Case Study - 34 MW generator, was not generating sufficient reactive power. Checked at FSNL, PMG current is 12 A (typically 4A). Checked all points, found main rotor IR = 0 MÎ©.
Rotor threaded out & DC voltage drop test conducted on rotor (DC to winding, checked drop between winding & ground). Rotor coils = 12. Large drop occurred between 2nd & 3rd coils, +ve to –ve changeover occurred at the 6th coil. Boroscope inspection showed melted copper between 2nd & 3rd coils under the turbine end retaining ring.
Next, passed 250 A DC current through the rotor body & checked drop between winding & body. Null point was detected between the 2nd & 3rd coils about 700 mm for the turbine end retaining ring. We have now started the repair activity.
Other observations: Both bearings have significant pitting marks on them, forming an axial band across the bearing lower halves. Rotor winding resistance has gone down from 0.235 Î© to 0.116 Î©. Bearing pedestal insulation is healthy.
1. Do we have a single or double rotor earth fault? Can we expect a fault at the 6th coil also?
2. Have the coils been shorted too? Or is the lower resistance due to the earth fault?
3. Why did the bearings get damaged by shaft current? As these are white metal bearings with no prox. probes, how does one detect this problem in advance? I have seen high amplitude, high frequency peaks in case of anti-friction bearings. Would these also show up for plain bearings?
4. A shaft grounding brush was provided & was found healthy. Why did it not take care of the shaft current? Could the earth fault be responsible?
5. Could electrical signature analysis have some early indication of this problem? Has anyone experience in monitoring generator rotors with it? We have an EMPATH unit that could be tried I guess. It did not pick up anything in the stator current & voltage when we checked two months back.
I would appreciate your views on this matter.