In high power rating, Diode Rectified Frontend VSD utilises a multi-pulse arrangement on the line-side rectifier stage. In this arrangement, a Phase Shift Transformer (PST) is connected on the line-side to produce a smooth DC link voltage. The number of pulse correlates to the number of ripples in the DC link per AC input cycle.
Phase Shift transformer can assist in mitigating the harmonics reaching the electrical network through the number of pulses of the VSDs. The harmonic distortion can be mitigated by increasing the number of rectifying sections used in the VSDs, as shown in Table 1. The larger the number of pulses, the smaller the distance between waves, which then create a shorter distance between wave crests and consequently lower harmonic distortion.