Stator Core Repair – Cooling System – Generators Troubleshooting
Stator Core Repair – Large Electrical Generators Troubleshooting
Depending on the extent of the damage, the stator core may receive one of the following interventions:
• Minor repair Partial restack
• ing • Complete core replacement
The minor repair is usually limited to a defined small area of the core, typically to replace protruding laminations, or reshape laminations distorted by minor rubbing of the rotor. The repair is typically limited to the tips of the core where high quality of insulating varnish has to be injected between the laminations.
When the damage covers an extended area, the stator core has to be partially restacked or even replaced with a new core. For partial restacking, it is recommended to use laminations having the same magnetic characteristics (permeability, insulation coating) as the existing core to get a uniform distribution of the flux density around the core.
Stator Core Clamping
When the reddish powder from Iron Oxide is noticed during the inspection, we can suspect that this is the result of loose core laminations and/or loose wedges. In the first case, the experience determined that the powder will be spread out over a large area determined by the cooling air flow. In the second case, the powder tends to concentrate between the wedges and the iron.
In fact, after years of operation, the thermal cycling and vibration may lead to the abrasion and looseness of the laminations, eventually causing possible hot spots. Particular attention should also be given to the core compression bolts, through bolt insulation, and core compression fingers. This will avoid other detrimental effects on the coil insulation due to core hot spots, increasing core losses, vibration, and audible sound levels.
Typically, during the inspection, a 0.25 mm blade can be used to check the looseness of the laminations. It is a common practice that if the blade enters more than 25 mm, the core must be retightened. Care must be taken not to damage the coil insulation, or break the blade and leave the broken piece between the laminations. When necessary, the core should be retorqued to the value recommended by the manufacturer (typically around 150 psi of core pressure).
Cooling System – Large Electrical Generators Troubleshooting
Heat Exchangers Inspection
Turbogenerators have their air-water or hydrogen-water heat exchangers inside the machine casing. They should be regularly inspected for cleanliness and leaks, as the two major concerns are the clogged water tubes and the water leaking. The water ducts could be clogged due to corrosion and deposit of minerals, leading to poor heat transfer properties and reduced water flow. General mechanical integrity should be inspected to avoid vibration and the loss of bolts, with potential harm to the rotating machine.
Cooling System Rehabilitation
The cooling system can be improved to support higher uprating levels, as it will be explained in the next section. This objective is achieved by modifying the existing thermal circuit configuration using one or more of the following actions:
• Altering air guides, shrouds, and openings.
• Adding fans when applicable.
• Retrofitting or changing the coolers; improving the cooling water flow; cleaning; changing the tubes and fins material.
• Cleaning the clogged stator core air vents.