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Alternator rotor earth fault

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



RE: Alternator rotor earth fault

What is the rotor IR value? You can test for ground with 230 V lamp test.

Muthu
www.edison.co.in

RE: Alternator rotor earth fault

(OP)
Rotor IR Value= 1.87 k ohm

RE: Alternator rotor earth fault

I suspect rotor has earth fault. It could be under the retaining rings.

Muthu
www.edison.co.in

RE: Alternator rotor earth fault

I agree with Edison123, under the end caps is a weak point.

What do you mean by "dismantled the rotor"?

RE: Alternator rotor earth fault

Minor point: post is a better fit in the Electric Motors & Generators Engineering forum.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

RE: Alternator rotor earth fault

(OP)
By dismantled the rotor, I mean we removed the rotor from the stator to be able to check visually if there is any damage on the winding of the rotor.

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

In 2 pole rotors, you pretty much cannot see the winding. Only a small portion of the end winding is visible under the retaining rings.

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

The rotor sees very large forces during a fault as the machine load angle rapidly changes relative to the grid. These forces act upon the conductors: within the rotor slots they are well controlled and the force is transmitted into the rotor forging, but at the rotor ends there is limited support for the winding and more scope for winding movement. Movement generally results in insulation failure given enough time. My guess is that your fault will be on the rotor end, close to where the bars leave the slot.

RE: Alternator rotor earth fault

I agree with Scotty. Also you may have two rotor earth faults, the resulting unbalanced magnetic field causing excess vibration. One rotor earth fault should not cause an unbalance and may go undetected - unless there is a rotor earth fault detector fitted.

Ps it is an ac generator, not an alternator

RE: Alternator rotor earth fault

A shorted turn would also give rise to vibration due to magnetic field asymmetry. You certainly appear to have at least one earth fault based on the IR readings, whether or not you also have a shorted turn will take a bit more investigation.

RE: Alternator rotor earth fault

I am happy to stand corrected!

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

No, no, you're correct - a double earth fault could certainly cause vibration, I was just offering another possibility. smile

This rotor certainly needs some TLC.

RE: Alternator rotor earth fault

How old is this machine? The rotor insulation could have died due to aging.

Muthu
www.edison.co.in

RE: Alternator rotor earth fault

(OP)
The machine is in operation since 2007. It is a 4 pole 1500 rpm.
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

@Randhir

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

(OP)

RE: Alternator rotor earth fault

Did it have metallic retaining rings over the end windings? I see only the insulation bandings, one of which is cut?

Muthu
www.edison.co.in

RE: Alternator rotor earth fault

(OP)
These are insulation bandings over the end windings.

The rotor is not new. it is in service since 2007

RE: Alternator rotor earth fault

So the steel retaining rings have been removed prior to the photograph being taken? I can't imagine anyone building a rotor of that size and not needing retaining rings in the design, even on a four-pole machine where the rotational forces are much lower than on a two-pole design.

RE: Alternator rotor earth fault

Scotty - I am guessing the diameter of rotor around 1.3 m. At that dia and speed and power, it has to have steel retaining rings. Insulation bindings not gonna cut it. OP has to post more info to help.

Muthu
www.edison.co.in

RE: Alternator rotor earth fault

Well if you're right then that makes it nearly twice the diameter I'm guessing. smile I reckon about 800mm. Still way too big to avoid the use of steel rings, at least in the machine designs I'm familiar with.

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

An anecdote to illustrate a problem evaluating an intermittent 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

I can well imagine it Bill. The difference in expansion coefficients for copper and steel is about 30% or so, plus the copper is normally the hotter of the two.

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

I realize that spinning a machine that big for a test may be unrealistic.
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

Bill

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

Yes. I agree. I forgot that you need a commutator. It may work to develop the differential heating.
The voltage applied to the stator would be much less than rated voltage.

Bill
--------------------
"Why not the best?"
Jimmy Carter

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