Some Problems in Dielectric-Loss Measurements

The measurement of dielectric loss in high-voltage cables presents many problems that are not obvious until such measurements have actually been attempted and the results checked and analyzed. Some of these problems are not simple, and their solution is obtained only after considerable effort and research. This paper presents some of the problems that the authors have encountered in this type of work, together with the methods used for their solution. The bridge method, which is used for the measurement of dielectric loss, is described in some detail. Details of the auxiliary equipment, such as galvanometer, amplifiers, air condenser, shielding, and special methods of bringing the shielding to the proper potential are also given. If accurate results are to be had in dielectric loss measurements, the entire bridge, including cable, air condenser, and leads, must be properly shielded and the capacitance between leads and shielding should be made small. Errors of large magnitude may be introduced in the results if these precautions are not taken. It is entirely insufficient to shield the bridge and ground the shielding directly. At times shielding cables may increase the measured losses. In measuring the capacitance of the air condenser, all stray capacitance to its guard rings and to surroundings must be eliminated. The paper gives a method of accomplishing this. The voltage across the voltmeter coil of a high-voltage transformer is frequently taken as a measure of the voltage across its secondary.