Yesterday we have faced Rotor Earth Fault stage -2 operation in Unit-1 followed by Stage-1 Alarm (2 Times) with in 10 Minutes. After tripping we have investigated thoroughly (Resistance Value rotor earth greater than 40 Mega Ohm, Brush and Cable Condition etc.). In first instant we have some doubt on reduction of rotor resistance due to moisture as unit was synchronized on yesterday 01:00 hrs after 50 days long shutdown.

After searching details in DCS trend we found following sequence of events:

1. Control Valve fluctuation
2. Resulted in Load fluctuation due to variation in valve position
3. To overcome above situation AVR excitation changes frequently,
4. Frequent sudden changes resulted in drop in Resistance across Field winding.
5. This has caused the Rotor Earth Fault alarm (Two Times) and consequently tripping of the Turbine.
The present value of Rotor Earth Fault Relay is here under
Alarm - 20 Kohm for 5 Sec, and
tripping - 5 Kohm for 5 Sec.
Based on above phenomenon we have conclusion on frequent change in Excitation system followed by load/CV fluctuation, which have finally converted in Rotor Earth Fault.
for your review & comment