The Influence of Phase Disposition of AC Grids on the Harmonic Distortion of Line-Commutated Converter based HVDC Systems

摘要

This paper deals with the analysis of the characteristic harmonics of currents and voltages of line-commutated converter (LCC) based high voltage direct current (HVDC) transmission systems. The generation of harmonics of AC and DC currents and voltages of an HVDC transmission system is carried out. The characteristic harmonics are analyzed in a simulation with the help of the Fast Fourier Transformation (FFT). It turns out that there is a dependency of the characteristic harmonics on the firing angle of the rectifier α_R, the firing angle of the inverter α_I and the phase shift between the AC grids δ. For specific harmonics of the current and voltage there are some combinations of firing angle and phase shift which minimize the amplitude of the considered harmonic. It can be shown that irrespective of the ordinal number of the harmonic there is always a linear correlation between the firing angle of the rectifier and the phase shift of the AC grids for a certain firing angle range. Plotting α_R over δ reveals that the gradient of the line representing the location of the maximum/minimum amplitudes of the harmonics is -0.5. Furthermore, the curve of the maxima/minima of the harmonics shows symmetry of 60°/k referring to the phase shift δ. The course of minimum and maximum amplitudes of harmonics of the AC current and voltage with same ordinal number are equivalent, but the course of the harmonics of DC current and voltage distinguish from each other. Analysing the results leads to the conclusion that LCC based HVDC systems could be run in certain operating points which allow a minimized harmonic distortion. This fact could again save of lot of costs for the installation of additional AC filters.