Non-characteristic harmonics and DC side capacitor calculation in VSC connected to a distribution system with unbalanced voltage

This paper presents a model of typical three-phase, three-wire and two-level PWM controlled voltage-source converters (VSC) suitable to investigate the non-characteristic harmonic generation at dc- and ac-side of the converter under sinusoidal unbalanced voltage condition, e.g., third order current harmonic component of positive-sequence in the VSC ac-side current. The proposed model, based on instantaneous power as well as switching functions concepts, permits to evaluate the oscillating voltage that appears on the VSC dc-side. The effect of the negative-sequence voltage component is considered on the dc capacitance calculation which consequently enables to size the dc capacitor accurately in order to keep the dc voltage ripple under pre-defined limits. The model also shows the exact dc-side voltage ripple due to the VSC ac-side negative-sequence voltage component. The dc capacitor size and the non-characteristic harmonics obtained based on the proposed model are assessed using time-domain simulation in the PSCAD program.