Shorted turns faults on synchronous rotor field windings

[genRman]

Can anyone suggest reading material or have some knowledge of investigating a main field shorted turns problem on synchronous machines?
I have a 3 phase synchronous machine, but am concerned that at some point during operation, it gets into a slip operating condition, such that it effectively pole slips, as would a generator.
I am considering the possibility of high voltage transient appearing across the complete rotor, either due to some action between the armature mmf & the now slipped rotor field mmf, or through premature commutation of the rectifier diodes at some point advanced of the natural zero volt commutation phase.
Any ideas / suggestions please?

 

[edison123]

genRman,

Is your machine salient pole or cylindrical (non-salient) generator ?

Is your generator running in island mode or is it running parallel with other machines or grid ?

What do you mean by pole slipping ?

If it is running in parallel (with other machines or grid), it stays running at the frequency of the system the machine is connected to.

If it is running in island mode, then if your prime over speed reduces, the frequency will fall down with corresponding compensation of voltage by your AVR, if the speed drop is not drastic.

For checking interturn field shorts in salient poles, AC drop test is used. You apply a single phase voltage (110/220/415 V) across the start and end of rotor field winding and measure the voltage drop across each pole, which should be equal in all the poles for a healthy rotor. A shorted pole coil shows the lowest voltage drop with adjacent pole coils also showing lower volt drops as compared to other poles.

For cylindrical rotors, the test is more complicated.

Also, note that if the field winding had inter-turn shorts, you will see increased vibrations with increasing field current.

 

[DougMSOE]

First, Never use an AC drop test. To evaluate this test there are no standards to hold the rotor to . Used a DC test with about 0.2p/u of the field current. (20%) Any dc field should be within +/-2.5% of the design value of the Vdc/pole. The other reason not to use the ac test is that you can get inductive coupling to the magnetic circuit AND the inductance of the synchronous field is far larger than the resistance you are trying to measure further clouding your efforts.
If the field is in reasonable shape (<=0.1p/u) equivalent poles shorted and you have proper excitation to the field, the rotor should turn at thr synchronous RPM. Slipping a pole is a very distructive episode to the machine.

 

[edison123]

Doug

I disagree with your view. AC drop test is universally used (refer Usbr practice codes) and is more definitive test than DC drop test. The very fact the field windings are highly inductive is the reason that AC drop test is more effective since even a few turns short will reveal significant voltage drops across the defective coil(s). The DC drop test measures only the resistive drop, which may not be significant with a few turns short.

Also, due to the high inductance of field coils/winding, the current drawn in AC drop test is negligible as compared to DC drop test.

I have been using AC drop test for more than eighteen years with correct prognosis of field faults.

 

[genRman]

Gentlemen,
perhaps I've miss stated my question!
My rotor - salient pole has indeed got shorted turns on it,detecting it is done through an AC resonance test with a comparator sometimes called a shorted turns indicator.
The rotor has 800 turns & so whilst rotational tests seperately excited do reveal a reduction in field resistance, the AC resonance test can descriminate down to a single shorted turn out of 80o turns, something which no other test has been able to achive to date throughout my own 22 years experience.

My question really should have focused more on the pole slip phenomenon, or any other source of transient voltage, developed across the main field. It is this that I am interested in.
I know that a pole slip causes a high zero to peak voltage to be developed across the rotor, in my case over our varistor protection device, say nominally 1400V, but would like some explaination of the phenomenon & any further theories where a similar or greater magnitude votlage transient might be developed.
Any potential theories?

 

[electricpete]

It sounds like you are looking in a lot more detail than what I understand. I will just mention some basic ideas that I think you are already familiar with.

Pole slip occurs when the load imposes a steady or transient torque exceeding pull-out torque. Machine pull-out torque can be decreased by reducing field voltage or reducing ac voltage, increasing susceptility to pole slip.

You could improve your resistance to pole slip in presence of momentary load torque pulses (impacts) if the inertia of the machine is increased (flywheel). Also possibly by selection of coupling (mechanical damping).

If the power system is robust, it probably doesn't affect pole slip (acts like infinite bus). If it is a small islanded power system, then swings of other machines during transient may affect stability of your machine during transient.

I suspect all of the above is elementary to you. But that's is about all I know on the subject.

 

[jbartos]

Suggestion: It could potentially be caused by electromagnetic irregularities. Visit

http://www.ab.com/drives/techpapers/ieee/04_1.pdf

for some; however, they are pertaining to induction motors. Spatial harmonics are common phenomena to induction and synchronous machines.

 

[jbartos]

Small encore: Visit

http://www.ofgem.gov.uk/dso/G75doc.pdf

for slipping protection, etc.
Also, any oscillatory interactions among synchronous machines can cause pole slipping.

 

[swgrmfg]

genRman:

In reply to your question about source of transients, we can say that one common cause is open circuiting the field anytime during operation. You need to examine your field circuitry carefully in order to ensure that the field circuit is never open.