Publication: Street Railway Journal
New York, NY, United States
The High-Tension Insulator
The importance of the high-tension insulator in the successful transmission of power over long distances has been emphasized considerably of late, and the demands of newmarkets for power utilization in the general vicinity of large hydro-electric developments have stimulated insulator manufacturers to the most active efforts to extend the range of transmission through the use of voltages in excess of 60,000. At the recent meeting of the Canadian Electrical Association in Montreal, this aspect of the situation was discussed at length, the consideration of the topic having been introduced by C. E. Delafield, who presented a paper on "High-Tension Insulators from an Engineering and Commercial Standpoint." The author considered the intimate dependence of long distance transmission upon the insulator, outlined the type of insulator demanded by the exacting conditions of to-day, and emphasized the importance of correct manufacturing processes and careful tests prior to the acceptance of the product in commercial service.
There is room for considerable difference of opinion as to the best way of securing a satisfactory insulator for very high potentials. The canons of insulator design are so far from fixed that the engineer who specifics materials and shapes in great detail takes considerable responsibility upon his shoulders, and perhaps pays a good deal more in the end for his line construction, not to mention maintenance, than the one who asks the manufacturer for an insulator of the latter's design capable of withstanding a definite potential stress in continuous service of a specified nature. A short abstract of the paper was published in these columns recently in an account of the meeting, but the increasing use of high voltage transmission makes the topic so interesting that attention will be called to certain of the points brought out in the paper or discussion.
The author points out that an increase in voltage from 60,000 to 150,000 would make it possible to deliver Niagara power economically in New York, Boston or Philadelphia, and states tha: the insulator is the principal hindrance to this consummation. Without accepting this assertion in detail — for the commercially profitable sale of Niagara power in competition with coal on the Atlantic Coast involves many other factors than the voltage question — it serves to fix the central responsibility of the insulator in the transmission field. It needs little demonstration at the present prices of copper and aluminum to show that by very high voltage only can the large powers now so successfully generated and safely handled in transformers and switching mechanism be distributed economically over very long distances.
The paper stated that wood can be safely accepted for insulator pins up to about 30,000 volts. Beyond this point, it seems advisable for mechanical reasons to use malleable iron, but the so-called pin type of insulator has reached such dimensions, in the endeavor to meet the requirements of higher voltages, that it appears to be the opinion of the leading high-tension engineers that this type of insulator has reached the limit of good line construction on account of its dimensions. It is a difficult matter, from a mechanical standpoint, to find a pin that will take the necessary stress incident to an insulator of very large size and weight, and the problem of manufacturers, from the standpoint of the pottery is one of great difficulty. It is probable, therefore, that a suspended form of insulator will be tried in the near future, as it is a comparatively simple matter mechanically to suspend any desired weight and, from an electrical standpoint, it seems possible to so design an insulator that it will be mechanically strong and a good dielectric as well.
The suspended type of insulator would have the advantage that ample arcing distance could be provided without making the insulator top-heavy and difficult to manufacture. It should be so designed, however, so that arcing cannot occur until the voltage is sufficient to rupture the air and cause the current to arc from end to end, this feature being of great importance in steel tower work. On high-tension lines where steel towers are used, the pin type of insulator for 100,000 volts or higher would seemingly be almost an impossibility owing to the size necessary to take care of surges and other line disturbances, and also because the earth potential is carried into the head of the insulator through the steel pin and towers. In proportion as the insulator accepted for a given transmission takes care of fogs, dust deposits and spray, is not handicapped by large still air spaces, exposes a large part of its surface to the wind and has a long leakage distance of small area, it is likely to be successful in the ultra high potential work of the future, and it has been pretty clearly shown that nothing but well vitrified porcelain should be used between points of opposite potential. Cemented parts should, if used, be under compression rather than tension. Insulators of the same electrical design but of different manufacture may vary greatly from one another. Thus, the one having the greatest electrolytic capacity and hence the greatest electrostatic field will be the first to suffer from brush discharge and arcing over. In another case the insulator possessing the greatest density in its body, and which is the most vitreous, will stand the more severe service.
The factor of safety which should be required in testing high-tension insulators has been the subject of much comment among engineers. Mr. Delafield urges that three times normal voltage be applied in the dry breakdown test between the insulator head inside and outside. This is a pretty strenuous requirement in the case of an insulator designed for operation at say 100,000 volts, and we question whether manufacturers are as yet willing to bid on a specification requiring their insulators to stand a 300,000-volt test. It is probable that a 200,000-volt test would not be prohibitive if the purchaser shows himself willing to pay the cost, and Mr. Delafield is clearly right in urging that economizing in high-tension insulators to save a few cents apiece in first cost is pretty poor policy. The operating engineer and the manufacturer are coming closer together on the insulator question, and both are beginning to realize that it is not the working voltage of a long, exposed line which demands the greatest precaution—the common enemy is an excessively high potential discharge, caused by lightning or resonance in the system. The latter factor will ultimately drive the breakdown test to the highest point consistent with manufacturing possibilities.
|Date completed:||August 16, 2010 by: Bob Stahr;|