Evaluation and maintenance testing of generator stator winding insulation

For the insulation of high-voltage windings of electrical apparatus two fundamental insulation systems and manufacturing methods have been in use during many years. In the U. S., a continuous hand wound mica tape insulation was preferred on the slot and coil end portion, which was impregnated with asphalt in vacuum in several operational steps. In Europe, to the contrary, many manufacturers insulated their machine windings with mica tubes on the slot portion, which were produced by wrapping on and pressing of shellac or asphalt micafolium in hot presses, and mica and varnished-cloth tapes on the end winding. In both types of insulation mica splittings have proved successful as a supporting component of the thermally, mechanically, and electrically high stressed slot insulation. On asphalt insulations difficulties arised from insufficient adhesivity of the bonding agent at excessive temperature rise thereby permitting displacement of the insulation. An improvement had become the more urgent as a higher reliability of the insulation was required with increasing machine rating.