Large transformers are cooled by an air blast supplied by motor driven blowers or by water pumped through a coil of pipe which is immersed in the insulating oil of the transformer. The large size oil insulated, water cooled transformers are used on circuits of 33,000 volts or more. In water turbine plants, the water may be piped to the transformer under pressure and the pump omitted which cuts down the cost of operating. Air blast transformers usually have a damper or shutter for air control.
Fig. 2,778.—Marine portable transformer station on Los Angeles Aqueduct. The view shows three 20 kva. Westinghouse out door transformers installed on a float, 33,000 volts high pressure; 440 volts low pressure; 50 cycles.
Ques. Explain the use of reactance coils in sub-stations.
Ans. In order that the direct current voltage of the ordinary rotary may be regulated by a field rheostat, which calls for a corresponding change in the alternating current voltage, a reactance coil is provided between the low tension winding and the converter.
Without such a reactance the maintenance of the same voltage at full load as at no load involves excessive leading and lagging currents and consequently excessive heating in the armature inductors, unless the resistance drop from the source of constant pressure is small, or the natural reactance of the circuit high.
Ques. What is the effect of weakening the converter field?
Ans. A lagging current is set up which causes a drop in the reactance coil.