Transmission-line insulators under deposit conditions

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Abstract
The paper is divided into two parts, Part 1 dealing with industrial deposits and Part 2 with saline deposits. For both parts the procedure followed is to give examples, from practice, of insulator troubles due to the deposit under consideration, and then to discuss, firstly, the mechanism of insulator failures, and, secondly, insulator designs to overcome or alleviate the troubles.In Part 1 an account is given of two 33-kV lines insulated with cap-and-pin type units. The lines are in areas where heavy industrial deposits are experienced—thus an ounce of solid matter has been collected from an insulator after it has been on the line for 6 weeks. Also the lines are in a river valley where mists are prevalent.The record of faults on these lines is given, together with an account of the procedure followed to reduce their number. It was found necessary to increase the original insulation, and the conclusion is reached that almost complete reliability of insulation has now been attained by using 4-unit suspensioninsulator strings and by carrying out one cleaning operation per year.A description and discussion are given of existing insulator designs which show improved performance under deposit conditions. New designs are described, particular attention being given to utilizing “wind-washed” surfaces. The features of design necessary to secure the maximum extent of wind-washed surface are considered. The degree of limitation imposed on the obtaining of high deposit characteristics by the existing wet-flashover requirements is discussed. Improved behaviour under industrial deposit can be obtained by sacrificing wet-flashover characteristics, but this is not recommended. Of the new designs, Designs 1, 2, and 3 have normal and Designs 5 and 6 sub-normal wet-flashover characteristics. The section dealing with new designs is, in the main, an account of development work still proceeding, rather than a record of final accomplishment. One design (Design 3) has, however, reached the stage of being tried out on a large scale on the British grid at 132 kV.Pin-type insulators are briefly considered.In Part 2, two typical examples are given of trouble due to salt on 33-kV pin-type insulators. The mechanism of failure is discussed, and it appears to be necessary to provide different designs for the two kinds of deposits. An account is given of development work resulting in an insulator which appears to have solved the trouble described in the first typical example. This insulator was unsatisfactory, however, when tried as a solution for the second example, and a suggestion is made to classify saline deposits, according to their severity, as “ordinary,” “severe,” and “extreme.” Designs of pintype insulators for extreme salt conditions are described.Ca-pand-pin type insulators are briefly considered.

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