Research On The Harmonic Characteristics Of MIDC System

With the energy strategy implementation of power transmission from west China to east China and rapid development of HVDC transmission technology,our country has formed two multi infeed AC and DC system,which are the east China power grid and China southern power grid.Compared with the AC system and single infeed AC and DC system,harmonic interaction of multi infeed DC(MIDC)transmission system will be more complex.In this paper,harmonic characteristics of MIDC system AC network and the possible interaction of multi harmonic sources are studied.Based on the Shanghai MIDC system,the harmonic characteristics has been discussed.Based on switch function,this paper considering the effects of two kinds of dominant harmonic currents modulation generated by the DC side harmonic current presents a new AC equivalent harmonic impedance of converter.Through the analysis of the influence of converter harmonics on the AC equivalent harmonic impedance,the time domain simulation calculation method of AC equivalent harmonic impedance of converter is presented.It has a high accuracy in the low frequency range by comparing calculation results of the mathematical model of the AC equivalent harmonic impedance of converter with time domain simulation results based on Yi Hua HVDC project.It can be used to replace the HVDC system in the harmonic analysis of the MIDC system.Based on the mechanism analysis of the harmonic interaction effect in the MIDC system,the concept of transfer Transfer harmonic Mutual impedance and two calculation methods.The strength of each harmonic coupling of the 4 HVDC system in Shanghai MIDC system is analyzed and it can be verified that the size of Transfer harmonic mutual impedance can characterize the strength of each harmonic interaction.Based on the PSASP simulation software,the model of the Shanghai power grid is built and the accuracy of the model is verified by comparing the simulation results with the measured results.By harmonic power flow calculation and the impedance frequency scanning method,the harmonic characteristics of 500 k V substation bus in Shanghai MIDC system are analyzed,the corresponding measures to improve the harmonic status of Shanghai power grid is proposed.