Rationalization of transmission system insulation strength

Experience on high-voltage transmission lines has shown numerous failures of apparatus which have indicated a decided lack of coordination of the insulation strengths of the various parts of the transmission system. Apparatus offered by manufacturers for a given service shows wide variations in insulation values. Again the flashover and the breakdown values are not at present sufficiently standardized to be comparable among manufacturers of the same piece of apparatus. The standard tests on different types of apparatus are not properly correlated. Besides discussing the above situation, this paper points out the causes for the present status and indicates the benefits to be derived by grading the insulation on the entire system. Predetermining the point of electrical breakdown on the system in the case of high voltage surges leads most logically to grading the insulation. This grading should result in fewer major service interruptions, with a localization of trouble on a link of the system where repairs can be made easily and inexpensively. The paper points out that additional information is required on surge-voltage breakdown of insulation to solve the problem completely but shows that with the present information available a start in grading can be made. The different links in the transmission chain are tabulated according to their relative importance and with this as a starting point, the entire grading scheme is developed to the point of showing relative 60-cycle insulation strength required of the different apparatus used on a transmission system. It is shown; 1, that at the present time transmission systems in general, are designed without proper consideration from the standpoint of surge voltages which may be imposed upon them; 2, that the grading scheme proposed is possible, although requiring additional operating data and data from the manufacturers to be fully effective; 3, that grading should result in less costly designs and installations; 4, that the net effect will be better performance of the transmission system in service.