Comparative effects of surge voltage waveforms on the insulation of power transformers fed by voltage sourced converters

Transformers connected to HVDC system using voltage sourced converter (VSC) arrangement are exposed to continuous converter AC voltage waveforms which are typically a rectangular wave shape with a high repetitive frequency. The most commonly encountered is a two-level VSC waveform. Steep voltage steps like these result in high stresses of the insulation due to non-uniform voltage distribution across the windings. The influence of two-level VSC waveforms on the voltage distribution across different winding and gap sections of a typical medium power transformer has been analytically studied. The practical implications of these results on the dielectric stress of transformer insulation is discussed in relation with those stresses generated under power-frequency and standard lightning impulse voltage excitations on which are transformer designs are generally based.