Study on harmonic propagation of VSC-based HVDC systems

In Voltage Sourced Converter (VSC) based HVDC systems, the existing harmonics on one side ac system will be transferred through the dc-link to the other side ac system and induce a series of harmonics that tend to be more than the origin. This is also true for the case of one side unsymmetrical ac system; the fundamental negative-sequence component will be transferred to the dc-link with the second harmonic is dominant, which in turn causing a series of harmonics on the other side ac system. This paper describes the operation of VSC using three switching functions, which include a fundamental component and carrier sideband components, using double-edge naturally sampled PWM to yield their harmonic spectra. The space vector representation of the VSC´s switching functions is used as a tool for analyzing and giving the detailed rules of VSC´s harmonic interaction, and from that, analyzing harmonic propagation through the dc-link Based on the methodological analysis, the simulation model of a suggested project for Northwest power grid - China is built and implemented using MATLAB/SIMULINK. In the investigation of harmonic propagation, simulation results show that, the amplitude of transferred harmonics through the dc-link may be amplified due to the resonance causing harmonic instability. This deep understanding of VSC-based HVDC system´s harmonic propagation characteristic could be beneficial to harmonic mitigation control.