Repair of a damaged 300 MVA machine

A stator ground fault occurred in a hydrogen cooled synchronous condenser rated 300 MVA. This was as a result of a small metallic object that had been left in the rotor during an overhaul. The object had entered the air gap and damaged the end arms of three stator bars, one of which failed. The OEM patched the three stators bars. A series of off-line electrical tests were performed to assess the adequacy of the repair. One patch failed and had to be reapplied. There was concern about possible damage to the stator core and the field poles. A robotic inspection vehicle and camera were rented to examine all 132 teeth of the core over its 3.95 m length as well as the poles. Areas were observed and photographed where there was visual evidence of possible damage. The inspection vehicle was used to move the sensing coil of our digital EL CID along stator core teeth where there was the most evidence of damage. The information obtained indicated that the damage was not sufficient to warrant removing the rotor. This decision greatly reduced the duration and cost of the repair. Upon return to service on-line partial discharge measurements were made from directional bus couplers as well as from stator slot couplers with the machine operating in air and in hydrogen.