The design here followed is thoroughly sim ple and normal. At higher hydraulic head the .somewhat unusual turbines here used would be replaced by impulse wheels in the same situa tion; at lower head by considerably bulkier turbines, which would require a wider tail race and power house for their accommodation. In well up to very much larger capacities. In case the necessary width becomes too great to be conveniently covered by a single roof, separate roofs are placed over the wheel rooms and gen erator rooms and in mild climates the wheel cases and tail race may sometimes be left uncovered.
In case of hydraulic plants operating at rather low heads one is often driven to the use of vertical shaft turbines, which results in a somewhat different arrangement of power house from that shown in Fig. 1. In such cases the forehay usually comes fairly up to the power house wall instead of the water being received from a reservoir at the end of a pipe line. The generators like the water wheels are vertical shaft machines, the weight of the revolving fields being carried by a combination of water pressure and step bearing. Fig. 2 shows a cross-section through one of the four 6,000 horse-power units in a recent hydro-electric plant operating under a head of 49 feet. The design is an interesting one from the fact that the wheels have hut a single runner which operates at 116 revolutions per minute, while the spiral wheel case and its entrance and the discharge draft tube are smoothly molded in the concrete foundation of the power house, which is really built as a sort of annex to the massive forebay wall, and in the monolithic foundations, of which the wheel cases and the water channels are a part. The bus bar and switchboard galleries occupy in this structure their wonted position, but the power-house in terior is otherwise very simple, containing around, instead of containing, the hydraulic units.
It is necessary even in hydraulic plants to make the power station, as nearly fireproof as possible and hence brick, steel and concrete are the most suitable materials of construction. Wood, even if treated by some fireproofing process, should be sparingly employed. For merely the four great vertical-shaft units ris ing above the floor over the wheels, the water wheel governors and a few other auxiliaries. This power-house design is the latest type of those designed for the utilization of large pow ers at fairly low head. The hydraulic con struction has the advantage of combining very high efficiency with compactness and extreme simplicity. It is really a power house built floors, concrete, reinforced when necessary, is probably the best material. The trans
formers and switchboard are the danger points, from the standpoint of fire risks. The former should be placed so that even if they burn out no exterior damage will result. If, as is usual in high voltage plants, they are oil-insulated, provision should be made for dis posing of the oil harmlessly in the rare case where it ignites from an extreme short-circuit. Transformer oil is very difficultly inflammable, but it can be fired, and oil transformers should be guarded by pits with drains, so that the oil may not run about if it escapes from the cases. Generally the transformer cases are provided with drain valves from which the oil can be quickly run off outside the station.
The connections between generators and switchboard are generally made by cables laid under the floor in tile ducts, so as to keep the overhead space free for the crane. The higher voltage wires from the transformers should be taken out of the building by a very short route overhead or through very capacious ducts. In any case great care should be taken to locate all cables carrying heavy currents where a burn out of one will not involve others.
In very large stations, both hydro-electric and steam-driven, the switchboard is fre quently placed in a gallery overlooking the generator room. This position is in itself un desirable as adding enormously to the com plication of the wiring, but often becomes advisable. In such case the larger switches are not manually operated, but are worked by electric or pneumatic power controlled from the switchboard. The switches them selves are then located under or behind the gallery and are commonly oil insulated and enclosed in masonry or concrete cells. With three or four generators and one or two main lines to be fed the switchboard is sufficiently complex, but when in addition numerous feed ing lines must be controlled the complication becomes something frightful and a large amount of space is required for the switching apparatus, so large and so subdivided for safety that manual control is practically out of the question. In large high voltage power sta tions the transformers are commonly in a building entirely separated from the main sta tion and provided with an independent high voltage switching equipment, a procedure which probably adds nearly as many risks as it avoids. On the whole a plant with few units and rela tively simple equipment is likely to give the most reliable service at the lowest cost.