Analysis of Insulation Electric Stress on Valve-Side Winding of Converter Transformer

Converter transformers are core equipment in DC transmission systems. The valve-side winding of these transformers is subjected to complex voltage conditions, with the main insulation enduring a composite electric field of $\mathrm{AC}, \mathrm{DC}$, and harmonics. The longitudinal insulation faces AC and harmonic superimposed electric stresses, which can lead to discharge and further insulation degradation. This paper establishes a simulation model of an ultra-high-voltage transmission line and analyzes the electric stress waveform endured by the main insulation of converter transformers under different operating conditions. An equivalent RLC model for the valve-side winding of the converter transformer is established based on the finite element method and analytical method. Nine different simulation scenarios are set: rated condition, operation at $60 \%, 70 \%, 80 \%, 90 \%$ of rated voltage, operation at $60 \%$, $\mathbf{7 0 \%}, \mathbf{8 0 \%}, \mathbf{9 0 \%}$ of rated power. The voltage to ground (main insulation waveform) of four converter transformer valve-side bushings are obtained under these nine scenarios. Additionally, a method based on pulse superposition is used to equivalent the inter-disc voltage waveform and extract the parameter range of the inter-disc voltage waveform.