Study On The Harmonic Analysis Model And Steady Characteristics Of VSC-HVDC

With the development of power electronics and modern power grid, voltage sourceconverter based high voltage DC transmission system (VSC-HVDC) will be appled morewidely. VSC-HVDC makes power system more flexible, but as a typical nonlinear device, itproduces harmonics and has bad effects on the power system at the same time. Therefore, it issignificant that a valid harmonic analysis model of VSC-HVDC to mitigate the impact onpower system. In this paper, the harmonic analysis model two-level VSC-HVDC based onnatural sampling PWM modulation control mode has been studied. The steady-state andtransmission characteristics of harmonics in VSC-HVDC have been shown theoretically. Themain studies is given as follows:Firstly, the harmonic analysis model of VSC-HVDC is established based on dynamicphasor and symmetrical component. The switching function model of VSC-HVDC in timedomain is transformed into the dynamic phasor model. Then the sequence components ofcurrent harmonics on ac side and voltage harmonics on dc side have been selected as statevariables. The VSC-HVDC harmonic analysis model is approached in state space. The modeltakes both the dynamic switching of power electronic devices and the interaction between acand dc side into consideration. After solving the model with uniform solution method, theharmonics of voltage on dc side and currents on ac side are presented.Secondly, VSC-HVDC electromagnetic transient simulation model is established withMATLAB/Simulink. With FFT module of the software, the harmonics of simulation resultscan be analyzed. The comparison of the simulation and calculation results can testify thecorrectness and effectiveness of the harmonic analysis model.Finally, based on harmonic analysis model VSC-HVDC, the harmonic interactionsbetween ac and dc side are analyzed in details. The reason of harmonic generation and theharmonic transmission characteristics on every side of VSC-HVDC have been given asresults.The results of the study give a simple, direct and valid approach to analyze theharmonics of VSC-HVDC, and it is a very useful base to mitigate the harmonic problems caused by VSC-HVDC.