On 08/11/12 UNIT#2, SGTPP tripped due to fault in outgoing feeder and the generator swings around 50 sec. After synchronization shaft vibration was found very high which come down to normal value with next tripping but the generator field current became higher around 300 Amps compared to earlier similar load.
RSO (Recurrent Surge Oscillograph) test on Rotor was carried out by Power Test Asia both in standstill condition and at various speeds during rolling condition and impedance test confirm inter turn fault in rotor winding.
They suspect following probable causes for inter turn fault:
1) Cracking/damage in the inter turn insulation.
2) Shifting of inter turn insulation in over hang portion.
3) Over heating of the inter turn insulation due to some blockage in the ventilation ducts.
4) Electrical shorting of turns due to some metallic dusts/chips present in the winding.
The suggested procedure for location of the fault is as follows:
1) Remove the retaining rings from the rotor shaft.
2) Carry out voltage distribution tests for the turns of the rotor windings. In this case first the voltage drop across each pole is to be checked. The pole with the lesser voltage drop will be having the inter turn fault.
3) Turn to turn voltage for all the turns in both the poles is to be measured and recorded for identification of the defective turns.
M/s Eron service opined that the unit can be run with the inter turn fault as long as shaft and bearing vibration are well within the limits. However they suggested that the excitation current should be limited to 90 % of the rated current.
Generator continued on load around 8 months with higher field current. No problem as such was faced except VAR control. (Normal full load field current 1350 amps and during fault it took upto 1850 amps. Rating of AVR is 2075 amps).
After dismantling Inter turn fault on overhang part of exciter end was observed. Some copper strip of winding got bend and shorted with other strip, somewhere it melted slightly and insulation removed.
Manually copper bar/strip straightened and reinsulated.
Retainer ring i.e end cap of both end of rotor were dismounted with induction heating and simultaneous pushing by hydraulic jack.
Before assembling of rotor testing like dc resistance, high voltage(1.5 kv) and impedance test before synchronization. All gives satisfactory result.
ELCID-(Electromagnetic core imperfection identification) Stator core imperfection test carried out and found stator condition healthy. Finally boxed up and synchronized to full load.