COOLING SYSTEM DESIGN OF TRANSFORMERS

Abstract

Power and distribution transformers form an important link between generation transmission and distribution of power. Prior to 1955 world's sub-station ratings were mainly based on `r transformer's name plate ratings. After 1955, however power engineers began to rate them on the thermal basis by using the full thermal capability of the substation transformers. The better understanding of the thermal capabilities and thermal problems of the transformers greatly enhances their utility for the user. , This work aims at understanding the effect of thermal losses on the temperature profile of the windings of the power and distribution transformer having transformer oil as their cooling medium in conjunction with .the insulations for different conditions of the coils. The hottest point in the transformers is a great concern for an elect.rical engineer, and it mainly occurs in the windings (coils) which are the heat generating elements. An attempt has. been made to find out the probable hottest point in the windings by utilizing the knowledge of "Heat transfer engineering". Various cases of the coils with different cooling conditions due to the position of insulation have been studied and the probable hot spot temperature has been found in each case. one dimensional approach has been used for all the cases and results have. been plotted on the graphs ( between temp. & radial dimensions of the coil) and compared. Two dimensional approach using finite difference method has been used for case having symmetrical. cooling on both the sides of the coil section and the position of the hotspot temperature is located in vertical as well radial directions. A tailor made software has been developed for the purpose. Also highest average winding temperature of the transformer coil leads us to the design of cooling system of the transformer i.e. external tubes (radiators) etc. required. These are illustrated by examples and a software for determining above has been developed.Due to loss of energy in the active parts, the transformers are subjected to thermal stresses. in oil immersed transformers, paper or cellulose material along oil forms major insulation. The ,life of insulation is the gauge for predicting the life of the transformer which depends on the hotspot temperature. Due to.hotspot temp. the paper or cellulose material insulation gets' brittle and develops cracks which are the vulnerable points. The higher the hotspot temp., faster is the insulation deterioration. The life of transformer has been predicted on the basis of this hotspot temperature taking into account the ambient temp. and changing load cycle. The loading guides have changed gradually to use more and more of the thermal capability of the transformers. It has been shown that it is possible to meet the Short-time peak load requirements even beyond name plate rating from the existing transformers without consuming the normal life of these transformers (1.S.6600). This has been illustrated by taking an example and suggestions are made for loading.