Transformer Loading Guideline?
Transformer Loading Guideline?
(OP)
I looking for a loading guide for distribution transformers. At what point is a transformer overloaded? At nameplate KVA, at 20% over nameplate? How does the seasonal peak factor in? Mfgs tend to recommend not loading beyond the nameplate KVA. What about "degrees C rise"? -- Thanks.






RE: Transformer Loading Guideline?
It Is not a trivial issue. For ANSI oil, IEEE Std C57.91-1995 Guide for Loading Mineral-Oil-Immersed Transformers
RE: Transformer Loading Guideline?
RE: Transformer Loading Guideline?
RE: Transformer Loading Guideline?
It is important in case of parallel transformers that one transformer carries the overload when the other trips till such time the load shedding takes place to brings down the load. If the transformer over current protection is not set to take this need in to account, it is likely that the second transformer also trips and no one would like it.
This is a critical requirement in case of grid transformers and many a grid failures are as a result of conservative settings adopted for transformer over current protection.
RE: Transformer Loading Guideline?
Even if you set the LT pickup at 150% so the trip will occur before 10 minutes, then the curve will allow overloads up to 150% (between 100% and 148% or so) for an indefinite time.
The only safe way to protect a transformer is by using the manufacturer's nameplate limits, or use temperature relays to trip out the secondary device when the temperature reaches the nameplate limits.
RE: Transformer Loading Guideline?
Trying to protect the transformer against overloads with O/C protection generally we get in to bigger problems and undesirable situations, this is my experience.
RE: Transformer Loading Guideline?
RE: Transformer Loading Guideline?
However, if your installatin falls under NEC (in the United States speciallly), you have to follow NEC..
As for the definition, overload will be anything beyond its rating..it is a different matter how much overlaod is tolerable for how long.
Utilities can overload trasnformers for the reasons raghunath gave and also the fact they can afford to..some reduction in life due to repeated overloading is not a concern for them..it may not be the case in your case.
RE: Transformer Loading Guideline?
I would consider any load over nameplate rating to be an overload. Whether or not the overload is acceptable is a different question.
For what it's worth -- I will generally size secondary transformer overcurrent protection at 125% of nameplate, which is in accordance with NEC. With 100%-rated breakers, that would permit a 125% overload to exist continuously. No code violation.
As mentioned above, utility requirements are different than us mere mortals. But the utility philosophies can be interesting and enlightening none the less. Around here, our utility wouldn't bat an eye at continuous 10 to 20% transformer overload. And they would not lose much sleep over 40%.
I've heard that it's more cost effective for utilities to replace transformers when they burn out from overload than it is for them to monitor the load and take care of overload situations.
Keep in mind that heat is the real problem that will kill the transformer -- so fans can be helpful in squeezing a bit more power out of a transformer beyond it's actual nameplate rating. Similarly, if you have the transformer stuffed into a 120-degree electrical room, you may find that nameplate IS an overload condition. . . .
RE: Transformer Loading Guideline?
That works as an operating philosophy when you have a stock yard with spares. For an owner with just a few transformers the economic picture would be just the opposite.
RE: Transformer Loading Guideline?
If anyone thinks the NEC does specify transformer size, please provide a code reference.
RE: Transformer Loading Guideline?
There is also NEC 110.3(B) to consider, are the nameplate FLA values "instructions"?
RE: Transformer Loading Guideline?
Statistics compiled In the pass 30 years by EPRI, IEEE and EEI shows not significant increase in transformer failure rates using the overloading criteria of 3% annual loss of life (ALOL) provided not exceeding the following constrains:
-115 deg C maximum top oil temperature
-160 deg C maximum hot spot temperature (55 deg C insulation)
-180 deg C maximum hot spot temperature (65 deg C insulation)
-LTC switch capability
-Bushing capability
-Insulation moisture content
-Tank oil expansion limits
See the enclose information for aditional reference in this subject:
http://www.maac-rc.org/reference/PJM%20Ratings%20G...
http://www.usbr.gov/power/data/fist/fist1_5/vol1-5...
http://elect.mrt.ac.lk/Estimation_optimum_1998.pdf
http://www.dstar.org/Public/Transformer_loading_an...;
RE: Transformer Loading Guideline?
That's kind of out of character for the NEC, where most everything else is sized at 100 or 125 or 250% of the load it carries.
And we could actually take that even further. Only the feeders are required to be sized to handle 100% of their demand load. There's no reason that the OC devices need to be sized as large as the feeder ampacity -- they, and their transformer, could be even further reduced in size.
RE: Transformer Loading Guideline?
RE: Transformer Loading Guideline?
1) I also size primary OC at 125% primary current at full nameplate load as a rule of thumb, whether or not I have OC protection on the secondary. (Based on NEC table 450 -3)
2) Any loading above nameplate is definitely an overload condition.
3) Acceptability of any overload that doesn't pop the primary OC protection, in terms of severity or duration, is all determined by temp rise. (don't exceed the nameplate rise rating)
Regards,
NEMA6P
RE: Transformer Loading Guideline?
RE: Transformer Loading Guideline?
+ Utilities seem to size their transformers at about 50% to 75% of what we would size them at (which is often based on connected load or 75-80% of connected load).
+ There are quite a few articles out there about the philosophy behind NEMA TP-1 transformers (Energy Star high-efficiency transformers). A lot of research has been performed on transformer loading, and most of it seems to indicate that most small transformers are loaded to something like 30% of nameplate.
You might want to contact some utility engineers and see if they can give you some pointers on transformer sizing (just curious -- GOTWW, are you a utility engineer? If not, how come you're sizing a utility transformer?).
Also, I'd forget about your 80*3=240A starting point for your calculations. I'd think a better bet would be to get some good rule of thumb numbers for w/sf for residential and start there. Unfortunately, I can't give you any guidance there, either, most of my work is in commercial/industrial. But, my guess would be about 3 w/sf. Assuming 3x 2500sf houses, that would be 15 kVA demand. DON'T BASE YOUR DESIGN ON THIS. But this does indicate that your calcs may be high. And maybe someone else can throw out some better w/sf numbers.
Actually, after taking a look at NEC Table 220.3(A), I'm starting to think even 3w/sf might be high for residential for utility purposes. . . .
RE: Transformer Loading Guideline?
This reminds me of this weekend, and the cascading transformer explosions in florida, probably due to high impedance secondary faults. And of course Mr Heroldo(?sp) explinating the virtues of UG electrical.
RE: Transformer Loading Guideline?
It is funny though having to do a full blown NEC load study for a commercial building upgrade to justify keeping the 400A main service, whereas the utility company leaves an existing 10KVA unit of the pole, no questions asked.
And No I am not a utility engineer, I have to wear allot of hats with varying degrees of incompetence
RE: Transformer Loading Guideline?
RE: Transformer Loading Guideline?
RE: Transformer Loading Guideline?
We also have to bear in mind the differences in "time constants" at play. A transformer takes hours, at worst tens of minutes, to be damaged by overload. An overcurrent relay (or fuse) takes fractions of seconds, at worst seconds, to operate. No point in "throttling" the transformer for short term overloads (in the order fo seconds) just because that's how the overcurrent curve is set. How would you deal with (for example) large air-con motor starting?
Bung
Life is non-linear...
RE: Transformer Loading Guideline?
However you may be overlooking a more important issue if you choose to follow this route. I strongly recommend that you look at your voltage drop calculations to the service that is most at risk of being served at a voltage below your mandated requirements (Typically 114V at the meter) You will also need to review your flicker criteria in the case of heat pumps and air conditioning.
In the end I would wager you will be limited to around 120% of nameplate as a result of the voltage issues you may incur.