Research On Harmonic Stability And Its Supression Strategy For Multi-Inverters Grid-Connected System

New power generation technologies,mainly represented by photovoltaics and wind power,are effective ways to deal with fossil energy crisis and environmental protection issues.As the penetration rate of new energy increases yearly,and the widespread use of other non-linear power electronic devices in the power grid,power quality problems such as harmonics and distortion are brought about.A highly-controllable and fast-response power quality control device is the ideal solution for improving power quality.With the increasing demand for power supply and power quality government,it is necessary to find effective expansion plans to achieve large-scale applications.Limited by the withstand voltage level and capacity limitation of power devices,the use of multi-inverter modular parallel connection is an effective way to solve the above problems.However,the coupling and interference between a large number of nonlinear power devices and grid impedances.In this paper,the stability analysis and control of multi-inverter parallel gridconnected systems are studied.Firstly,the basic structure of the parallel-connected multi-inverter system is described and the circuit principle is analyzed.It is pointed out that in order to establish a simple impedance model,the inverter must be modeled in the two-phase stationary coordinate αβ system.Based on this,taking the grid-side current feedback as an example,it is pointed out that the same circuit topology and the control strategy of the inverter can all obtain the standard type of the closed-loop transfer function block diagram through the equivalent transformation method.On this basis,the factors that affect the stability of the stand-alone machine are analyzed.Therefore,the control strategy,delay links and other links can be expressed in the form of circuits,and then the admittance model of the stand-alone system can be obtained.The single-machine admittance model is extended to a multi-machine parallel system,and the stability mechanism in the case of multiple machines is described.Secondly,this paper describes and compares several methods of stability analysis,including time domain analysis,modal analysis,and impedance analysis.Then,from the point of view of global admittance frequency domain characteristics,the essential cause of the stability problem of the multi-inverter parallel system is revealed;based on this,an improved Nyqusit stability Criterion for multi-inverter parallel gridconnected system is proposed.The idea is applied to the stability design guidance,and the evaluation and determination of parallel inverter resonance instability were proposed.The influence of each inverter on the instability of the parallel system is quantified.Finally,this paper focuses on stability control methods.In order to solve the stability problem,this paper introduces the analysis method of wavelet packet decomposition and decomposes the instability signal.The wavelet packet decomposition method can only obtain the frequency band where the resonance signal is located,and cannot accurately obtain the resonance frequency.Therefore,a series SOGI resonant frequency extraction method is proposed,and the self-adaptability of the extracted frequency is verified by simulation.In addition,combined with the stability criteria of this paper,the stability control objectives are emphasized,and for this goal,stability indirect control and stability direct control suppression methods are proposed.At the same time,a global active suppression scheme is introduced to achieve stability control.