Reliability assessment of insulation system for dry type transformers

The insulation system of a dry-type transformer under the combined thermal, electrical, mechanical, and environmental stresses undergoes gradual deterioration, which ultimately leads to transformer failure. It is hence important to test specimen insulation of transformer coil so as to explore the aging law and the associated characteristic parameters related to the insulation condition. This paper proposes a method to evaluate reliability of the insulation system of a dry-type transformer based on the three models of degradation, regression and expert judgment. The three models are developed based on the quality parameters (QPs) from the lab tests. The method is illustrated by estimating the reliability of the insulation system of a mining flameproof dry-type transformer. The accelerated aging tests on the Nomex paper (i.e. the turn-to-turn insulation of the transformer) are conducted. The QPs of the Nomex samples of degree of polymerization, partial discharges, and photomicrograph were measured periodically during aging test. The effectiveness of the proposed method was proved through the lab experiments. The results show that the parameters from the aging test can be used to represent the condition of the insulation system.