TGM® has developed a process to install a magnetic flux probe without removing the rotor from the generator. A flux probe can detect the degree and location of shortened turns in generator rotor windings, down to a specific pole and coil, without taking the generator off line. An installed flux probe allows generator specialists to monitor the accumulation of these shorts and develop remediation plans and timelines to forestall load degeneration or forced outages.
Shorts are commonly caused by the dielectric breakdown of the turn-to-turn insulation system within the main field windings. This failure can result from movement and fretting of the conductors caused by coil foreshortening, end-strap elongation, or inadequate end-turn blocking. Metal particles from erosion or rubbing (copper dusting) can also form new and undesirable pathways between turns.
The magnitude and location of turn-to-turn shorts play heavily in whether or not they will even be noticed. Indeed a great many shorted rotors have operated without issue for years. It is those rotors with turn shorts nearest the poles that are most likely to become thermally sensitive; this caused by asymmetrical heating, rotor bowing, and associated vibration. Shorts can also produce unbalanced magnetic forces, which will increase mechanical stresses. This combination of thermal and mechanical stresses can create more shorts and an accelerating pattern of degradation. On-line monitoring can measure this degradation and signal the increasing need for remediation before a forced outage occurs.
Previously, the rotor had to be removed to gain access to install a flux probe. TGM® has developed special tooling and procedures for installing flux probes with the rotor in place. Placement of the probe is no easy task. Imagine installing an instrument several inches inside the generator air-gap, under conditions so restrictive that you cannot even reach in and touch the inboard side of the retaining ring. TGM® has developed a pneumatically actuated device which inserts the flux probe sensor into the air-gap and presses the sensor against the top of the core iron tooth. The pneumatic tool expands, using the OD of the rotor as a backstop and holds the flux probe in position while the epoxy cures. Amazing and ingenious.
For more information, or for a proposal to install a flux probe in your generator, please contact your TGM® Regional Account Manager at 800-226-7557