Rotor Thermal Sensitivity experience on 143.4 MVA hydrogen-cooled turbo generator with direct-cooled rotor design

Tambak Lorok Power Station operates two blocks of combined cycle units. Each block consists of three gas turbine-driven generators with 143.4 MVA base capacities. Those six generators serve the Jawa-Madura-Bali (Jamali) power grid as the start-stop unit. The generators use a hydrogen gas as their cooling medium. The hydrogen gas also responsible to cools the rotor winding which has a direct-cooled rotor design. In the early of 2008, one generator in the station, namely GTG 2.3 generator, experienced a high vibration event. Dominantly high vibration values have been measured on generator's bearing and the highest value was on bearing No. 5. This high vibration event was usually happened during peak-load operation time, 07.00 to 09.00 PM, when the unit was loaded with base-load MW and high MVAR. Actions have been taken following this high vibration condition, including Rotor Thermal Sensitivity (RTS) Test and several analyses on vibration test and operation data. From these actions, the GTG 2.3 Generator has been diagnosed for experiencing a RTS condition. Base on this condition, a short-term operating decision has been made to maintain a safe operation until the next unit planned outage. Maintenance decision in the next planned outage, by pulling out the rotor, has been made to have a closer look for rotor condition. The root cause of the RTS problem in GTG 2.3 Generator has been found, which was blocked cooling holes/unsymmetrical cooling on rotor. Furthermore, necessary actions have been taken to repair the problem and putted back GTG 2.3 generator into normal service condition.