A blackout occurred on August 14, 2003 that affected portions of the northeastern United States, including a hydroelectric generating facility, which itself lost electrical power. The loss of electrical power initiated an automatic shutdown of two hydroelectric generators. During the shutdown, an unregulated flow of water entered one of the hydroturbines, resulting in an over-speed incident.
EDT was asked to examine individual components of the hydroelectric generating unit to determine the scope of damage and the reparability of certain components. The arrangement of the hydroelectric generating unit was unique in that the generator, speed reducer, and turbine output shaft were contained in a submerged water-tight chamber.
EDT Analysis and Findings
The turbine was disassembled and examined. Fortunately, there was no damage. However, the activities involving the disassembly, cleaning and reassembly of the turbine were included in the scope of damage.
The speed increaser, or gearbox, allowed the slow speed turbine to drive a synchronous shaft. Non-destructive examination of the bull and pin gears revealed no surface-penetrating indications of damage.
However, examination of the bull gear shaft revealed surface damage at a bearing location. A proposed repair recommendation involved machining the shaft to accept a smaller bearing, thereby removing the damaged surface area. Implementation of the repair option, however, would have required design verification for the new smaller shaft diameter. Instead, the damaged area was repaired by machining, conducting a weld build-up of the machined area, and then heat treating and re-machining the shaft to the original profile. This repair option also proved to be the more economical choice: the savings amounted to $20,000 without compromising the structural integrity of the shaft.
Examination of the generator revealed that there was no contact between the rotor and stator during the incident. Although the generator shaft was not damaged, there was evidence of bearing damage caused by overheating.
A recommendation was made to replace the damaged bearings. Activities involving disassembly, bearing replacement, and electrical testing of the generator were included in the scope of damage.
The installation included a wicket gate assembly to regulate the flow of water through the turbines. The wicket gate was examined and determined to be undamaged by the incident. Here again, activities involving disassembly, cleaning and reassembly were included in the scope of damage.