Decoupling Method For Sequence Admittance Model Of Grid-Connected Converter

As the proportion of new energy in power grid becomes higher and higher,sub/super synchronous oscillation caused by power electronic equipment has attracted more and more attention.However,the converter modeled in dq axis is a multi-input multi-output(MIMO)system,which requires the generalized Nyquist method to analyze the system stability,which increases the difficulty of stability analysis.After ignoring the frequency coupling in the converter,the sequence impedance method can treat the converter as two independent systems of positive sequence and negative sequence and analyze its stability separately,thus simplifying the stability analysis of the converter system.However,when the frequency coupling of the converter is large,ignoring the frequency coupling will make the sequence impedance method have a large error in judging the stability.In order to analyze the stability and oscillation mechanism.of the converter accurately and simply,it has obvious engineering significance to transform the converter into a single input single output system.This paper focuses on the frequency coupling problem and decoupling method of converters.The main work is as follows:For frequency coupling in converter,In this paper,a small signal model of the converter’s sequence admittance considering phase-locked loop,inner current loop and outer DC voltage loop is established in DQ,and the sequence admittance matrix of the converter is obtained.Using the sequence admittance matrix of the converter,the cause of frequency coupling of the converter is analyzed.Secondly,the constraint conditions for decoupling in the admittance matrix are obtained,and according to the constraint conditions,the transfer function for realizing the full frequency decoupling of the converter is calculated.On the basis of the full frequency decoupling,the middle and low frequency decoupling method is proposed,and the sequence admittance matrix of the converter after the full frequency and middle and low frequency decoupling is obtained.The principle of decoupling is analyzed from the sequence admittance matrix.Finally,the simulation model is built in MATLAB software,and the simulation and experiment are carried out on the simulation and physical experiment platformSimulation and experiments verify that there is a large frequency coupling in the coupled converter,and the frequency coupling in the converter is almost eliminated after the proposed decoupling method is adopted,indicating that the decoupling method can eliminate the frequency coupling in the converter well.And compared before and after decoupling,converter sequence impedance method is used to analyze the difference of the system stability,the results show that the coupling term can lead to sequence impedance in the admittance of the program there was a large error to judge system stability,the decoupling control to eliminate the frequency converter after coupling,sequence impedance method can accurately determine system stability,Meanwhile,the proposed decoupling method has little effect on the dynamic performance of the converter.