Electric Transmission of Water Power

by Alton D. Adams

Fig. 75.—Cable Terminal House at Piscataqua River Crossing.

Where the two, three, or more conductors that form a complete circuit for alternating current are included in a single metallic sheath, the inductive effects of currents in the several conductors tend to neutralize each other and the waste of energy in the sheath is in large part avoided. To neutralize more completely the tendency to local currents in their metal sheath, the several insulated conductors of an alternating circuit are sometimes twisted together, after being separately insulated, before the sheath is put on. Distribution of power at Niagara Falls was at first carried out through single-conductor, lead-covered cables with two-phase current at 2,200 volts. One objection to this plan was the loss of energy by induced currents in the lead coverings of the cables. It was later decided to adopt three-phase distribution at 10,000 volts for points distant more than two miles from the power-station. Each three-phase circuit for this purpose was made up of three conductors separately insulated and then covered with a single lead sheath, so as to avoid losses through induced currents in the latter. Underground and submarine cables for operation at high voltages are generally covered with a continuous lead sheath and sometimes with a spiral layer of galvanized iron wire. For high-voltage work underground the lead covering is generally preferred without iron wire, but in submarine work coverings of both sorts are employed. The lead sheath of a cable being continuous completely protects the insulation from contact with gases or liquids. As ducts of either tile, wood, or iron form a good mechanical protection for a cable, the rather small strength of a lead sheath is not a serious objection in conduit work. Submarine cables, on the other hand, depend on their own outer coverings for mechanical protection, and may be exposed to forces that would rapidly cut through a lead sheath. Cables for operation under water should usually be covered, therefore, with a layer of galvanized iron wires outside of the lead sheath. These wires are laid closely about the cable in spiral form and are usually between 0.12 and 0.25 inch in diameter each, depending on the size of the cable and its location.

Underground conduits cannot be relied on to exclude moisture and acids of the soil from the cables which they contain, and either of these agents may lead to destructive results. If cables insulated with rubber, but without a protecting covering outside of it, are laid in underground conduits, the rubber is apt to be rapidly destroyed by fluids and gases that find their way into the conduit. If a plain lead-covered cable is employed the acids of the soil attack it, and if stray electric currents from an electric railway find the lead a convenient conductor it is rapidly eaten away where they flow out of it. To avoid both of these results the underground cable should have a lead sheath, and this sheath may be protected by an outside layer of hemp or jute treated with asphaltum.

Rubber, paper, and cotton are extensively used as insulation for underground and submarine cables, but the three are not usually employed together. As a rule, the insulation is applied separately to each conductor, and then an additional layer of insulation may be located about the group of conductors that go to make up the cable. Where rubber insulation is employed, a lead sheath may or may not be added, but where insulation depends on cotton or paper the outer covering of lead is absolutely necessary to keep out moisture. The radial thickness of insulation on each conductor and of that about the group of conductors in a cable should vary according to the voltage of operation.

Cables employed between the generating and sub-stations of the Manhattan Elevated Railway, to distribute three-phase current at 11,000 volts, are of the three-conductor type, rubber insulated, lead covered, and laid in tile conduits. Each cable contains three No. 000 stranded conductors, and each conductor has its own insulation of rubber. Jute is laid on to give the group of conductors an outer circular form, and outside of the group a layer of insulation and then a lead sheath is placed. Outside diameter of this cable is nearly three inches, and the weight nine pounds per linear foot.

The 11,000-volt, three-phase current from Niagara Falls is distributed from the terminal house to seven sub-stations in Buffalo through about 30 miles of rubber-insulated and 18 miles of paper-insulated, three-conductor, lead-covered cables, all in tile conduits. In each cable the three No. 000 stranded conductors are separately insulated and then twisted into a rope with jute yarn laid in to give an even round surface for the lead sheath to rest on. A part of the rubber-insulated cables have each conductor covered with ⁹⁄₃₂-inch of 30 per cent pure rubber compound, and the remaining rubber cables have ⁸⁄₃₂-inch covering on each conductor of 40 per cent pure rubber compound. The paper-insulated cable has ¹³⁄₆₄-inch of paper around each conductor, and also another ¹³⁄₆₄-inch of paper covering about the group of three conductors and next to the lead sheath. In outside diameter the rubber-insulated cable is 2³⁄₈ inches, and of the paper-insulated cable 2⁵⁄₈ inches, the radial thickness of the lead sheath being ¹⁄₈-inch in each case. It is reported that the cables insulated with ⁹⁄₃₂-inch of the mixture, said to be 30 per cent pure rubber, have proved to be more reliable than the cables insulated with ⁸⁄₃₂-inch of a mixture said to be 40 per cent pure rubber. Vol. xviii., A. I. E. E., 136, 836.

The six miles of underground cables that carry three-phase, 25,000-volt current in St. Paul are of the three-conductor type, lead covered, and laid in a tile conduit. One of the two three-mile cables is insulated with rubber and the other with paper. In the former cable each conductor is separately insulated with a compound containing about 35 per cent of pure rubber and having a radial thickness of ⁷⁄₃₂-inch. The three conductors after being insulated are laid up with jute to give a round surface, tape being used to hold them together, and then a rubber cover ⁵⁄₃₂-inch thick is placed about the group, after which comes the lead sheath over all. In the three miles of paper-insulated cable each conductor is separately covered with paper to a thickness of ⁹⁄₃₂-inch, then the three conductors are laid together with jute and taped, and next a layer of paper ⁴⁄₃₂-inch thick is put on over the group. Outside of all comes the lead sheath, which has an outer coating of tin. The paper insulation in these cables was saturated with a secret insulating compound. The lead sheath on both the rubber and paper insulated cables is ¹⁄₈-inch thick and the sheath of the former contains 3 per cent of tin. Each of the three conductors in each cable consists of 7 copper strands and has an area of 66,000 circular mils. Outside of the lead sheath each of these cables has a diameter of about 2¹⁄₄ inches. By the manufacturer’s contract these cables were tested up to 40,000 volts before shipment, and might be tested up to 30,000 volts in their conduits during any time within five years from their purchase. In first cost the cable with rubber insulation was said to be about 50 per cent more expensive than the cable in which paper was used. Vol. xvii., A. I. E. E., 650.

Underground cables in which the separate conductors are covered with cotton braid treated with an insulating compound, and then the group of conductors going to make up the cable enclosed in a lead sheath, are extensively used in Austria and Germany. For cables that operate at 10,000 to 12,000 volts the radial thickness of cotton insulation on each conductor is said to be within ³⁄₁₆-inch, and these cables are tested up to 25,000 volts by placing all of the cable except its ends in water, and then connecting one end of the 25,000-volt circuit to the water and the other end to the conductors of the cable.

A test on the paper-insulated cable at St. Paul showed its charging current to be 1.1 amperes at 25,000 volts for each mile of its length. For the cable with rubber insulation the charging current per mile of length was found to be about twice as great as the like current for the paper-insulated cable. Each of the two overhead transmission lines connected with these cables consisted of three solid copper wires with an area of 66,000 circular mils each, and all three so mounted on the poles as to form the corners of an equilateral triangle twenty-four inches apart. The charging current of one of these three-wire, overhead circuits was found to be about 0.103 ampere per mile, at 25,000 volts, or a little less than one-tenth of the like current for the paper cable. These tests were made with three-phase current of sixty cycles per second.

Where overhead transmission lines join underground or submarine cables, either with or without the intervention of transformers, lightning arresters should be provided to intercept discharges of this sort that come over the overhead wires. Lightning arresters were provided in the terminal house at Buffalo, where the 22,000-volt overhead lines feed the 11,000-volt cables through transformers, also at the terminal house in St. Paul, where the 25,000-volt overhead lines are electrically connected to the underground cables. If an underground or submarine cable connects two portions of an overhead line, as in the Portsmouth and Dover transmission above mentioned, lightning arresters should be provided at each end of the cable, as was done in that case. One advantage of a high rather than a low voltage on underground cables, where power is to be transmitted at any given rate, lies in the fact that the amperes flowing at a fault in the cable determine the destructive effect there, rather than the voltage of the transmission. It is reported that a fault or short-circuit in one of the 11,000-volt cables at Buffalo usually melts off but little lead at the sheath and does not have enough explosive force to injure the cable or its duct.