The temperature dependence of the dielectric strength of transformer oil and pressboard insulation at HVDC polarity reversal stress

This work reports on measurements investigating the dielectric strength of insulating oil and Transformerboard, from very low (-20°C) to increased temperature (+60°C). Different insulation structure models-board puncture and creepage-were stressed with 1 hour DC step-by-step voltage increase of reversed polarity each step, until breakdown. The impact of adhesives and their orientation relative to the electric field was investigated. As a reference, the AC strength and the moisture content of the oil was measured. With the results from this study, important knowledge of the DC behavior of oil/cellulose insulation is gained. The findings constitute a profound basis for improved HVDC power transformer insulation technology.