Use of ramp test to expedite return to service of damaged stator winding

The fracture of a clamping finger resulted in major damage to the stator core and winding of a 13.8 kV 120 MVA hydro generator. Immediately following the fault the fire suppression water deluge system operated for a period of time. The polyester bonded mica insulation system of this stator winding is known to be vulnerable to tape separations in the end arm area. High direct-voltage ramp tests were used on four occasions to: ldr Identify stator coils needing replacement ldr Determine adequacy of hot air drying ldr Determine adequacy of drying under short circuit conditions ldr Monitor subsequent improvement in service The objective was to return the unit to service as quickly as possible in the expectation that the condition of the groundwall insulation would continue to improve. To date this approach has been successful.