Research On Grid-Connection Stability Of Parallel Inverters Under Different Grid Conditions

In order to solve the problem of energy shortage in the world,photovoltaic power generation and wind power generation are widely used in the new energy grid system.However,because of various reasons,the voltage of the grid connection point may be lower than the rated voltage,which will directly affect the power quality.Therefore,in order to improve the the grid connection point's voltage,the reactive compensation devices are generally incorporated.Nonetheless,in this case,the external impedance characteristics of the grid side will change,which will bring about the problem of gridconnected stability.The traditional "over-compensation" method,which has enough margin for compensating the capacitance value,avoids this problem,but it will lead to a large system operation loss,the large equipment volume and the low interest rate.In addition,in traditional researches,the equivalent capacitance value of the compensation device or the capacitance value of the capacitor are considered to be constant.Yet in fact,with the system running for a long time,the capacitor has attenuation characteristics,so its capacitance value will continuously decrease,which will also threaten the stability of the system operation.By considering the role of reactive compensation capacitor,this dissertation will establish the grid-connected stability model of the inverter,and fully consider the capacitor aging attenuation characteristics,and propose the corresponding reactive compensation capacitor value selection strategy.In this dissertation,the stability principle of traditional grid-connection will be first expounded,and the existing problems and the research method of this dissertation will be pointed out.According to the current research conclusions of grid-connected stability,the stability results are calculated by using appropriate parameters,which will be compared with the following studies.The dissertation will analyze the grid-connection stability of the actual system by the dominant pole method.By modeling the grid-connected system of a single inverter,the grid-connected stability law considering the effect of compensation capacitance will be obtained,and the compensation capacitance selection strategy under constant value capacitance will be proposed as well.The single-inverter grid-connected operation will be modeled under the grid current feedback(GCF)and converter current feedback(CCF)control respectively.After obtaining the corresponding grid-connected stability,assuming the compensation capacitance value is unchanged,in order to meet the requirement of increasing the voltage of grid-connected point,the compensation capacitance value selection strategy that meets the grid-connected stability will be proposed.By modeling the multiple-invertered grid connection under the GCF control,the gridconnected stability law considering the effect of compensation capacitance will be obtained,and the selection strategy of compensation capacitance considering the effect of aging and attenuation of capacitors is about to be proposed.Considering aging of capacitor leads to the value of capacitance attenuation,the system may run from the stable region to the unstable region,therefore when selecting compensation capacitance needs to both guarantee that voltage of grid-connected point can meet the specified requirements,and guarantee that the capacitance value will not enter the unstable region due to the attenuation characteristic of the capacitor by giving a certain margin to the compensation capacitance value.Based on the grid-connected stability law of the multi-inverter grid-connected system controlled by CCF,the compensation capacitance selection strategy considering the attenuation effect of aging of capacitor will be proposed,and the corresponding improvement method will be proposed because of the fact that the limited range of compensation capacity.Under the CCF control,the stability region of capacitance value has an upper limit,which will result in an insufficient compensation voltage capacity.It is necessary to optimize the operation parameters to improve the upper limit value of the stable region,so as to improve its compensation capability.