The Position of Central-Station Lighting.

Uncontrolled financial speculation, aided by the stringent clauses of the Electric Lighting Act of 1882, have been a great deterrent to the extension of old or the introduction of new schemes for the supply of electricity to the public in the same manner as gas. The President of the Board of Trade, replying to a question in the House of Commons, said that, “since the passing of the Electric Lighting Act of 1882, fifty-nine provisional orders and five licences had been granted to companies, and fifteen provisional orders and two licences to local authorities. He was not aware that, in any single case where these powers had been obtained, they had been exercised.” Up to the present time no company supplying electricity has been under the necessity of applying for compulsory powers, and has either obtained permission from the local authorities to take up the streets, or has carried the electric mains over the houses, and, regardless of the question of overhead wires, has depended on wayleaves granted by the too-confiding householder, who has no idea that his roof is supporting a cable weighing 1¾ tons to the mile.

An amendment of the Act of 1882 has passed both Houses without hindrance, and has received the Royal assent. It provides that in the case of Provisional Orders the period after which the undertaking may be bought up by the Local Authority shall be extended from twenty-one to forty-two years, and that portion of the previous Act which referred to the compulsory purchase of the undertaking by a local authority at the end of the term has been altered, and more favourable terms given to the electric companies.

On the Continent, and in the United States, where each city may be said to legislate for itself in matters relating to the general welfare of its citizens, the electric lighting industry is in a very different position, and central-stations are either established or about to be started in every important town. There were, in 1887, 121 Edison central-stations alone, supplying over 323,000 incandescent lamps, and paying dividends from 6 to 14 per cent. The Westinghouse Company, who use a transformer system which is a modification of the Goulard and Gibbs, have a hundred stations, maintaining 191,000 lamps, although the first Westinghouse plant was put down only three years ago. The progress in the United States is so rapid, and there are so many successful applications of central-station lighting, that the subject becomes too large to be even summarised, so that it is proposed to treat in the following pages with some of the principal installations on the Continent and at home.

Travellers abroad are accustomed to find electric lighting installed in the most out-of-the-way places, especially in Switzerland, where water power is abundant and is utilised to generate electricity, so that in small hamlets arc lighting is often employed, and the visitors to the local hotel will find it lit throughout by electricity. Electric light stations in England are, with one exception, small in comparison with those on the Continent. The most important is that at the Grosvenor Gallery, London, which has increased from small beginnings until it now supplies 20,000 glow lamps on sixteen circuits, the total length of which is seventy miles. The next largest, which have been in practical work for some time, are the Brighton and Eastbourne stations, from which small installations of glow and arc lights are maintained in various districts of the two towns. That the question of cost or trouble, and the annoyance of machinery when erected in a dwelling-house, do not altogether prevent the adoption of a superior light, is clearly proved by the increasing number of householders, who, after waiting in vain for electricity to be brought to their doors, have set up the plant necessary to produce it themselves, and find no practical difficulty in doing so. There are also many important public works where electric light has been exclusively adopted. For instance, at the Tilbury Docks there are 1350 glow and 80 arc lamps, distributed over an area of 300 acres, and including the lighting of an hotel, dock sheds, warehouse, signal-boxes, and offices. The London, Chatham, and Dover station at Victoria has also been electrically lighted for the past three years, the current being obtained from a central-station, which was erected for the purpose of supplying electricity to the Victoria district, and for which a provisional order was obtained. This, however, has since been abandoned, although £16,000 had been expended on plant and buildings by the promoters, who preferred to postpone the scheme rather than to submit to the onerous 27th clause of the Electric Lighting Act. Another still larger installation has been put down to supply electricity to the Paddington station and district of the Great Western Railway, as far as Westbourne Park. It embraces an area of sixty-seven acres, and is lighted by 4115 glow and 98 arc lamps. The system adopted is that designed by Mr. J. E. Gordon, and has now been successfully worked for some time; but the many accessories which are introduced, such as telephones, telegraphs, and indicators, make it complicated in comparison with gas, or even with the ordinary electric light systems. The current is generated by two dynamos, each weighing 45 tons, and having revolving magnet wheels 9 feet 8 inches in diameter, 22 tons in weight, a third machine being kept in reserve. These dynamos are separately excited, and produce alternating currents. The electricity is led to a large switch-board for distribution throughout the district by means of five sub-stations; and from this board there branches a double system of mains, which run everywhere side by side, one-half the mains being connected to the first machine and one-half to the second, so forming an excellent arrangement for the prevention of total extinction of the light. The mains running to the sub-stations are on the divided system, which is introduced for the purpose of saving copper, as in a solid cable the loss of pressure is greatest when the full number of lamps is on, and decreases as the lamps are extinguished. With the divided main system it is intended to follow out Sir William Thomson’s formula, which equates the value of the loss of head, and the interest on the saving on the copper. If for a certain main this formula shows that a fall of 20 per cent. is the most economical condition for working, then, since by the divided main the pressure can be kept within a variation of 2 per cent. at the distant end, it follows that a considerable saving can be effected over an ordinary solid main. Special arrangements are adopted at Paddington to keep the pressure constant, a fall of potential being allowed for; thus at the engine-house the pressure is 150 volts, in the passenger station it is 120 volts, and at Westbourne Park it is 100 volts. The arc lamps are fed by the same mains, and are arranged two in series.

A small installation at Kensington Court, erected two years ago, for the purpose of supplying the houses in the immediate neighbourhood, has rapidly developed, and underground mains have been led in many directions from the station, and a constant service of electricity is provided for by means of secondary batteries. As this is the first practical exposition of the secondary batteries’ system of distribution, it is proposed to describe the installation under that head. Central-stations are also at work in Liverpool, Leamington, Taunton, Exeter, and there are also five large installations nearly completed in London, besides the Kensington Court station, all of which will probably be in full swing before the end of the year.

Electric Lighting from Central-Stations is now practically carried out on five different methods.

CLASS I.
Supply by Secondary Generators
or Transformers.

The problem of electric lighting from central-stations is comparatively easy if an area can be obtained immediately surrounding, and within a short distance of, the station, with a right of way for laying down the electric mains direct. This happy state of affairs has not yet been attained, consequently the generating station has more often to be in an out-of-the-way corner of the district to be lighted, and it would be financially impossible to use low-tension currents with correspondingly large mains. The difficulty has been overcome in several ways by the use of high-tension currents in the mains, and has led to the adoption of secondary generators or transformers of electricity, which by induction supply a current of low potential in the house-service. The first to make this plan a practical success was Mr. Goulard, to whom the honour of the introduction is due, although his claims as the first inventor have been recently upset by the decision of the Courts.

The relative economy of the supply of electricity by the use of a transformer is clearly shown by the following diagram, [Fig. 7]. A, B, and C give proportionately the area of cross section of the total mass of copper necessary to supply 5000 16 candle-power glow lamps situated at a mean distance of 4000 feet from the dynamo. A refers to the Edison “three-wire” system, working at a potential or electrical pressure of 200 volts with a fall of potential or loss of energy in the distributing feeders of 10 per cent., average distance from dynamo 4000 feet—the usual conditions on which this system is worked. B shows the size of conductor required for the same work in an installation based on the transformer system, potential 1000 volts, allowing 2·5 per cent. loss in the supply mains—only one-fourth as much as in the direct Edison system at the same average distance from dynamo. If this loss were increased to 10 per cent., and made equal to that in the direct system, viz. 10 per cent., the size of the conductor would be that shown at C.

Fig. 7.

Fig. 8.

The graphic diagram, [Fig. 8], demonstrates what the relative cost would be with each of the three conditions just named.

Mr. Goulard’s first practical application of the secondary generator in this country was the lighting of the Underground Railway Stations, in 1883, from Edgware Road to High Street, the generating dynamo being fixed at the former place. These experiments, which were made by Mr. Kenneth Mackenzie, attracted considerable attention at the time, but it was not until the report of the jurors to the Turin International Exhibition in 1885 was published that companies were formed to instal the Goulard system for lighting an extensive district.

Fig. 9.

The principle underlying all transformers is that of the induction coil invented by Ruhmkorff in 1842, but described by Faraday in his “Experimental Researches,” published in 1831-2.

[Fig. 9] is a diagram of the ordinary induction coil; on a central core is wound a short length of thick wire called the primary, and again over this is wound a greater length of fine insulated copper wire which forms the secondary coil. On sending a low-pressure current from the generator round the thick wire, a much smaller high-tension current is induced in the secondary. A contact breaker is employed to make and break the current, or, as in the early instruments, a commutator may be used to produce the alternations. When used as a transformer the action is reversed, that is, a high-tension current is passed through the primary coil, which is composed of a wire of small sectional area, the high-pressure main connected to the dynamo also being small as compared with the distributing cable leading from the transformer, which, acting as a step-down induction coil, converts the electricity into a safe working pressure.

Fig. 9a.

[Fig. 9 a] shows the arrangement of the two separate and complete circuits. D is the dynamo, P the primary coil, S the secondary, and L the lamps arranged in parallel.

It is hardly necessary to go into the technical details of the various improvements which have led up to the modern type of transformers; they are summarised by Mr. Kapp into two classes:—

No. I. are those in which the copper coils are spread over the surface of the iron core enveloping the latter more or less completely; and No. II. in which the core is spread over the surface of the copper coils forming a shell over the winding.

Fig. 10.

The original Goulard and Gibbs secondary generator was of the core transformer type, it had an open magnetic circuit and cores which could more or less be inserted into the coils so as to regulate the electro-motive force of the secondary circuit. The transformers were constructed with a number of copper disks or washers; these were placed alternately primary and secondary in a vertical frame, through the centre of which an iron core was fixed, consisting of a bundle of straight iron wires. The core was movable in the coil in the manner of the well-known induction coils, and thereby the electro-motive force of the secondary current could be adjusted. In their latest design the coils are circular in plan and rectangular in section and are surrounded by groups of U-shaped soft iron stampings slipped over from both sides and held together by two circular cast-iron plates with a central bolt. The magnetic lines of force pass through the core, in at one end and out at the other, and are then more or less disseminated through space; it will thus be seen that the path of the lines lies partly in iron and partly in air, and, since air has about seven hundred times more magnetic resistance than iron, it is evident that the number of lines created with a given current must be considerably smaller than would be the case if the path of the lines contained iron only. This constitutes the improvement in the Zippernowsky-Deri-Blathy system of transformer, which has coils similar to the Goulard, but with the iron of the core applied in the form of a ring-shaped shell, surrounding both coils completely. This arrangement can best be described by comparing it to a Gramme armature, in which the copper and the iron have changed places. Imagine what is usually the core in an armature replaced by the primary and secondary coils, and, instead of the winding of insulated copper wire, wind iron wire around the coils, and one of these transformers is the result. In consequence of the lower magnetic resistance of the Class II. transformer, as compared to that of Class I., the electrical output obtainable with equal weights of copper and iron appears to be considerably greater in the former apparatus. Professor Feraris, of Turin, has published some of the results of comparative experiments made with Classes I. and II. and finds that the coefficient of induction is 3·6 times as great with the latter as with the former. There are many varieties of transformers in the market which closely resemble each other; one of the most practical is that designed by Kapp and Snell, of which [Fig. 10] is an illustration. U-shaped stampings form the shell and the cores are laid in the double trough. The cover of these troughs is formed from the metal removed from the interior of the stampings, and the whole is held together in a cast-iron frame so arranged as to allow air to circulate through the core and round the coils. The price of these transformers is about £4 per indicated horse-power, and the efficiency under the best conditions, namely, with full load, is, according to Professor Ayrton, as high as 96 per cent., and when it is doing one quarter of the full work 89 per cent.