Insulation Co-Ordination of Gas-Insulated Transformers

Gas-insulated transformers are now available to the industry. Since their insulation characteristics differ from the conventional oil units, the opportunity is presented to revise the concepts which were necessary in establishing the original BILs. It appears that when these units are protected with modern lightning arresters, their basic insulation levels can safely be reduced one step below the normal BIL established for liquid-insulated units. At the same time, maintaining full, standardized, low-frequency high-potential tests are maintained, with considerable margins of safety under both the lightning and switching surge conditions. It is suggested that the values shown in Table I be used as a preliminary basis for establishing standards for this new product.