Generator Bearing Temperature rises with increase in Excitation

One of our 24MW,13.8KV STG was tripped due to the stator earth fault causing the blackout of the plant and resulting in the damage of the stator windings(U,W).
Our grid is and island grid with 3 STGs connected to the grid (2 of 24MW and one of 10MW)

The stator winding were repaired and one turn of each phase (U,V,W) was bypassed and new rotor was installed.
Rotor healthiness was tested through RSO test. However, stator windings was checked by megger test and Hi-Pot test at 17KV for 1 minute and was found ok.

At startup, we decided to check the generator through open and short circuit test. While performing open circuit test , the AVR voltages were increased manually with some steps. When the terminal voltage (Vt ) of the generator reached 11KV , the NDE (non-Driven End) Bearing Temperature increases to 425 C resulting in the tripping of turbine.

While, same test was repeated, and the same result was obtained but this time we decreased the excitation voltages which decreases the temperature of the NDE bearing protecting the turbine from tripping.

Kindly suggest what can be the real causes on the issues on the generator and what checks need to be taken before attempting another start-up??
 
What kind of stator earth fault is....
Is that covering 95%or 100% of stator windings..
Do you know why stator earth fault appeared..?
You can check protection relays and see if 95%or 100%of stator earth fault...
 
Moizfarooqui
Welcome to Control.com
1) What is the value of the earthing resistor?
2) When not running what is the motor carcass to earth resistance?
3) How old is machine?
4) Is it a Brushless or Slip-ring machine
5) If Stator-fault was the initial problem, why was rotor changed?
6) Was step-change in Stator-kV linear with respect to bearing-temp change?
7) Was step-change in Stator-kV linear with respect to excitation-current?
8) Was current in Earthing-Resistor noticed?
Regards, Phil Corso
 
Moizfarooqui
Welcome to Control.com
1) What is the value of the earthing resistor?
2) When not running what is the motor carcass to earth resistance?
3) How old is machine?
4) Is it a Brushless or Slip-ring machine
5) If Stator-fault was the initial problem, why was rotor changed?
6) Was step-change in Stator-kV linear with respect to bearing-temp change?
7) Was step-change in Stator-kV linear with respect to excitation-current?
8) Was current in Earthing-Resistor noticed?
Regards, Phil Corso
1) earthing resistor 0.3 ohms
3) 6 years
4) brushless
5) because the rotor was responsible for the stator fault as two of the bolts were found broken from rotor that caused stator insulation to get damaged.
6) NO bearing temp were at 60C when stator kv was at around 10 kv and drastically shoots up to 472C at 11kv
7) YES
8) NO
 
Moizfarooqui...
We can now rule out a mechanical problem.

I want you to carry out what I call the Radio-test. I've used it in the past to detect problems with induction motors having Open Rotor-Bar-to-Ring problems. It's in this forum's archives, sometime in 2013! But, it's very simple to do:

a) Use a small portable AM-radio!
b) Tune it "off-station"!
c) Raise the Generator Stator-kV so that NDE-bearing temp is higher than ambient, but less than trip-setting!
d) Hold Radio about a 2-3 cm from the NDE bearing!
e) If safely reachable do the same with DE bearing!
f) If an electrical current is present, regardless of magnitude, the radio will emit a garbled/scratchy sound, different than the off-station noise!
Regards, Phil
 
Moizfarooqui...
In 1980 I was in the EXXON Chemical Plant in Daharki. Wrote a 24 page Report on the plant's early problems.
New topic: You should do something about increasing the 13.8 kV system's Neutral-Grounding-Resistance!
Phil
 
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