Electric Transmission of Water Power

by Alton D. Adams

Sometimes two or more water-powers on the same river that are to be united are so far apart that any attempt to construct a canal between them would be impracticable. This is illustrated by the Spier and Mechanicsville Falls on the Hudson River, which are 25 miles apart in a direct line and at a greater distance by the course of the stream. At Spier Falls the head is 80 feet, and at Mechanicsville it is 18 feet. Union of the power of these two falls is thus out of the question for physical reasons alone. Electrical transmission, however, brings energy from both of these water-powers to the same sub-stations in Schenectady, Albany, and Troy.

In another class of cases electrical transmission does what could not be done by any system of canals, however expensive; that is, unites the water-powers of distinct and distant rivers at any desired point. Thus power from both the Merrimac and the Piscataquog rivers is distributed over the same wires in Manchester; the Yuba and the Mokelumne contribute to electrical supply along the streets of San Francisco; and the Monte Alto and Tlalnepantla yield energy in the City of Mexico.

It does not follow from the foregoing that it is always more economical to develop two or more smaller water-powers at different points along a river for transmission to some common centre than it is to concentrate the water at a single larger station by more elaborate hydraulic construction, and then deliver all of the energy over a single transmission line. The single larger hydraulic and electric plant will usually have a first cost larger than that of the several smaller ones, because of the required canals or pipe lines. A partial offset to this larger hydraulic investment is the difference in cost between one and several transmission lines, or at least the cost of the lines that would be necessary between the several smaller stations in order to combine their energy output before its transmission over a single line to the point of use.

Against the total excess of cost for the single larger hydraulic and electrical plant there should be set the greater expense of operation at several smaller and separate plants. Even a small water-driven electric station that can be operated by a single attendant at any one time must have two attendants if it is to deliver energy during the greater part or all of every twenty-four hours. But a single attendant can care for a water-power plant of 2,000 horse-power or more capacity, so that two plants of 750 horse-power each will require double the operating force of one plant of 1,500 horse-power. If two such plants are constructed instead of one that has their combined capacity, the monthly wages of the two additional operators will amount to at least one hundred dollars. If money is worth six per cent yearly, it follows that an additional investment of $1,200 ÷ 0.06 = $20,000 might be made in hydraulic work to concentrate the power at one point before the annual interest charge would equal the increase of wages made necessary by two plants.

Reliability of operation is one of the most important requirements in an electric water-power plant, and its construction should be carried out with this in view. Anchor ice is a serious menace to the regular operation of water-wheels in cold climates, because it clogs up the openings in the racks and in the wheel passages. Anchor ice is formed in small particles in the water of shallow and fast-flowing streams, and tends to form into masses on solid substances with which the water comes in contact.

At the entrance to penstocks or wheel chambers, steel racks with long, narrow openings, say one and one-quarter inches wide, are regularly placed to keep all floating objects away from the wheels. When water bearing fine anchor or frazil ice comes in contact with these racks, it rapidly clogs up the narrow openings between the bars, unless men are kept at work raking off the ice as it forms. At the Niagara Falls electric station, in some instances, when the racks become clogged, they have been lifted, and the anchor ice allowed to pass down through the wheels. This is said to have proved an effective remedy, but it would obviously be of no avail in a case where the ice clogged the passages of the wheels themselves.

The best safeguard against anchor ice is a large, deep forebay next to the racks, where the water, being comparatively quiet, will soon freeze over after cold weather commences. The anchor ice coming down to this forebay and losing most of its forward motion, soon rises to the surface or to the under side of the top coating of solid ice, and the warmer water sinks to the bottom. Good construction puts the entrance ends of penstocks well below the surface of water in the forebay, so that they may receive the warmer water that contains little or no anchor ice.

Fig. 7.—Cross Section of Dike on Chicopee River at Red Bridge.