70,000 volt line in Montana

A 70,000-volt Transmission Line.†

By CHARLES F. SCOTT.

"If we consider both high-tension commercial service and time, I believe we must accord to Mr. Gerry the honor of having operated at the highest voltage over the longest time. He has been operating nominally at 50,000 volts, but actually at 55,000 volts in continuous commercial service for two years and a half. His line is about 65 miles from the power, house of the Missouri Power Company, near Helena, to Butte, Mont. Other plants have operated at a little higher voltage; others have operated at longer distances, but taking all together — high voltage, length of time, continuity of service, amount of power — his plant may be taken as one of the foremost, if not the foremost example of high-tension transmission at this time. I believe he told me that he had not lost an insulator through breakdown on the line due to electrical causes."

The foregoing paragraph is my comment taken from the Transactions of the International Electrical Congress, September, 1904, in the discussion of papers upon high-tension lines. I had visited the plant of the Missouri Power Company during the preceding month.

A year later I again visited Helena and made specific-inquiry as to the operation of the system during the year which had elapsed since I had been there before. I found that the record of the plant for the year has been a remarkably successful one. The service is for 24 hours a day, seven days a week, and the power delivered at Butte for mining and smelting operations shows a very high load factor. There had been a few interruptions, due to the high-voltage system. These were four or five in number; one had occurred in the sub-station, and was a discharge between line and ground across an apparatus terminal which had been recently installed. The discharge, however, did not cause a short circuit and the service was not interrupted. There were several line short circuits, which were attributed to lightning. When this occurred the circuit at the power house was opened and immediately closed again. The resulting inconvenience was simply the restarting of such of the motors as had stopped as the result of the momentary interruption.

The users of this power have found that it is more reliable than steam power generated on their own premises.

The endorsement of the success of this plant, both from its commercial and electrical standpoints, is proved by the present plans for extension. A new dam and power house will be constructed a few miles below the Canyon Ferry plant. The old and new power houses will be operated in parallel over the present lines to a new sub-station at Butte and an additional sub-station at Anaconda.

The transmission voltage will be increased to 70,000 volts at the power houses, and new transformers are to be installed in the old power house for the higher voltage. The maximum distance of transmission will be approximately 100 miles. There are at preseent two pole lines which will be used for the higher voltage.

The additional lines, which will join the two power houses at one end and the two sub-stations at the other end, will employ insulators of the same kind which have been used, although the pins will be increased slightly in length.

The new power house will contain three banks of raising transformers, each consisting of three 2,000-kilowatt units; two of these will be for regular service, and the additional one will be available as reserve, also for use in line tests, and further may become a part of the regular working equipment when additional generators are installed.

An important adjunct to the system will be a steam turbine auxiliary plant at Butte. During seasons of low water the steam plant will be used to supply power; at other seasons, when water power is abundant, and the limit of the amount of power transmitted is the capacity" of thee generators and transmission circuits," the turbo-generators will be uncoupled from the turbines in order that they may be run as idle synchronous motors for increasing the power factor of the current from the transmission lines.

The contracts for the electrical equipment of the new plant have been placed with the Westinghouse company, which furnished the apparatus for the first plant.

The particularly interesting feature of the new plant is the adoption of a higher voltage than has ever been employed commercially by the engineer who already holds an enviable record for operating at high voltage. It also is of interest to note the form of construction used.

A brief description of the insulators and pole construction, which have been found so well adapted to conditions in Montana, is given in the following extract from a paper before the International Electrical Congress by Mr. M. H. Gerry, Jr., who has had under his direction the engineering of the early plant and of the extensions which are being made by the new company, the Helena Power Transmission Company.

"As a further illustration of current practice, the high-tension of the Missouri River Power Company, built under the direction of the writer, is here briefly described.

"This transmission has been in service for over three years, operating at 57,000 volts, delivering power at a distance of over 63 miles in a satisfactory manner. The country through which it passes is very rough.

"The lines leave the generating station at an elevation of about 3,700 feet, pass over three distinct summits, including the Continental divide, at which point they reach an elevation of 7,300 feet above sea level. There are two parallel lines expending from the generating station on the Missouri River to the Butte sub-station. They are located, in the main, on a private right-of-way 200 feet in width, from which all timber was removed. Each of the lines carries three copper cables, arranged in a triangular position, 78 inches apart. The cables are composed of seven strands and have an area of 106,000 circular mils.

The poles are of Idaho cedar, the cross-arms of Oregon fir, the braces and pins of white oak and the insulators and sleeves of glass. The crossarms, braces and pins are held in place by means of through bolts. The pins in the top of the poles are of larger size and of greater length than those in the cross-arms, to provide for the greater strains there present. The pins were prepared by being first dried and then treated in paraffine until all moisture was removed, and were then tested to 60,000 volts. The glass sleeves are not fastened to the insulators and merely rest on a shoulder of the pins.

"The circuits are transposed five times, making two complete turns between the generating station and the sub-station. The switching arrangements are such that the circuits may be operated either singly or in multiple. A telephone circuit is located on one of the lines and gives good results in service. The poles are from 35 to 75 feet in length, and the pole tops are from nine to 12 inches in diameter. The poles are set from six to eight feet in the ground, according to height, and the standard spacing is no feet, with a maximum spacing of 150 feet."

† From The Electric Journal for November, 1905.