A sufficiently concentrated fall for power de velopment may occur in a stream naturally, or it may be created artificially. Fall is obtained artificially by ( I) a dam. or (2) a canal or con duit, or (3) by both combined. Where a dam is built the fall which naturally occurred in the stream from the dam up to the head of the pond which it forms is concentrated at the dam, and form and smooth as possible with just enough grade to convey the required volume of water to the power house slowly and smoothly. The stream having a much steeper grade than the canal, the difference of level in the canal and stream. or. in other words the head, pro gressively increases proceeding downstream. Where the water is conveyed from the dam to the power house through a closed conduit, the principle is the same as where a canal is used, fee akhough the conduit may lie down near stream level, the water would rise in a vertical open pipe. connected to the conduit at any point, to a height nearly equal to the level at the entrance to the conduit. The difference between the height of the water at the source and the bright to which it would rise in such a pipe is the bead lost by friction in the conduit. In the earlier days of water-power development in the United States it was considered impracticable to utilize a head of more than about 30 feet on a single turbine. Accordingly, where the fall ai a stream was greater than this at a given location, the total fall was subdivided between different canals, and the water drawn from the level supplied turbines which dis the second or next lower level, etc.
This type of canal development was success fully utilized at Holyoke, Mass., Cohoes, N. Y., and elsewhere, but it often happened that mills on lower levels did not require the full quantity of water discharged from the upper levels, and a large waste would occur. One of the results which is being brought about by modern ad van;ed methods of power development is the substitution of single hydro-decide stations for such canal systems, thus conserving a consider able portion of the available power.
Turbines are usually set at some height, sever more than 25 feet, above the bottom of the fall from which they derive their power. The utilization of the power of the water in falling through a height equal to the distance from the centre of the turbine down to the Irsel of the tailrace is effected by means of the draft tube. The water in falling through the draft tube creates a suction head, which in creases the pressure or power of the water operating the turbine exactly as if the turbine were placed at the foot.of the fall, thus render ing the full use of"the bead practicable. Plac ing the turbine above the tan water has many advantages; among others electrical generators can he directly connected to horizontal turbines so placed without danger of being flooded dur ise high water. Theory requires that in order to utilize the power of a stream of water to the fullest degree, the water must be disch:rged back into the stream after its use, with the least possible velocity. It is never possible to dis charge the water from a turbine wholly without selocity. The velocity of discharge can be much further reduced by passing the water through a ly expanding draft tube after it leaves gradually The velocity of the water enter in the draft tube is mostly converted back into draft head. which in turn increases the effec tive bead and power of the turbine.
One of the most difficult and at the same like the most important factors in successful safer power development for the purpose of speaerating electricity is the matter of speed regulation.
la many hydro-electric plants, sudden and large changes in load on the hydraulic turbine, or in the amount of power required, take place. To generate electricity for commercial use, the toltage of the current must remain practically ennstant. In order that this shall be the case, the speed at which the generator is driven by tie hydraulic turbine must be regulated and unattained as nearly constant as possible. If the taxes regulating the supply of water to the fortune were set at a constant or fixed position, then each time that a change in load to the turbine occurs, there would be a corresponding change in the speed of the turbine and the gen erator which it drives. The problem of regu lating the speed of an hydro-electric power unit is much more difficult than that of regu lating a steam-engine or steam-turbine driven electric generator, for a variety of reasons.
Steam is a highly expansive elastic fluid. In order to regulate the speed of the steam engine, it is only necessary to partially open or close the ports or valves admitting steam to the engine cylinder. This is accomplished in plain
slide valve engines by the familiar fly-ball governor, and in more advanced types of steam engines by closely related mechanisms. In order to regulate the speed of an hydraulic turbine with changing load, it is necessary to open or close the turbine gates. The weight of these gates and other connected parts which must be moved is enormously greater than the weight of the valves to be moved in regulating a steam engine. Thus the problem of quickly over coming the inertia of heavy weights is in In the case of the steam engine, the steam pressure at the boiler is usually nearly constant. In the case of an hydraulic turbine, the head operating the turbine may vary within relatively wide limits at different seasons of the year. Thus when the head is reduced by back water or otherwise, the gate openings must be wider to maintain a given load and speed than at other times. In hydro-electric plants supplied with water by means of closed conduits or flumes, especially if the head is high, sudden reduction in the gate opening of the turbine produces a pressure wave in the penstock simi lar to the water hammer effect produced when house faucets under considerable pressure are suddenly closed. This pressure wave tends to produce a sudden large increase in the head and pressure, and consequent .ov erspeeding of the turbine at the instant when the gates are closed, or shortly after. This difficulty does not exist in the case of steam engine regula tion. These problems have been solved by the use of modern governors for water wheels, to such an extent that the degree of speed regulation obtainable in hydro-electric plants closely approximates that attained in the best of steam driven power stations. The principle of operation of a water wheel governor is es sentially the same as that of a steam governor. An incipient change of speed operating on a pair of fly•balls similar to those of a steam engine governor operates what is known as a pilot valve. This is a small valve, easily moved. and consequently sensitive to changes of speed. which admits oil under high pressure to a valve or system of valves known as the •relay." Oil under high pressure, flowing through the secondary valves, actuates an hydraulic piston, which is articulated to the water wheel gates. Thus the work of opening or dosing the gates is accomplished by oil under high pressure from an accumulator, which is kept constantly supplied by means of an auxiliary pump. An other important feature of a water wheel governor is the compensator. If controlled by the fly-halls alone, the opening or closing of gates could take place only as rapidly as the speed changes. In that event, speed regulation iur hydraulic turbines would he relatively slow and inefficient, furthermore its value might be largely vitiated by the effect of surges. The purpose of the compensator is to immediately set the governor in operation approximately to the full change of gate opening which will be required for a given change of speed or load without waiting until the full change of load or speed occurs. Governors with compensators have been developed to such a degree of perfec tion that they will anticipate with but small error the total effect of a sudden change in load on the generator almost the instant that the change in load and resulting change in speed begins to take place. Slight errors in the first adjustment are quickly corrected by successive smaller movements of the governor mechanism, as the gate openings become adjusted to the new load conditions Pondage and storage may be distinguished in that the former is derived directly from a pond formed by the dam supplying the power plant. It is usually used only for the purpose of sup plying a quantity of water greater than the average flow of the stream for the day, dur ing hours when the load or demand for power is maximum. The pond is then allowed to re fill during hours of the day when the supply exceeds the demand. When such a pond is drawn down to any degree to supply power. the available head is accordingly reduced. and there is generally an economic limit of the amount of the draft from pondage, since the drawing down of the pond to a greater extent reduces the head to such a degree that the power output is not increased by any further Fig. 6 illustrates a typical water wheel gov ernor or actuator of the oil pressure relay com pensating type. Such an apparatus is almost human in its operation. in apparently antic, paling pending load changes, the moment they begin, and correcting the gate opening in ad vance of the complete load change so as to maintain a constant speed.