With the increasing penetration rate of grid-connected converters at power generation and consumption ends in the power grid,the impedance of the power grid presents complex and changeable characteristics,resulting in the phenomenon and risk of broadband oscillation of grid-connected converters as the interface of new energy power generation systems and power equipment,which seriously threatens the safe and stable operation of the power grid.At the same time,due to the influence of nonlinear load in the power grid,the voltage of the public connection point of the power grid has serious background harmonics,which will affect the grid-connected output power quality of the power generation system.Converter-side current control has the advantage of overcurrent protection for converter-side devices,and is widely used in practical systems,however,converter-side current control has the defect of harmonic suppression ability of grid-connected current.Therefore,this paper aims to study how to design a stable converter-side current control strategy and improve the power quality of the grid-connected current in a complex power grid environment,and the specific work completed is summarized as follows:Firstly,the working principle and mathematical model of the converter side current control LCL grid-connected converter are analyzed,and then the equivalent modulation link and system delay are analyzed and modeled to obtain the equivalent circuit of the system.According to the equivalent circuit,the impedance model of the system is established,and two stability criteria based on the impedance model are given,which are the stability criterion based on frequency passivity and the stability criterion based on impedance ratio,which provides a basis for the design and optimization of the control structure of the grid-connected converter controlled by the current control on the converter side.Then,the current loop is designed based on passivity to achieve high-precision control.For the stability of the current loop,the system was impedance reshaped using active damping.To achieve high-precision control,a repeat controller is added to the current loop,but its infinite gain reduces the passivity of the system output admittance.In order to compensate for this shortcoming,the repetitive control parameters are re-optimized.Then,the harmonic suppression ability of the design method is demonstrated by harmonic suppression theoretical analysis,and the correctness of the analysis is proved by experiments.Finally,because the current control on the converter side itself has the defect of harmonic suppression ability of the current entering the grid,this paper proposes an improved capacitor current compensation scheme to optimize the control,so that the overall control has harmonic suppression ability.In order to reduce the economic cost,an improvement in the use of digital differentiators to obtain capacitor current is proposed.However,after analysis,the digital differentiator causes a phase lag that has to be accepted,causing the system to admittrate and a non-passive interval around the Nyquist frequency,and this paper tries two methods to narrow the non-passive interval.Finally,through experiments,the correctness of the analysis and the effectiveness of the overall method are verified.