Fig. 25.
The Kensington Court installation has been previously quoted as an example of what promises to be one of the most successful methods of distributing a constant supply of electricity through a large area, a description of the station may therefore be interesting. The accompanying elevation, [Fig. 25],[5] shows the unpretending design of the building, and the very compact arrangement of the generating machinery and batteries. When the illustration was made the plant consisted of one Willan’s single-crank triple-expansion engine in combination with a Crompton dynamo provided with vertical inverted single magnets, the output being 250 ampères at 140 volts when running at 500 revolutions per minute, the steam pressure being 160 lbs. on the square inch. A complete duplicate plant has already been installed, and three more sets of engines and dynamos are shortly to be erected. The draught from the boiler is led downwards by an underground flue, with the object of economising the very limited space as much as possible. As a rule, the dynamo and accumulators are used in parallel, the current enters and leaves the regulating cells by the same contact, in other words, there is only one switch which serves for charging and discharging the batteries. This switch has nine contacts, so as to give nine degrees of regulation of the light; when the dynamo and accumulators are working together, the lights are parallel with either 41, 42, or 43 cells, according to the amount of charge in the cells and current required, while, when the dynamo is out of circuit, the lights are worked off, 50, 51, 52, or 53 cells. The current passes through the usual measuring instruments, and each main conductor is protected by safety fuses mounted in a Hedges duplex cut-out. The accumulators are of the Planté type, but instead of being plain lead are sawn out of ingots which are cast porous on the Howell process. Each cell contains 35 plates, 8 in. × 8 in., and, as each plate when fully formed is said to be capable of yielding five ampère-hours per pound of lead, the cell has about 600 ampère-hours total capacity. In the event of a serious breakdown the whole of the work would fall on the accumulators, which could furnish a steady current for perhaps an hour or more; and herein lies the novelty of the arrangement. For the first time we have an accumulator put in not only as a fly-wheel to the whole system and to give the advantage of supplying current throughout the day and the small hours when the engine is not running, but also to act as an actual reserve. The routine is as follows:—the dynamo will start charging the accumulators a few hours before dusk; for a short time after lighting hours commence, the dynamo alone will supply sufficient current, but later on the demand will gain on the dynamo, and a certain portion of the discharge will be from the accumulators. At eleven o’clock at night the engine will be stopped and the accumulators will alone supply the demand for the rest of the night. In the small area occupied by the station there is ample room for a plant of six times the present capacity, and it is intended to erect sub-distributing stations at points at the outskirts of the district where accumulators to act as transformers will be fixed, which will be charged by a special main with a current of 500 volts, the outgoing wires from the sub-station taking electricity at the usual E.M.F pressure for incandescent lamps in houses of 100 volts.
Thirteen candle lamps are used in the district, having been found to be more convenient than 16 or 20 candle-power, the 13 candle is obtained for 36 watts, or 2·75 watts per candle. The price charged to consumers is 8d. per Board of Trade unit, or equivalent to gas at about 4s. 7d. the 1,000 cubic feet. Meters on the Aron plan, [Fig. 17], are used, a card being supplied on which the readings are entered exactly similar to the method adopted with gas. The service mains terminate at the meter, where the company fix for their own purposes a double pole switch of the author’s design, [Fig. 26], which enables both wires to be disconnected, a spring shut-off, marked S S, prevents the switch being left partly on.
System of Distribution.
The mains from the Kensington Court Station are laid underground in a culvert 18 in. by 12 in., which is built with brickwork and cement under the pavement. A double conductor of flat copper, 0·25 square inches section, is stretched from shackle insulators attached to iron bars, which are firmly built into the culvert; the continuity of the circuit is provided by means of stranded wire, which connects each section; the flat copper rests on the top of porcelain insulators, fixed on vertical iron pieces, which are built into the floor. Connections with the sewers are left for drainage, and six surface boxes are provided for every hundred yards. Where house connections have to be made, the branch wires are united by soldering to the bare copper mains. For crossing under the streets a heavily insulated cable is employed, and is led through cast-iron pipes.
Fig. 26.
Until a larger amount of mileage is actually at work, it is difficult to express an opinion as to which is the cheapest and most efficient method of laying conductors in the streets. The relative cost of two plans tried at Kensington Court—the insulated and the bare cable in a culvert—was given by Mr. Crompton in the following Tables, [No. 1] and [No. 2], which are taken from a paper read before the Society of Telegraph Engineers and Electricians on April 12th, 1888.
[Table No. 1] refers more particularly to what is known as the Callender-Webber system of using bitumen concrete, which is compressed into blocks or cases usually about 6 ft. long, 8 in. by 5½ in. section, having two-inch holes through which the insulated copper cable is led.