Health indexes for power transformers: a case study

Proper operation of power transformers is critical to ensuring transmission and distribution of electrical power. Most transformers have an electrical insulation system based on oil and paper. The state of the insulation system is the major factor influencing the state of the transformer. During service the dielectric materials within the transformer deteriorate, and small concentrations of impurities such as water, carbon monoxide, carbon dioxide, and furan compounds accumulate in the oil. Since it is easy to obtain oil samples from power transformers, the information most commonly collected by transformer fleet managers relates to the physical and dielectric properties of the oil. These properties include dielectric strength, dissipation factor, color, interfacial tension, and concentrations of dissolved gases, furans, acids, and moisture. Using these properties it is possible to determine whether a transformer has developed certain specific faults, e.g., partial discharges, arcing, sparking, overheating, etc. On the other hand, various health indexes have been proposed to characterize the general condition of a transformer [1]-[3]. The factors taken into account in these indexes vary, and are given different statistical weightings depending on their influence on the general condition of the transformer. In this article we evaluate the condition of a fleet of operating power transformers, using two recently proposed health indexes, and compare the results.