Hydro Generator High Voltage Stator Windings: Part 3 - Stator Winding Slot Support Systems

Abstract

Appreciable radial electromagnetic forces are exerted on the slot sections embedded in the stator slots (Calvert, 1931). On rapidly loaded hydro generators used for peaking duty, there may be significant differences in axial and radial thermal expansion between quickly heated stator windings and the stator core (which takes a longer time to heat up), imposing significant compressive stresses on winding insulations and possibly causing insulation creep, with resulting reduction in size (Grabner & Kofler, 2001). If a stator winding is allowed to become loose within the core slots, the radial electromagnetic forces will cause winding slot section vibration. The slot radial electromagnetic forces are acting at twice the electrical frequency of the machine, so that loose winding slot sections may experience up to 8.64 million cycles of force in any day of full operation. If even the smallest winding slot section looseness is present, the slot electromagnetic forces will cause radial movement/vibration of winding at double frequency, causing rapid abrasive wear between winding insulation and stator core laminations. If corrective measures are not implemented, rapid insulation failure will result. The slot support systems consisting of slot wedging system, radial slot strips and slot section side packing are designed to prevent in-slot winding vibrations, and to ensure positive slot section grounding for prevention of in-slot partial discharges.