Sajal Kumar Harmukh with adviser K. Haran
This research aims to diagnose arcing observed in the retaining rings of power plant turbogenerators. The main culprit behind arcing appears to be secondary currents induced on the rotor surface during unbalanced faults and operation. Two plausible hypotheses for the arcing in the shrink-fit region are being tested. The first states that the contact resistance between the ring and rotor is sufficient to produce voltage for the media breakdown. The power supplied to a hydrogen atmosphere should be at least 250 V. To test this hypothesis, mock-up experiments were performed in Talbot Laboratory where we determined the contact resistance between an actual retaining ring sample and a rotor-forging sample taken from the generator (see Figures 10 and 11).
The resistance values obtained indicate that even a very high fault current (say 30kA) will not produce the voltage required for breakdown of the medium. The second hypothesis, that arcing results from a make-and-break contact, implies it can happen at a much lower voltage. Contact breaking is an outcome of pulsating torque and the heat due to secondary currents. To test this theory, a finite element analysis is being carried out to see the voltage that develops when a contact is suddenly opened. We will try to create arcing in the lab by applying a sudden impulse to one of the test samples while passing high current through it. This research is supported by the Electric Power Research Institute.