Coil tests of partially conducting filler materials for generator stator coils

Power factor “tip-up”, i.e. increasing power factor with increasing voltage, is commonly observed in generator stator coils and is usually attributed to partial discharge activity in the coil insulation system. A major contributor to partial discharge activity is the discharge of voids in the insulating Roebel filler material used in the coil top and bottom. In measurements on aluminum test bars, the authors have previously shown that use of partially conducting materials results in a significant reduction in the partial discharge and lower power factor tip-up. More recently, they have extended this work to coil sections and full-size coils. Comparison tests were conducted on full-size coil sections using an insulating Roebel filler and a partially conducting Roebel filler based on poly(pyrrole). Significant reduction in power factor tip-up resulted when using the conductive Roebel filler. The tip-up was related to the coil construction, and to the conductivity of the Roebel filler material