Turbo Alternator Rotor bars arcing

[data74]

We've got a 38 year old Brush Electrical Turbo alternator with the following equipment.

Control & Protection

• GE MULTILIN SR489, differential protection system (87/2G 1200:1 Class X).
• AVR has 1250/5 CT on A1 - A2 winding, Earth fault protection is through a floating 600/1 Neutral CT

Excitation System

Pilot Exciter is a 90Volt 1.35kVA 190Hz shaft mounted
Main Exciter is a 90kW, 1220V 407A with a field exciter of 27V, 5.4A

Main Alternator
23.375 MVA, 18.7 MW, 1225A, 11 KV , pf 0.8

The problem is that the rotor bars have extensive arcing marks on the copper bar windings. Also the end retaining rings show marks of pitting.

 

[ScottyUK]

This machine sounds like it is in distress. Arcing on the rotor is never a good sign: any chance of some photographs of the damage? Specifically where is the rotor bar arcing, and what is it arcing to?

Pitted end rings sounds like a corrosion problem. Rings are normally a forging for strength, so the base metal is of good and uniform quality. I'm not sure what testing methods were in use at Loughborough 40 years ago but I suspect they would ge good enough to pick up any gross problem with the end ring material.

What maintenance tests have been performed? Have you carried out an RSO recently? How does it compare to ones from earlier in the machine's life?


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If we learn from our mistakes I'm getting a great education!

 

[Hoxton]

I agree with Scotty, we need photos to assess this one further. The end ring material should be ok, other manufacturers had problems in the 60's i think with material problems and i think there was an industry change out for upgraded material.

The ring forgings were delivered with a forgemasters certificate and i think a separate piece from the forging for factory analysis.

 

[edison123]

Where do we have 'rotor bars' and bar windings in a 2 pole alternator ?

Muthu

http://www.edison.co.in

 

[ScottyUK]

On the rotor? The DC field windings are usually bars in machines of this size.


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If we learn from our mistakes I'm getting a great education!

 

[rmw]

You use the term retaining rings rather than the British term end caps. Interesting. I speak both languages.

Have the retaining rings (end caps) ever been changed from original 18Mn/5Cr metallurgy? If not the older metallurgy is highly subject to SCC and especially if you are in an environment where moisture and/or (and especially) salt can be present (industrial or marine) ((industrial in USA venacular connotes plants with industrial type gases or processes nearby as to opposed to British venacular where industrial means electric utility)). Could it be cracking that you are seeing that you are seeing on the retaining rings? I promise you that if your rings are 38 years old, that they are 18/5. Brush didn't even start to change until the mid to late '80's.

If they were replaced prior to that, they still may be 18/5.

The newer 18/8 metallurgy retaining rings are said to be immune to SCC cracking.

Google the term "retaining ring metallurgy" and do some heavy reading.

I raise the same question as raised earlier with respect to the arcing comment. Arcing from where to where? Rotor bar to end cap or bar to bar? Did you (do you) have a rotor earth fault detector? And if so, did it take you out?

What about the insulation over the end windings? (between the end windings and the retaining rings.) Is the integrity of that still good? If it hasn't been changed in 38 years, I'd be suspicious of that.

Is the information you have given about the rotor condition with retaining rings in place or removed.

If the latter, is there signs of fretting in the rotor tooth to end cap fit area?

rmw

 

[data74]

Unfortunately before eneogh investigations were made we have had to make a call to rewind the rotor. The excitation system, stator and rotor are all not earthed. The stator has oc/ef & diffrential protection. The alternator neutral is equiped with a neutral CT for protection but is not being used. Would it be worth it to buy a seperate relay for stator and rotor earth fault protection.

 

[rmw]

I hope included as part of the rewind is new retaining rings if they were the original 18/5. I don't think a respectable rewinder would go back with the old ones if they had to guarantee the rotor.

 

[data74]

Any Generator Protection Guru's out there? We are using a MultiLin489 relay to protect this generator.....the generator is not earthed i.e (Floating).What categories of protection do i require??

 

[ScottyUK]

Why is it not earthed?

As a starter for a machine that size:

Stator differential
Phase overcurrent
Underexcitation
Overexcitation
Voltage-restrained or voltage-controlled overcurrent
Negative phase sequence
Rotor earth fault
Over-fluxing (might be on the GSU transformer)
Over-voltage
Pole-slip
Loss of excitation

Probably a few I can't think of right now. There are plenty of good texts - have a look in the FAQs.


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If we learn from our mistakes I'm getting a great education!

 

[electricpete]

Scotty’s probably referring to a power forum FAQ:
faq238-1287
There are several free items, including art and science of protective relaying


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(2B)+(2B)' ?

 

[ScottyUK]

Thanks ePete. That's the one.

Add reverse power to my previous list.


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If we learn from our mistakes I'm getting a great education!

 

[data74]

Thanks for that info Scotty, will certainly do some research!

 

[data74]

I'm making an effort to edit the settings of the MultiLin489, How does one determine the synchronous reactance? i require this in order to determine the settings for the MoH circles?? The machine name-plate gives a value of 0.8 for the power factor.....is this the optimal power factor under FL conditions?? i wud expect this to vary wityh load conditions....does the manufacturer supply such info??

 

[davidbeach]

Probably should hire a local consultant that understands generation protection and can walk you through the process. Totally on your own (and that includes getting help from invisible members of an on-line forum) is no way to do your first generator relay settings.

If you don't understand what synchronous reactance is and how it is found, and don't understand what the nameplate power factor is telling you, there are probably a lot of other questions you should be asking but don't know that you don't know. Your machine will thank you for getting local assistance.

 

[waross]

A power factor of 0.8 indicates that the KVA rating is 1.25 x the kW rating. You are correct that the actual power factor is determined by the load. The generator is suitable for full kW output at a PF of between unity and 0.8
If the PF drops below 0.8 the kW output should be reduced accordingly to avoid overcurrent on the stator winding.

Note, many diesel sets rated for prime power "can supply 10% overload power for 1 hour in 12 hours."*
That equates to a PF of about 0.73, for rated kW output "for 1 hour in 12 hours."
I would be very comfortable running a prime rated diesel set at full kW output and 0.73 power factor "for 1 hour in 12 hours."
How does this compare with the rating methods on the big sets, Scotty?

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

 

[davidbeach]

I'm not Scotty, but I'd be a bit leery of your example of 0.73 pf. If the generator capability curve were a complete semi-circle, sure go right ahead. But generally the capability curve starts to pull way back from the semi-circle when power factor gets lower than the name plate power factor. (And the bigger the machine, the more pronounced.) VAr overloads can induce far more heating than Watt overloads. I'd be surprised if that "10% overload power" was referring simply to a 10% Watt overload.

 

[waross]

Hi David.
I checked this out on the F G Wilson website before I posted to be sure I had it right.
A number of years ago F.G. Wilson was number three in the world as a diesel gen-set builder. They have since been bought by Caterpillar.
They often use the same machine for a prime rated set as for a standby set. The prime set may have some added engine accessories such as an oil cooler, and/or larger oil sump.
This is the set that I based my comments on:

http://www.fgwilson.com/cda/files/653281/7/S250P3-S275E3(1PP)GB(1007).pdf

You can see from the specs that this is the same generator end and the same diesel engine.
As a prime rated set it is capable of 200 kW and 250 KVA. Model number S250P3.
As a prime rated set it is capable of 220 kW and 275 KVA but no overload is permitted. Model number S275E3.
So this set may safely handle 275 KVA. As a prime rated set, 200 kW/ 275 KVA = 0.727272.
Your comment is welcome David. Scotty and I often discover discrepancies between the characteristics of small sets versus larger sets. That is why I posted the question.
Yours
Bill



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

 

[ScottyUK]

Hi Bill,

Big sets don't normally have a declared overload margin in the manner that smaller units do. The machine capability is dependant on external factors such as ambient and coolant temperature, but it is normally stated as a single rating at specified conditions of temperature, PF, etc. Normally machine protection is set to trip the unit a little below the thermal rating of the insulation system. This is to ensure the longevity of the insulation because many customers will specify a minimum service life of (say) 25 years.

David is right - the capability diagram does pull in quite a bit from the stator current limit, primarily due to the difficulty of extracting heat from the rotor which imposes a limit on rotor current, and thus a limit on reactive power export. The heat removal problem gets worse as units get bigger, and while this isn't a large generator by utility standards it is still quite a big machine.

data74,

Please heed David's advice on setting machine protection. That's an expensive machine, even if it is an old one, and you really need to get external help. Some of those protection functions are tricky to set even for someone with experience. I imagine Brush probably issued a relay setting sheet when the machine was built. Brush do have a pretty good archive at Loughborough so they might be able to help. Are you trying to replace an old electro-mechanical relay scheme by any chance?


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If we learn from our mistakes I'm getting a great education!

 

[waross]

Thank you for your contributions to my continuing education, David and Scotty.
Yours
Bill

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