Publication: The Journal of Electricity, Power and Gas
San Francisco, CA, United States
"PROVO" HIGH-VOLTAGE INSULATORS.
THE interesting brochure just issued by the Hemingray Glass Company of Covington, Ky., bearing the title above given, opens its introductory paragraph with the significant statement that, "If there is anything that the purchaser of electrical apparatus abhors, it is to be the introducer of a new article, or, in other words, " ' to be experimented upon.' " Then it proceeds with the assurance that, "while the 'Provo' type insulators were once new, the manufacturers can assure prospective customers that they have never had to change the design or construction of these insulators since their introduction." The conclusion given to this reasoning is that the continued and increased use of "Provo " insulators during the past five years would indicate that they have become accepted as a standard article.
Brief descriptions of these insulators, which, as is well known, are all glass, then follows, from which there is compiled the following table of physical data of "Provo" insulators:
In the opinion of the manufacturers of "Provo" insulators, it is advisable that wooden pins be used, because metal pins, or pins having metal in their construction, are objectionable to the insulation of a very high voltage line, without regard to the make of the insulator used. The treating of wooden pins with paraffine wax or oil is also advised, as this tends to preserve the life and insulating qualities. This treating is usually done by placing a quantity of pins in a tank of wax or oil of a low temperature, and heating the insulating material up to a temperature approximating 250 F. The pins should be boiled in this manner for an hour or more until the air and water are driven from the wood. The insulating material should then be allowed to cool until the pins can lie removed without losing the paraffine or oil that is in the grain of the wood. No definite information can be given as to the exact temperature or the length of treatment, as these depend upon the kind and quality of wood used in the pins. Too long a boiling or at too high a temperature will destroy the strength of the wood.
The "Provo" type insulators were named after the city of Provo, Utah, where the main generating station and offices of the Utah department of The Telluride Power Company are located and where the insulators were first put into use. The insulator was designed by Mr. V. G. Converse, the No. 1 type being designed especially for the Provo plant after a long study of high voltages and aided particularly by exhaustive high voltage experiments of a practical nature at Telluride, Colo.* These experiments were conducted by The Telluride Power Transmission Company for the express purpose of determining a safe maximum voltage for its Utah plant, and it was only after the complete success of the Provo plant that the Hemingray Glass Company acquired, in 1898, the right to manufacture "Provo" type insulators. The transmission in question was originally planned to run at 40,000 volts, ** but it is stated that its present running voltage is nearer 50,000 volts. As to leakage on "Provo" insulators, it is stated that the maximum losses observed at Telluride on any type of insulator which would withstand puncture were found to be inconsiderable when running at a pressure below 45,000 volts. These measurements showed further that when subjected to a high voltage there was no appreciable difference noted between the losses over glass insulators and porcelain insulators.
In continuing the discussion of this subject the brochure observes that it seems to be a very difficult matter to obtain electrical measuring instruments which will distinguish true leakage with accuracy. Furthermore, there has never been any measurements made of a practical nature which separated the losses from the lines and the losses over lightning arresters from the losses either through or over the surface of the insulators. The only definite knowledge which the Hemingray company possesses regarding leakage is on the No. 1 "Provo'' insulator, and is contained in a statement given to the public in 1900 by Mr. P. N. Nunn, chief engineer of The Telluride Power Company, which said:
"In connection with the transmission system of The Telluride Power Transmission Company in Utah, there has been in constant service for nearly two years a single circuit more than 100 miles in length, and equipped with more than 12,000 No. 1 "Provo" type insulators. The circuit is three-wire, three-phase, grounded center, ' star ' connected and operated at 42,000 to 44,000 volts. In truth, this loss is usually negligible, never material, and during the most severe conditions ever known in any country, the total of line losses of all kinds has never exceeded 10 per cent, of this amount." The amount referred to was 134 watts per insulator, as published upon another occasion.
Thanks are due to the Hemingray Glass Company and to The Charles F. Sloane Company, its Pacific Slope sales agent for the "Provo" type insulators, for the privilege of making excerpts from the interesting catalogue referred to prior to its issue in completed form.
* Detailed in THE JOURNAL, Volume VI, page 113, December, 1898.
** See illustrated description in THE JOURNAL, Volume V, page 168, June, 1898.