LOSS OF LIFE ASSESSMENT OF DISTRIBUTION TRANSFORMERS IN ACTIVE DISTRIBUTION SYSTEMS

Abstract

A transformer is a costly and critical piece of equipment for a distribution utility. A large population of transformers is near to the end of designed life in many distribution utilities around the world. The life of existing transformers can be increased with proper operation and maintenance to maximize the return on the investment. However, the field experience suggests that a lot of transformer failures happen before their designed lives. The failure of a distribution transformer leads to consequences such as revenue loss due to forced outages, repair and maintenance or replacement cost, environmental impacts, etc. The research work of this thesis aims to estimate the remnant life of distribution transformers (DTs) in active distribution systems. The research work is divided into four parts. The first part of the research work concentrates on the effect of load model selection and harmonics on the loss of life (LOL) calculation of the transformer. The study of the impact of load models on transformer insulation life is essential for an accurate aging calculation. In most of the studies related to the assessment of transformer aging, distribution loads are treated as constant power type. However, the practical loads are of mixed type (a combination of constant power, constant current, constant impedance). Moreover, the residential, commercial, and industrial loads comprise the different percentages of constant power, constant current, and constant impedance loads. In this research, different load models are investigated to quantify their impact on the aging calculation of a transformer under various loading scenarios and environmental conditions. The study includes different classes of the loads with corresponding load shapes, ambient temperatures, determined for a typical week of each season. It considers highly unbalanced transformer loading conditions and network configurations. The study also considers the effect of harmonics on the transformer aging. Aging acceleration rate and percentage loss of life of the substation transformer connected to the IEEE 13-bus system have been calculated for different loading and climatic conditions. The same procedure has been adopted for the modified IEEE 123-bus system. The trends of the results have been compared with the IEEE 13-bus system. It has been shown that the load models and harmonics have a considerable effect on the loss of life (LOL) calculation of a transformer. Therefore, the selection of an appropriate load model and consideration of harmonics is important for an accurate estimation of the aging of a transformer.