On distributing multilevel SHE-PWM waveforms in HVDC systems built with conventional three-phase VSC modules

A high-voltage direct current (HVDC) power transmission system based on eight voltage-source converter (VSC) modules is discussed in this paper. The system is operated with a selective harmonic elimination (SHE) pulse-width modulation (PWM) scheme, which is pre-calculated as a single multilevel waveform. There are many degrees of freedom on how such single multilevel waveform can be used to derive level switching signals for the individual converter modules and a selected number of available solutions are presented. The multimodule converter of the HVDC system can synthesize high voltage AC waveforms with controlled harmonic bandwidth and hence the AC-side filtering requirements are not excessive. The switching frequency of every VSC module is kept very low and therefore the overall module and system switching losses are kept to a minimum level. The theoretical findings are verified through simulation studies using PSCAD/EMTDC.