account of power houses would be incomplete did it not take notice of sub-stations. The modern power house is al most always fitted with alternating current gen eraors which transmit current either for gen eral distribution or for conversion into direct current for supplying electric railways or the old direct current lighting systems. Such con version must probably be regarded as a make shift, but at present a necessary one. The high voltage current from the power house is trans mitted through cables or over aerial lines to the sub-stations, and there the voltage is lowered by reducing transformers to the amount appro priate for the synchronous converters. The sub-station must, therefore, accommodate the reducing transformers, the converters, regu lating apparatus, switchboards for both alter nating and direct currents, and sometimes a storage battery and its accessories. The structure, therefore, becomes practically a power house for general service, less only the prime movers. Fig. 4 shows in eleva tion a sub-station for city lighting and power service which is a masterpiece of compact ness. It is a modern steel building two The same principles of location and subdivision of units hold for sub-stations as for power houses in general, with the advantage that the location is not dependent on water and fuel supply. Sub-stations for the receiving ends of power transmissions for general service differ from those just described in the lessened im portance of synchronous converters. The ordi nary distribution from a high voltage transmis sion service is very largely by alternating cur rents, and is accomplished by substations de signed accordingly. The high voltage current is received in large reducing transformers which in number, size and location are to be treated as so many alternating current generators fortu nately exempt from the necessity of rotation. Regulating apparatus is an important feature of stations of this class, and being inductive can be applied with very small loss of energy to the exact regulation of voltage on individual feeders.
stories above grade and with a basement for the cable entrances and exits. The main floor contains, as Fig. 4 shows, arranged in order, the high-tension bus and main switches, the three-phase transformers, the regulators, and the rotary converters, of which there are four, each of 1,000 kilowatt capacity. The same room contains the booster set for charging the batteries at a variable voltage somewhat higher than that of the rotaries, and the switchboard, also a blower set for ventilation. Large aper tures are left under the rotary converters and transformers so that a liberal air supply may be drawn in from below and exhausted by the blower. The second floor contains the storage batteries and their special switching arrange ments. On account of possible leakage of acid from the batteries the floor construction is pe culiar. The basis is tile arch over steel beams. Then comes a layer of concrete coated with asphalt, then a floor of hard-baked brick, glazed all over, laid with narrow interstices which are finally filled with hot asphalt.
Sub-stations for railway service do not so commonly have the storage battery, and are gen erally less compactly arranged, but after about the same general plan. The sub-station shown here covers only 3,150 square feet for its 4,000 kilowatts capacity. Ordinarily the switching ar rangements in a sub-station are so intricate as to require a rather liberal allowance of space.
For arc light service constant current trans formers are very widely used, taking energy from the lower voltage side of the reducing transformers. This service requires a special switchboard, and is generally set apart in the sub-station. Owing to the absence of rotating machinery in any considerable amount the sub station for general service is compacter and requires less in the way of foundations and special construction than the sub-station of Fig. 4. Otherwise the principles of location and design are unchanged.
Out Door needs of high voltage transmission have evolved in recent years an interesting and important form of sub station which is coming into very wide use. One of the main difficulties of high tension practice is the large space required for switch ing apparatus and the great difficulty of getting high voltage wires into and out of buildings while retaining adequate insulation. The re suit is that a sob-station building and equip ment rises to very burdensome cost in case the output required is not large. There has, there fore, been evolved the outdoor sub-station de rived by natural process of growth from the transformer mounted on a pole, and supplied with the necessary auxiliaries for protection and switching. To meet these requirements weather proof apparatus has been designed which can be safely left in the open under all climatic conditions. The common form taken by the outdoor sub-station is a concrete platform to bear the weight of the transformers and above it a quadrilateral of steel or wooden poles braced together and carrying switches, cut-outs and other auxiliary apparatus, as well as serv ing to support the incoming and outgoing cir cuits. In stations of the larger capacities elec trolytic cell or similar lightning arresters, and oil switches form part of the platform equip ment, while in the smaller stations air-break switches with very long swing and protected by horn-gaps, and horn-gap lightning arresters often take the place of the more expensive apparatus. Fig. 5 shows in section and plan a typical station of this kind and of medium capacity. Here the transformers are of the usual oil-filled type, cooled by natural draft, and the lightning arresters are of the elec trolytic form installed on a platform by them selves. The switching equipment is of oil switches carried on a platform like that which supports the transformers. Above these con crete bases for the heavy apparatus rises a latticed steel structure well braced together and supporting the busses, transformer connections and ingoing and outgoing lines. These last as well as the busses are borne by disc insulators carried in tension between the supporting mem bers. In larger installations it is not uncom mon to find the high-tension equipment in the open and the low-tension switches and appara tus in a suitable adjacent shelter. The purpose of the outdoor sub-station in every case is to cut down the heavy expense of housing high tension circuits and apparatus, and so success ful has the expedient proved that a large pro portion of all service sub-stations on the great transmission systems are now of this form. It is needless to say that they are protected against interlopers by thorough fencing in, and are otherwise treated with the respect due to theil. voltage.
Louts BELL, Consulting Electrical Engineer, Boston, Mass.