Effects of overloading on life of generators

[jan63]

I am working in an hydro-electric power plant where units were installed from 1977 to 1984.At time of installation the manufacturer specifications allowed 15% overloading and accordingly units were overloaded over the past 30 to 35 years.

Now we are facing problems in exciters and windings etc.Can any one guide me as to from where can i get effects of overloading on life of generators.Recommendations of international repute like IEC will be preferred.

 

[ashtree]

Can the manufacturer(if they still exist) give you some data.

I would assume that if they made that recommendation then it was supported by some data. The fact that you have gotten 30 yrs of overload would make you think it was very conservatively designed.

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[waross]

This may not be applicable, but prime power diesel sets are de-rated about 9.1%. Then a 10% overload is allowed for one hour out of twelve hours.
eg: a 1.1 MW set will be de-rated to 1 MW. Then a 10% overload will be allowed taking it back to 1.1 MW.
The same generator end will be rated at the full 1.1 MW for standby service.
30 to 35 years service with overloading; The sets don't owe you anything. Even with no history of overloads the sets are probably past the end of their expected life.
An oft repeated rule of thumb is that every 10 degrees Celsius above rated temperature will result in a 50% loss of life.
So, if an overload raises the temperature 10 degrees C above the rated temperature, every hour of operation at that temperature will result in a two hour loss of life.
Determine the temperature during an overload and extrapolate from the exponential curve.
In the meantime bite the bullet and arrange for re-winds. The units deserve it after 30 or more years of service.

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

 

[jraef]

Overloading anything with windings will increase the heat in those windings, and heat attacks the integrity of the winding insulation, making them break down faster. Ergo, heat X time = failure. Beyond that, I don't think it's quantifiable even if you knew the exact amounts of heat for the exact amounts of time. There are just too many variables. The 50:10 rule is a universally accepted truism with the intent of illustrating that basic formula; the more heat you produce in the machine, the less time it takes to fail. But the obverse is also true, as anyone in the cryogenic pump industry will tell you. Super cooling a motor means being able to get more power from it without raising the temperatures of the components to levels that degrade them. There are other issues of course, I'm just using that as an illustration of the overloading issue.

And I agree with Ashtree and Bill, 30+ years of service including repeated overloading deserves an award, not recriminations over having been overloaded. I doubt you would have attained a much higher lifespan had there been zero overloading because time is an ever present detractor, for us all...


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[electricpete]

I agree with all above. There are no exact general answers to this type question. Even armed with full information and specifics, answers are still guesses. But perhaps better guesses.

Can you provide any details of the insulation class (B, F), the insulation system type (asphaltic, epoxy) the rated temperature rise (on nameplate), the actual winding temperatures measured. Also obviously your observations during operation, inspections, testing would also feed into any judgement about expected longevity (you mentioned that you are facing some problems but don't tell us what exactly).

EDIT - I re-read your op and I think you are looking for standards to help develop or support conclusions regarding effects of overload. There are standards that define the thermal class of the insulation system. References for insulation temperature classification:
- NEMA MG-1 (motors and generators) paragraph 1.66 - classification of insulation systems
- IEEE 1 temperature ratings of electrical insulation systems

Sometimes there is a lot of margin built into the machine and sometimes not. By comparing winding temperature to insulation temperature rating, you'd have more basis to determine whether the margin was sufficient to accommodate the overload without exceeding the insulation rating.


=====================================
(2B)+(2B)' ?

 

[catserveng]

I ran into this article a few years ago while researching similar issues,

http://www.hydroworld.com/articles/...e-remaining-life-of-generator-components.html

While it doesn't specifically address your question completely you may find it useful, and this site has a number of excellent articles.

MikeL.

 

[davidbeach]

30 years between rewinds seems like a good, full, happy life for the stator coils. Looks like you got there. Even with no overloading those coils have a finite life. Be glad they got you as far as they did. Since you can "overload" the machine, you should get it rewound and then rerated. What was an overload can become the new normal. That may mean enough extra moolah over the next few years to pay for the rewind anyway.

 

[edison123]

Your generators are having typical end-of-life failures (bathtub curve). Overloading accelerates aging. It's a good policy to refurbish the generator windings after about 25 years so as to prevent unplanned outages. As David Beach says, the whole cost can be recovered quickly.

Muthu

http://www.edison.co.in