. Development and electrical distribution of water power . former case, the case being filled with oil,and circulating water through the pipe coil and thereby abstractingthe heat from the oil. Fig. 48 shows the last-named type withthe casing removed. The coil of pipe for the circulation of coolingwater is clearly shown. The air-blast transformers are generally used in the sub-stationsat the end of the line, while the oil-filled transformers, cooled by acoil of water-filled pipe inside the casing, are used at the powerstation for raising the transmission voltage, it being usually the casethat p
. Development and electrical distribution of water power . former case, the case being filled with oil,and circulating water through the pipe coil and thereby abstractingthe heat from the oil. Fig. 48 shows the last-named type withthe casing removed. The coil of pipe for the circulation of coolingwater is clearly shown. The air-blast transformers are generally used in the sub-stationsat the end of the line, while the oil-filled transformers, cooled by acoil of water-filled pipe inside the casing, are used at the powerstation for raising the transmission voltage, it being usually the casethat plenty of water for cooling purposes is available at the powerstation, while little or none is obtainable at the sub-station unlesspurchased from a water-supply company at prohibitive rates. At the power station, the cost of maintaining the water cir- TRANSFORMERS 95 culation is nil, as the head on the water-wheels will also force waterthrough the cooling coils. When the transformers are placedabove the level of the head water, a siphon arrangement can be. Fig. 48. used if the maximum lift of the water is not over ten feet above thelevel of the head water and the head itself is twenty feet or more. The oil in the transformer case acts also as an insulator pre-venting break-downs and re-insulating any puncture that mayoccur due to abnormal voltages from surges on the line. It alsoprolongs the life of the insulating materials used in the construc-tion of the coils, so that its value is twofold. 96 DEVELOPMENT AND DISTRIBUTION OF WATER POWER As in the case of generators, transformers have a certain effi-ciency and regulation and these are dependent on the amounts ofcopper and iron used in their construction and, therefore, on the cost. Good transformers have efficiencies ranging from 96 to 98per cent., depending on the size and design. The regulation isfrom 3 to 7 per cent. The desirable efficiency is a commercialquestion and determined in the same manner that the efficiencyof the
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