C57.12.56-1986 - IEEE Standard Test Procedure for Thermal Evaluation of Insulation Systems for Ventilated Dry-Type Power and Distribution Transformers
Description: This standard is intended to establish a uniform method for determining the temperature classification of ventilated dry-type power and distribution transformer insulation systems by test rather than by chemical composition. These insulation systems are intended for use in transformers listed in ANSI C57.12.50-1981 and ANSI C57.12.51-1981, and whose highest voltages exceed a nominal 600 V. Two test methods are developed to provide a means for evaluating insulation systems as a function of thermal aging and are an extension of AIEE 65-1956. Thermal evaluation of ventilated dry-type power and distribution transformers. One method is based on retention of a dielectric withstand voltage equal to a percentage of the initial 50/60 Hz dielectric withstand capability of the test sample. The second method is based on the retention of the basic impulse insulation level by impulse testing, or by related 50/60 Hz voltage withstand capability tests on models.