Ananlysis Of AC Impedance Model And Oscillation Modes In A VSC-HVDC System

With the growing demand for electric power in the process of urbanization,combined with the enhancement of people's environmental awareness,our country is speeding up the exploitation and utilization of renewable energy.But because the energy resources are dispersed and far away from the center of the load,so the technology of long distance and high power transmission becomes necessary.Subject to traditional high-voltage direct current transmission technology's high reactive power losses and it can not feed power to weak AC grid,the technology of voltage source converter(VSC)based high-voltage direct current transmission(VSC-HVDC)overcomes the disadvantages of the traditional high-voltage dc transmission,it can control voltage and power rapidly and flexiblely and it can improve power quality and the stability of the power system.Because the satisfactory control performance and operational characteristics of the system,the technology of VSC-HVDC has certain prospects in the utilization of renewable energy,the construction of urban DC distribution system and feeding power to passive grid.On the other hand,due to integration of VSC into grid,in some situations,the interactions between the VSC and grid can lead to sustained oscillation of grid voltage and current,even resonance may occurs.Therefore,developing corresponding impedance models for the converter station to analysis is feasible and necessary,which ensures the safe and stable operation of VSC-HVDC system and provides theoretical references for the controller parameter design.It certainly has practical engineering value.Since VSC-HVDC is a nonlinear system,which consists of complex controllers.The inner current control or outer voltage/power control loop are often ignored in most of the literature,that have impacts on the output impedance of VSC.This paper is based on previous studies,the impedance models that are seen from ac system for the rectifier station are derived.The output impedance models(which are expressed in d-q frame are unsymmetric al)take the inner current control loop,the outer power control loop and filter parameters into account.The obtained impedance models are then used to analysis the oscillation mode s of the VSC-HVDC system.Simulation is built in the time-domain PSCAD/EMTDC program,an d sustained oscillations of the gridvoltage and curre nt were observed in time-domain simulation,at the same time,the impacts of different types of filter on the system 's oscillation mode and oscillation frequency are compared.The oscillation frequencies that simulation results obtained corroborate with the theoretical analysis in frequency domain,which verified the effectivene ss of complex torque coefficient method to identify the oscillation frequency of the system.