Influence of Voltage Sourced Converter Waveforms on the Dielectric Strength of Transformer Insulation

Transformers connected to new HVDC system u s - ing VSC technology 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 results in high stresses of the insulation due to non- uniform voltage distribution across the windings. Such stresses represent a new class of stress on transformer insula- tion, which has been less investigated and understood. In this study, the influence of two-level VSC waveforms on the voltage distribution across different windings and winding sections on a large power transformer of industrial design has been analytically studied. The importance aspect of the ana- lytical simulation was that the entire dielectric structure of the transformer (iron core, windings, insulating barriers, spacers, shields, etc) was simulated as one-entity using a custom-made program. Consequently the results can be considered to be representative of similar type of transformers operating in this environment. 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 trans- former designs are generally based.