Over the past decade, since industry deregulation, the typical utility power plant has lengthened the maintenance intervals of its major systems to minimize downtime and costs. Many plants schedule spring and fall outages based on the inspection and maintenance needs of turbines. In the process of juggling maintenance activities and scarce dollars, generators and exciters may get less attention than prime movers. Consider the following "baker's dozen" list of typical failures when developing your checklist for maintaining generators and exciters.
Migrating rotor turn insulation
Problem. Across the industry, a number of generator rotors have experienced the problem of turn insulation migration. The insulation between the copper turns slips out of position because the adhesive bond between the insulation and the copper breaks down. Short-circuits develop as bare copper is exposed. Coils with shorted turns run cooler, and this causes a temperature imbalance. If enough turns develop shorts, rotor vibration can increase to unacceptable levels.
Prevention. Inserting a borescope under the retaining rings can help identify existing insulation migration. Removing the retaining rings can confirm the extent of the problem (Figure 1). Completely rewinding the rotor with Class F insulation and using a proven adhesive to hold it in place is the best long-term solution. Partial rewinds have proven less successful.
1. Creeping out. Exposing the end windings of this GE Frame 7's generator by removing its retaining rings reveals that insulation has crept out from between the turns. Courtesy: National Electric Coil
Partial discharge
Problem. Partial discharge (PD) failures have been experienced by some newer air-cooled generators after fewer than 10 years of service. PD occurs internal to the coil if the insulation is manufactured with the presence of voids. PD external to the core often occurs in the gaps between the coil and the stator core or in the end turns if the coils are in close proximity. Because the breakdown is partial, the condition does not create a full electrical ground. However, over time, PD can cause enough cumulative deterioration to create a full ground, tripping the generator off line. If PD activity is severe, it can totally destroy the resins and binders of slot filler materials. PD activity is often observed as a discoloration, usually white, of the coil insulation surface (Figure 2).
2. Telltale sign. Evidence of partial discharge activity is the white discoloration on the coil insulation surface of this stator winding of a 9,490-kVA, 600-rpm generator from the UK's Brush Electrical Machines Ltd. In this case, the PD likely was caused by the lack of a gradient coating in the end turn cell bend area. Courtesy: National Electric Coil
Prevention. PD activity can be monitored while a generator is on-line. But doing so requires inserting stator slot couplers under the stator slot wedges to track the magnitude and frequency of the discharges. Merely detecting PD activity alone isn't as beneficial as trending it over time, because different machines have different baseline values. For example, if PD levels double over a six-month period, a generator should be opened up and visually inspected. Once PD has damaged a unit, rewinding it with properly designed coils or bars may be the only permanent solution.
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