Alternator stator winding failure 4

[SA07]

Hi
Our alternator 950 kW 400 Vac stator has failed. The protection relay Micom tripped by Restricted Earth Fault.
Up to now we can see the damaged part of the stator winding as attached picture.
There was no disturbance on the utility network or internal fault on any motor. We have restarted the plant with the boiler operating now.

There are 4 cables per phase coming from the stator winding which is connected to the output L1,L2,L3
One of these cables on each phase is short with the earth. The environment is full of dust particles. The dust particles are residue of coal ash after it is burned in the boiler. There were dust inside the stator. The alternator is air cooled with no water refrigerant on top.

What might be possible cause of the stator winding failure?

 

[SA07]

Plz see attached picture

 

[Hoxton]

Contamination of generator or motor windings is never good!

Get your local repair company to repair the winding, if there is no other damage.

You need to agree a programme of removal of the generator on a regular basis for washout and baking (heating in an oven) etc.

What was the duty, standby or base load?

What is the prime mover? Diesl engine? Steam turbine?

Would it be practicable to move the generating set to a clean area?

Could clean air be ducted to the area of the generator?

Was the generator just starting up as the fault happened?

 

[SA07]

It is a steam turbine. It supplies slightly more than auxiliary load.
It is very difficult to relocate.
Environment is full of dust.
Fault occurs while plant was in normal operation.
Can a water coolant on top of alternator be an economical solution?
Any other solution?

 

[waross]

Air to air on top of the generator?
Water cooling will be more effective, but you should monitor for leaks.
Replacing dust with leaking water may be counter-productive.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[dArsonval]

After the wound rotor is extracted from the stator, there may be more "telling"
evidence for further root cause failure analysis.
Based on the one photograph, the failure appears to have affected 3,
possibly 4 slots confined to one coil group from the same phase.

There could be further slot damage visible in the stator bore once the rotor is removed.

Without a few more photos of potentially affected areas, it's a tough call to make a good guess.

It looks like a cascading failure spanning more than one slot. The cause of this failure is possibly
hidden from view until the rotor is removed.

John

 

[catserveng]

To get a more conclusive root cause determination we'd need a lot more info. But some general comments may help you.

Corrosive environments like coal ash, cement dust, H2S, etc, will eventually take its toll on the generator windings, especially in higher humidity areas. In general, at least based on my experience, random wound (or "mush wound" as called by some) stator windings seem way less tolerant to extreme environments or continuous service than form would stators.

TEWC (Totally Enclosed Water Cooled) generators exist, but are specially designed and I have never seen an air cooled generator successfully field retrofitted before (have seen it tired a couple times, none were successful long term).

Some generator manufacturer's offer filter boxes for air inlets, and some users have fabricated their own in many places I have worked. But the filter design must allow for proper air flow to the generator and has monitoring/protection in the event the filters plug. The most successful systems I've seen use stainless steel mesh filter media with differential pressure monitoring across the filter media.

I have, in some cases, used ducting systems to draw cleaner air from outside the generator space, but these modifications require a duct fan to assure proper air volume available at the generator cooling air inlet. So that fan can become a source of failure. At a couple sites we installed a manually operated louver assembly that could be opened in the event of fan failure, drawback was it pulled "dirty" air into the generator until the fan could be fixed.

So you have a few choices,

Get your generator repaired, if the stator iron was not damaged. If you go this route, than as mentioned by Hoxton above, you need to implement a more aggressive maintenance policy, that would include regular testing and inspection. If your primary contaminant is dust and moisture is not an ongoing issue (like coastal or high humidity environments), then regular cleaning (vacuuming and blowing out with dry compressed air) can be helpful in improving service life.

Find a replacement air cooled generator with form wound coils and a marine grade insulation package, possibly with a factory designed and installed filter box. These are becoming available on the surplus market due to decline in offshore oil and gas industries. Not sure where in the world you are but you may find an available unit of the correct frame size as a surplus new, reconditioned or low hour takeout.

You could try to buy a new replacement unit, but currently, at least in my part of the world lead times are huge right now and costs are very high.

You may also be able to find an available replacement TEWC generator, but even with the same base frame size, the enclosures and housing make them quite a bit larger than air cooled units, and most of these units were designed for marine and offshore applications and require fairly low cooling water temperatures. So going that route may require extensive modifications to your site.

If you stay with an air cooled unit, you may consider a ducting solution, but that requires a pretty careful look at your site and generator space to see if it is a viable solution. I have successfully done duction solutions on several sites over the years, but they do add additional maintenance requirements and possible points of failure.

Hope that helps, MikeL.

 

[edison123]

Going by your photos, I would say the dust contamination is not that severe. The winding failed due to a)aging which is normal or b)poor winding quality if it's a machine less than 15 years old.

Almost all of the DG's of that size are open ventilated. If you feel dust is a major problem, you could try to put in a silent enclosure which would derate the generator due to reduced cooling.

Muthu

http://www.edison.co.in

 

[RRaghunath]

DG set room can be kept closed with proper sealing to prevent dust entry and ducting clean/filtered cool air in. This I suppose would be cheaper solution.

 

[Hoxton]

I agree with cat serveng

Base load sets like this should have closed air circuit generators.

It is too late now to retrofit an external cooler.

This is an engineering problems which must be managed. The electrical fault may not be the result of contamination... This time!

 

[SA07]

The winding has been cleaned. That is why it may not look that dirty. we are waiting for insurance instruction to proceed further.

 

[esperantes]

I had been doing many insurance surveys for the gas and petrochemical industries. Be well prepared for the visit of the insurer. In many countries, the insured is obliged to provide the insurer with accurate information regarding the machine and equipment to be insured and the insurer bears all responsibility for erroneous information. Machinery insurance provides protection for sudden and unforeseen physical loss. The terminology unforeseen or rather accidental denotes damage which the insured neither remarked in time nor could have had anticipated. The insured is normally obliged to carefully monitor machine operation and to respond forthwith to prevent or avoid machine failure. Honestly, from an insurer perspective ,the damage looks predictable to me and therefore does not seem to qualify as an indemnifiable event under machinery insurance. The insurer would look at your maintenance plan and would like to hear from you how you maintain/monitor the insulation of the alternator. Best of luck.
Esperantes.

 

[zlatkodo]

My first guess: the reason is contamination + aging.
But, there is also a very good predisposition for premature failure.
It is the shape of an old coils.
If the coil comes out of the slot at such an angle, there is a good chance to damage or weaken the slot insulation already during coil-inserting .

A diamond shaped coils should be made instead (see picture below).
If the dimensions of such a winding are chosen correctly, the width and height of the end winding will be less than before.

Motor Repair Support

 

[edison123]

@zlatkodo

The original winding looks like a multi-pitch winding while yours shows single pitch coils. I doubt such ground clearances, while preferred, can be achieved in multi-pitch coils.

Muthu

http://www.edison.co.in

 

[zlatkodo]

It can be better even if it is a concentric winding.
Such a size and shape of coils plus hitting with a mallet is a recipe for a bad job.
After all, the winding does not have to be concentric.
If diamond-shaped coils are used, a mallet is almost not needed.
Winding Design and Analysis

 

[edison123]

@zlatkodo

Agreed. In our shop, except for 2 pole machines, we convert all multi-pitch windings to single pitch windings and we hardly ever use any mallets.

Muthu

http://www.edison.co.in

 

[SA07]

Hi

The stator has been rewinded and alternator put into service. After about 2 weeks in service, we see there are some dust inside the alternator.
The dust is from coal ash. There is a box around the alternator with 2 filters as shown in attached picture.



We want to improve the system. There are holes at the cables entry and the drive end shaft. Can we remove the box completely and put a filter directly on the opening at the alternator non drive end where it sucks in air? The air passes over the winding and comes out at the drive end opening. Is there another way to improve the cooling?

We cannot run the alternator at max. load due to temperature rise now.

The supplier has confirmed that we cannot add an air cooler or water cooler on top of the alternator.

 

[edison123]

They look like like louvers to me than filters. The enclosure will definitely reduce the airflow needed to cool the machine. With that type of basically open enclosure with no filters to prevent dust, you might as well remove it for better cooling.

For open type machines, the dust itself is not a problem, it's the accumulation of dust over windings that is a problem. Any filters you add to the air intake will reduce the air flow and hence the cooling of the machine.

Properly sealed air to air or air to water machine heat exchanger is the only way to prevent dust entry into the machine.

Muthu

http://www.edison.co.in

 

[TugboatEng]

Is it not reasonable to supply the enclosure with a fan ducted from a clean source or filter?

 

[SA07]

There is a filter behind the louvre as attached.