Ground Wall Insulation
Early detection of deterioration of insulation can help in extending the life of the machine. Dusting or powdering of the insulation along the slot wedges or in the ventilation ducts may indicate damage to the insulation by mechanical abrasion. This powder should not be con- fused with the reddish powder which is caused by core problems, or copper dusting (which occurs due to fretting of the rotor winding when the machine is on turning gear). Another white or gray powder is caused by partial discharge. This powder is only found in bars and coils near the line ends of the winding. This powder should not be confused with the one caused by abrasion which is found throughout the winding.
Other signs of mechanical distress are debris at slot exit, stretch marks and cracks in sur- face paint in the slot area or in the mechanical supports in the end winding area. Thermal aging is indicated normally by discoloration or undue darkening of the insulation surface.
Electrical effects are normally indicated by carbonized tracking paths to grounded components. External partial discharge, which normally occurs in line end coils, is confirmed by a gray or white powder. Chemical analysis of the dust will show a high percentage of salts.
Rotor Winding
Mirrors and boroscopes are used to perform rotor inspection without disassembly. In some cases, it may be necessary to disassemble the rotor partially by removing the retaining rings and some wedges.
Turn Insulation
The location of the turn faults in the rotor cannot usually be determined by visual means without some disassembly. Turn faults in the end windings become visible after removing the end caps. Those in the slot become visible by removing the wedges and lifting the turns.
Faults caused by copper dusting can be verified by small copper particles in the slots and vent ducts.
Slot Wedges and Bracing
The movement of slot packing under wedges in gas-cooled rotors can be detected without disassembly. This problem can be identified by examining the gas exhaust holes in the wedges to see if packing has been moved to block the flow of cooling gas out through them. The problem is also indicated by rotor thermal unbalance.
The rotor should be disassembled if there is evidence of early signs of deterioration in slot wedges.
Stator and Rotor Cores
Severe overheating and melting may occur at the surfaces of laminated cores due to insu- lation faults. These can easily be detected by a visual examination. Faults that occur in the slot region are normally hidden by the winding and slot wedges. These are normally indi- cated by signs of burning in the vicinity of the insulation and wedges.