Alternator rotor earth fault 1

[SA07]

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

 

[edison123]

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

Muthu

http://www.edison.co.in

 

[SA07]

Rotor IR Value= 1.87 k ohm

 

[edison123]

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

Muthu

http://www.edison.co.in

 

[Hoxton]

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

What do you mean by "dismantled the rotor"?

 

[crshears]

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]

 

[SA07]

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?

 

[edison123]

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

http://www.edison.co.in

 

[ScottyUK]

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.

 

[Hoxton]

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

 

[ScottyUK]

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.

 

[Hoxton]

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)

 

[ScottyUK]

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

This rotor certainly needs some TLC.

 

[edison123]

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

Muthu

http://www.edison.co.in

 

[SA07]

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.

 

[edison123]

@Randhir

Could you post a photo of the rotor here? Use the 'upload link' green button above the comment box.

Muthu

http://www.edison.co.in

 

[SA07]

 

[edison123]

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

Muthu

http://www.edison.co.in

 

[Sazirul]

It looks like Rotor is new!

Electrical Engineering | LED | Fuse Properties

 

[SA07]

These are insulation bandings over the end windings.

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