Internal Fault Characteristics of Gas-Insulated Transformers

Recently developed gas-insulated transformers are taking a prominent position in a field where existing apparatus has a long record of reliable and almost faultless service. The outstanding position of the gas-insulated transformer is due to some fundamental differences between the gases and liquids that are used for the cooling and insulation of transformers. These differences can be summarized as follows: 1. The gases being used or proposed are not combustible. 2. Although gases may decompose during accidental internal arcs, the pressures developed during these faults are only a fraction of those produced by the same arcs in liquid-filled apparatus. 3. In a liquid-insulated transformer the operation of pressure-relief devices depends upon the position of the relief device with respect to the fault position and on the rate of pressure rise. However, in a gasinsulated unit the operation of the pressurerelief device is much more reliable because of the small mass which must be moved during the fault and of the compressible characteristics of the gases. The purpose of this paper is to discuss in some detail the behavior of gas-insulated transformers under fault conditions (internal arcs) and to report results of tests made on representative apparatus. Noteworthy is the method of obtaining the pressure rise in these tests. After correlating the energy of the electric arc during fault operation with that of the energy developed by an explosive (Pentolite), interesting results were obtained by initiating the explosive charge in containers with liquids and gases.