Empaques

1278

IEEE Transactions

on

Power Apparatus and Systems, Vol. PAS-102, No. 5, May 1983

GAS PRESSURE CALCULATIONS FOR SEALED TRANSFORMERS UNDER VARYING LOAD CONDITIONS
T. V. Oomnen, Member, IEEE

Westinghouse Electric Corporation Large Power Transformer Division 23rd Street and Cowan Road Muncie, Indiana 47302

Abstract: Pressure -changes in the gas space of sealed transformers arediscussed, and the necessary formulas to compute the equilibrium pressure corresponding to a given load condition are derived. It is shown that when gas solubility effects are included, the equilibrium pressure computed is appreciably different from the pressure computed otherwise. Computation of gas space pressure from gas-in-oil analysis data is also discussed.
INTRODUCTION Sealed transformerswith gas space above oil have been in use for at least half a century. However, the popularity of this system is diminishing with the development of other systems such as the oil expansion tank with breather bag; it is still a viable option due to its simplicity of design and effectiveness in maintaining a protective environment. The gas cushion consists of dry nitrogen maintained at a slightpositive pressure, usually 0.5 psig with the aid of an auxiliary nitrogen supply system. A relief valve would open if the pressure exceeds a safe upper limit, usually set between 6 and 8 psig. In normal operation the pressure in the gas space would be between these upper and lower limits. In spite of the widespread use of this system, the factors that influence gas space pressure have not been fully investigated. In a classic study conducted in 1950 by E. W. Tipton, gas space pressure changes under varying load conditions were measured for several sealed transformers [l. The observed pressures were compared with the pressures calculated using the simple Gas Law. It was found that the measured pressure was lower than the calculated pressure by as much as 30 percent at the maximum oiltemperature. The difference was attributed to gas solubility in oil. As the oil expands and compresses the gas space, more gas would dissolve in oil, hence the pressure would be less than what is expected from the Gas Law. Conversely, when the oil contracts the gas space pressure would decrease, but some gas would return to the gas space and partly compensate for the pressure drop. Tipton did not

82 SM371-3 A paper recommended and approved by the IEEE Transformers Committee of the IEEE Power Engineering Society for presentation at the IEEE PES 1982 Summer Meeting, San Francisco, California, July 18-23, 1982. Manuscript submitted February 2, 1982; made available for printing April 19, 1982.

attempt to compute the solubility effect on gas space pressure because the necessary formulas were notavailable. Five years later, in 1955, E. B. Franklin derived some formulas to compute pressure changes due to solubility effect, but ignored the change in solubility with temperature [2]. Franklin's equations are somewhat cumbersome to use, and the omission of the temperature effect on gas solubility limits their practical application. In a recent paper the author presented formulas to compute gasconcentration in oil due to solubility changes accompanying temperature variations in noknal operation of transformers [3]. The response to this paper showed that the work should be extended to computing gas space pressure changes under varying load conditions, taking into account the solubility changes due to temperature variations. This is what the present paper hopes to accomplish. Forconvenience the task of computation is divided into two categories. First, formulas necessary to compute pressure in gas space are discussed on the assumption that the gas space consists of only one gas (nitrogen). This is justified because under normal operations more than 99 percent of the gas space composition would be nitrogen. In the second part, all the gas components in the gas space will be...