Research On Impedance Characteristics And Stability Of Grid-tied Inverters Considering Frequency Coupling Characteristics

At present,wind power and photovoltaic power generation continuous increase in the installing capacity proportion of new energy.The Grid-tied inverter plays a key role in the new energy Grid-tied system,however,under the weak grid conditions,the oscillation phenomenon generated by the interaction between the inverter and the grid causes the Grid-tied inverter system instability,which seriously endangers the normal operation of the regional grid.This thesis aims to study the impedance characteristics and the mechanism of wide-band oscillation of the in the Grid-tied inverter,for this reason,the influence of system parameters and frequency coupling on the stability of the Grid-tied inverter were analyzed,furthermore,the degree of influence of system parameter changes on the oscillation occurring in the Grid-tied inverter were studied,and a parameter adjustment scheme which suppress the oscillation of the interconnected system was proposed.Firstly,in this thesis,the frequency coupling phenomenon caused by the control links of the phase locked loop,current loop and voltage loop were taken into account,and the analytical equation of the Grid-tied inverter’s sequence impedance model was established using the harmonic linearization method.Furthermore,a model of inverter Grid-tied system considering frequency coupling was sted and the accuracy of the proposed model through simulation was verified.From the established inverter impedance model,it can be seen that the output impedance is not only related to its topology and control strategy,but also to the system control parameters.Secondly,based on the established frequency-coupled impedance model,the relationship of the control parameters and electrical parameters of the Grid-tied inverter on the impedance characteristics of the Grid-tied inverter were studied in comparison with the case of ignoring and considering the frequency-coupled characteristics,and the main factors affecting the frequency coupling characteristics were derived.On this basis,the influence of the Grid-tied inverter parameters on the system stability is also studied from the perspective of the sequential impedance considering the frequency coupling characteristics.Finally,this thesis combines the sensitivity analysis method in the time domain with the impedance analysis method in the frequency domain to determine the degree of influence of parameter changes on the oscillation characteristics of the interconnected system.By calculating the sensitivity of the Grid-tied inverter impedance to the system electrical parameters and the control parameters of each controlling unit.Based on the calculation results of the impedance sensitivity of the system parameters,a control parameter regulation scheme for suppressing the oscillation of the Grid-tied inverter,which is dominated by the regulation of the control parameters of the Grid-tied inverter,was proposed.The dominant parameters were analysed in conjunction with the impedance stability criterion,which provides a practical and reliable theoretical basis for the regulation of the grid-connected system parameters of the inverter during oscillation.