Alternator rotor earth fault
Alternator rotor earth fault
(OP)
Hi
Our alternator 11kV 60 MVA rotor has a rotor earth fault. The alternator exports energy on the national grid through a step-up transformer. There was lightning and a 3 phase fault on the grid. The alternator islanded but tripped after a few seconds due to high vibrations. Vibrations has rose to 349 micron. The relay rotor earth fault was activated. When we megger the rotor the insulation is low.
The stator, rectifying diodes, excitation winding have been checked and are good. We have dismantled the rotor. There is no sign of burnt winding.
Recording of grid fault in relay SEL 311 is attached.
What might have caused this rotor earth fault? Thks
Regards
Our alternator 11kV 60 MVA rotor has a rotor earth fault. The alternator exports energy on the national grid through a step-up transformer. There was lightning and a 3 phase fault on the grid. The alternator islanded but tripped after a few seconds due to high vibrations. Vibrations has rose to 349 micron. The relay rotor earth fault was activated. When we megger the rotor the insulation is low.
The stator, rectifying diodes, excitation winding have been checked and are good. We have dismantled the rotor. There is no sign of burnt winding.
Recording of grid fault in relay SEL 311 is attached.
What might have caused this rotor earth fault? Thks
Regards






RE: Alternator rotor earth fault
Muthu
www.edison.co.in
RE: Alternator rotor earth fault
RE: Alternator rotor earth fault
Muthu
www.edison.co.in
RE: Alternator rotor earth fault
What do you mean by "dismantled the rotor"?
RE: Alternator rotor earth fault
CR
"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]
RE: Alternator rotor earth fault
Any idea what is the cause of the rotor earth fault?
Can lightning on the grid cause the rotor earth fault?
RE: Alternator rotor earth fault
When there are serious grid disturbances/short-circuits, the generator rotor suffers the most voltage spikes. That's why the new/rewound rotors are tested for ten times the rated voltage as against twice+1 for the stators.
Muthu
www.edison.co.in
RE: Alternator rotor earth fault
RE: Alternator rotor earth fault
Ps it is an ac generator, not an alternator
RE: Alternator rotor earth fault
RE: Alternator rotor earth fault
The OP has admitted to an earth fault and vibration.
This could be the result of two separate earth faults or one earth fault and an inter turn fault (possibly all at the same location)
RE: Alternator rotor earth fault
This rotor certainly needs some TLC.
RE: Alternator rotor earth fault
Muthu
www.edison.co.in
RE: Alternator rotor earth fault
Can lightning on the grid cause the rotor earth fault?
Can a fault in the avr/ excitation circuit cause the rotor earth fault? The alternator is of make Jeumont, France. The AVR is also from Jeumont. model Jiren.
I have attached the excitation wiring drawing.
RE: Alternator rotor earth fault
Could you post a photo of the rotor here? Use the 'upload link' green button above the comment box.
Muthu
www.edison.co.in
RE: Alternator rotor earth fault
RE: Alternator rotor earth fault
Muthu
www.edison.co.in
RE: Alternator rotor earth fault
Electrical Engineering | LED | Fuse Properties
RE: Alternator rotor earth fault
The rotor is not new. it is in service since 2007
RE: Alternator rotor earth fault
RE: Alternator rotor earth fault
Muthu
www.edison.co.in
RE: Alternator rotor earth fault
If there's a shorted turn caused by winding movement due to centrifugal forces or thermal expansion then it may not be apparent until the rotor is at operating speed and temperature, with the fault going open-circuit at standstill and/or as the metals contract back to their rest positions. That will make fault location more awkward to say the least.
RE: Alternator rotor earth fault
This was the exciter on a very small machine but the principles of differential expansion and/or centrifugal forces are the same.
We had an exciter on a small machine that would fail once the machine came up to operating temperature.
We took the rotor into the shop.
It tested good.
The shop heated the rotor in an oven.
When hot, it tested good.
The shop was hesitant to rewind a rotor that tested good and suggested that the problem may be with the field or exciter.
We installed the rotor and ran it.
It failed again.
I did a brush null test.
This is applying AC to the stator winding and measuring the AC voltage appearing at the brushes.
The brushes are then shifted until the null point (zero voltage) is found. That is the best starting setting for the brushes.
We found that the voltage varied as we rotated the rotor. This is a clear indication that there is a fault causing the windings to be asymmetrical.
We took the rotor back to the shop and described the test and the findings.
The shop agreed that the test did show a fault in the winding.
The oven heated the rotor evenly.
When the rotor was in service the heat was generated in the windings and conducted to the core.
This caused heat expansion of the windings relative to the core.
The differential expansion happened only in service, not in the oven.
I don't know if this test will be feasible in your situation, but I hope that the information will be of use.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Alternator rotor earth fault
It will require a fairly big DC source to bring that winding up to operating temperature, and even if such a test was set up there would be second-order effects such as a thermal bend caused by heat rising to the top of the rotor forging causing it to expand more than the bottom, in turn causing the rotor to develop a slightly arched shape which will introduce additional stresses not seen in normal operation.
RE: Alternator rotor earth fault
How about installing the rotor and putting a suitable voltage on the stator field?
The rotor could then be shorted to bring the wire temp up quickly. Remove the short and measure the induced voltage over one revolution.
Do you think that we could get the windings hot enough, quick enough to avoid too much temperature rise in the iron?
Turning gear would be an asset.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Alternator rotor earth fault
Your test seems to be for DC machines (which we normally use to check interpole polarity without dismantling the machine). Don't see how it will work in this case.
Muthu
www.edison.co.in
RE: Alternator rotor earth fault
The voltage applied to the stator would be much less than rated voltage.
Bill
--------------------
"Why not the best?"
Jimmy Carter