Resonance Characteristic Analysis And Mitigation Strategies Of Distribution Network With Multiple VSCs

Voltage Source Converter(VSC) is a typical non-linear power electronic devices, which will produce high order harmonic in normal condition and low order harmonic in in normal condition. VSC is used widely in modern grid, which will make harmonic problems showing new features. The resonance characteristic had changed because of the extensive using of cable and compensative device and LCL-filter, which will increase the risk of resonance. Therefore, it is significant that avoid resonance occurring in modern grid and promote the use of VSC by study on resonance characteristic in distribution network including VSC and its suppression strategy.Firstly, based on the harmonic analysis model of sequence component dynamic phasors(SCDPs), this thesis transformed the harmonic analysis into the linear algebra equations solving and solved the linear algebraic equation by using sparse vector method. With the analysis of path sets and the description of weighted directed factor graph, this approach can discover the reason of the harmonics production of VSC and describe the interactions between ac side and dc side in VSC directly. At the same time, this thesis can testify that the harmonic interactions between the ac and dc side in VSC have some independence.Secondly, this thesis built the high-order harmonic equivalent circuit of the VSC to research the problem of distribution network resonance with multiple VSCs, then built the high-order harmonic equivalent circuit of distribution network with multiple VSCs. Through analysis the frequency characteristics of the system side port impedance and VSC side port impedance, this thesis found the high order resonance characteristics of the distribution network with multiple VSCs. Then, the impacts of the system impedance and the reactive power compensation capacity and the number of VSCs and the form of filter to the high order resonance characteristics was being researched.Thirdly, this thesis built the low-order harmonic equivalent circuit of the VSC to research the problem of distribution network resonance with multiple VSCs, then built the low-order harmonic equivalent circuit of distribution network with multiple VSCs. Through analysis the frequency characteristics of the system side port impedance and VSC side port impedance, this thesis found the low order resonance characteristics of the distribution network with multiple VSCs. The thesis focused on low-frequency resonance problems caused by the background of harmonics and phase unbalance.Finally, from the aspect of changing the system resonance points, this thesis proposed configuration guidelines of the system parameters. From the aspect of reducing harmonic source, this thesis proposed using the wave phase coordination method to inhibit high order resonance and using the compensating negative sequence voltage method to inhibit low order resonance.Under certain assumptions, this thesis found out characteristics of the low order resonance and the high order resonance, which are in the distribution network with multiple VSCs. The impacts of the resonance were found and the strategy of inhibiting resonance were being giving. This results provides a theoretical basis and research methods for researching resonance in the distribution network with multiple VSCs.