Research On Methods For Stability Enhancement And Unstable Inverter Identification In Multi-parallel Grid-connected Inverter Systems

With the rapid development of renewable energy,grid-connected inverters,as the core device for connecting distributed renewable energy to the grid,are receiving more and more attention.In addition,in order to increase the capacity of distributed power stations,multiple inverters are often connected to the grid in parallel.However,with the increase in the number of parallel inverters and the large-scale change of grid impedance,the stability of the multi-parallel inverter system has been greatly challenged.At the same time,when multiple inverters are connected in parallel,the interactive coupling between the multiple parallel inverters and the grid will also generate new resonances,which may cause system instability in severe cases.Therefore,it is very necessary to analyze the causes of instability of multi-paralleled inverters system and how to improve stability of the system.Active damper is often used to improve the stability of multi-paralleled inverters system.It simulates a resistor through current control and is connected to the point of common coupling(PCC)of the system to correct the output impedance of the multi-paralleled inverters system.Thus,it enhances the stability of the system.However,the traditional active damper needs to obtain the resonant frequency at the PCC in real time,which requires the system to perform a lot of calculations.In addition,active dampers with fixed damping coefficients are difficult to adapt to complex and variable power grid conditions.In practical industrial applications,due to the coupling of grid impedance,the instability in a multi-paralleled inverters system finally results in that each of the gridinjected currents of the inverters and the voltage at the PCC are oscillating.From the waveforms of output voltage and current,it is difficult to identify which inverter(or inverters)causes the instability in the overall interconnected system.Therefore,it is very important to quickly and accurately find the inverter that causes the instability of the entire system and remove it from the system.It is necessary to analyze the influence of each inverter in the multi-paralleled inverters system on the stability of the system.In response to the above problems,this article first established the mathematical model of the multi-paralleled grid-connected inverters system,and analyzed its stability and the influencing factors that affect the stability.Then,to improve the stability of the system,the traditional active damper was improved.This includes:(1)Using a high-pass filter to replace the resonance detection module and the corresponding band-pass filter that need to extract the system’s resonance frequency in real time.(2)The adaptive module is used to automatically adjust the damping coefficient of the active damper according to the content of voltage harmonics at PCC.Secondly,how to identify the inverter problem that causes system instability from the system when the system is internally stable.This paper analyzes and compares the Root Mean Square(RMS)value of harmonic voltage at the point of common coupling(PCC)and the harmonic voltages across the filter capacitors and designs auto identification of unstable inverter for internal instability in multi-paralleled inverters system.Auto identification method proposed in this paper can be used not only when the system contains an unstable inverter,but also when there are multiple unstable inverters in the system.In order to verify that the proposed identification method is correct and practical,the corresponding simulation models are built under the MATLAB/Simulink modeling environment,and an experimental platform for three paralleled inverters system is built based on the experimental platform of d SPACE1202.Both simulation and experimental data and results verify the effectiveness of the proposed method.