National Electric Coil
W Howard Moudy, director of operations for NEC, rarely misses a user-group meeting or an opportunity to speak about the practical aspects of electric generator operation, inspection, maintenance, and problem-solving. At the 2018 meeting of the 501F Users Group, he presented on the top issues affecting generator reliability in this fleet, including: stator spark erosion (SE), stator partial discharge (PD), stator endwinding looseness and resonance, and rotor pole-to-pole failures.
SE occurs when there is intermittent loss of contact between one or more stator bars and core iron. Because bar vibration is at fault, this deterioration mechanism sometimes is called vibration sparking. Moudy said SE is unique to manufacturers that do not use side ripple springs to maintain side pressure on the coil. He added that even if flat side filler is installed properly, insulation shrinkage over time creates additional gaps allowing SE to occur.
A borescope inspection is, perhaps, the best way to see first-hand if there’s any damage and to trend damage over time. Inspection frequency, Moudy said, depends on how the machine is operated and its condition. Access to the back of the core for borescoping is through panels (photo 1); core vents are accessible from the back of the core (2). Level 3 SE deterioration—slag/rough surface appearance over the vent duct opening is shown in (3).
Possible corrective actions: flat filler replacement with ripple springs, epoxy injection, or stator rewind with the latest preventive measures.
Partial discharge exists on virtually stator windings and is a good telltale for several common deterioration modes—including SE. With good PD sensors and monitors useful data can be captured. However, interpreting the significance of those data can be challenging. The method selected for PD repairs, Moudy said, depends on the location and extent of damage. Repairs are likely an ongoing effort. A rewind with a coil of better design and a superior insulation system should be considered.
Endwinding looseness, which can be identified by dusting, greasing, and broken ties, is a common aliment in the 501F fleet. Damage inflicted by endwinding vibration can be severe. Replace or repair loose or compromised components promptly.
Moudy noted that resonant conditions have been found in many of the endwinding components of generators coupled to 501F engines. Windings should be tuned to maintain their natural frequency below 110 Hz or above 135 Hz by changing mass or stiffness by use of additional ties or spacers, extra blocking, etc. Use a bump test to verify results.