Field Preheating for Large Turbine Generators [includes discussion]

Preheating of the field windings of large turbine generators is a procedure for warming up and expanding the coils somewhat during the start-up operation by heating them with direct current while the machine is at a relatively low speed. This is done to reduce the mechanical stresses in the coils and the abrasion effects on the coil insulation resulting from thermal expansion. This paper presents an analysis of the various forces which act on the coils in normal service, including a shortening effect in the cylindrical field body caused by the rotational stresses. It proceeds then to show how the coil stresses and insulation abrasion can be reduced by preheating. The procedure recommended is based on the transient heating characteristics of the rotor and requires a relatively short period of time which can normally be co-ordinated with practical turbine starting cycles. With suitable equipment, preheating can be carried out quite easily and gives promise of prolonging the useful life of generator field windings. This paper is concerned only with fields employing conventional ventilation as used for many years. In the future, preheating may well be applied to fields employing direct cooling, at least, in cases where this improved cooling is capitalized to the point where the temperature rise of the winding is comparable with present practice. However, the preheating cycle for directly cooled fields will probably require some modification on account of the more rapid heating of the winding.