Research On Low Frequency Oscillation Of Grid-connected Converter Based On Impedance Model

With the large-scale interconnection of new energy sources,the electronization of power system becomes more and more obvious,which brings a series of complex oscillation problems,including low frequency oscillation.The occurrence of low frequency oscillation seriously threatens the stable operation of new energy grid-connected system,and also has a negative impact on the further promotion and application of new energy.Therefore,it is of theoretical and practical significance to study the mechanism of low frequency oscillation and its suppression methods.As the core power electronic equipment in the new energy system,grid-connected converter plays the roles of power conversion,grid-connected synchronization and power quality control.In this thesis,grid-connected converter is taken as the object to study the low frequency oscillation problem as follows:The low frequency oscillation mechanism of single-machine is analyzed,and the key control factor causing low frequency oscillation is obtained.Based on the impedance theory of cascade system,the small signal impedance models of single current and double voltage current loop control modes of the converter in d q coordinate system under weak grid are established respectively.The dominant admittance elements of the converter under two control modes are given,and the influence of current and voltage loop parameters on the low frequency characteristics of dominant admittance elements is analyzed.The main factor causing low frequency oscillation is the current loop control.The correctness of the conclusions is verified by simulation and experiment.The interaction between weak grid and multi-grid converter and between converter and converter under parallel operation of multi-converter are studied.Studing the interaction between weak grid and multi converter,the cascade system of weak grid-equivalent grid-connected converter is analyzed,and the influence of output impedance and parallel number of converters on low frequency oscillation is obtained.Studing the interaction between converters and converters,the cascade system of equivalent weak grid-converter is analyzed,which is admitted by a converter in parallel system.Starting from the characteristics,the influence of the admittance changes of other converters on the low frequency oscillation characteristics of the converter is analyzed.The correctness of the conclusions is verified by simulation and experiment.According to the low frequency oscillation mechanism,a low frequency oscillation suppression method for grid-connected converter is proposed,which combines non-cross-feedback virtual impedance with IIR filter.Based on the small signal impedance model of the converter after introducing virtual impedance,it is concluded that the non-cross-feedback virtual impedance can effectively suppress low frequency oscillation,but at the same time it has adverse effects on the dynamic response characteristics.The combination of non-cross-feedback virtual impedance and IIR filter can effectively suppress low frequency oscillation and improve the system cut-off frequency.The simulation and experiment results show that the proposed method can suppress the low frequency oscillation quickly and effectively without affecting the original dynamic and steady-state performance.At the same time,the reliability of the method to suppress the low frequency oscillation under the operating conditions of the multi-converter is verified by experiment.