Can a Transformer Handle 125% of it's rated KVA?

[theophilus88]

I've heard from many experienced electricians and engineers that transformers are designed to handle 125% of their rated load. IE) A 112.5 KVA transformer which is normally good for 312A's @ 208V, 3PH can actually handle 1.25x this load or 390A's. Is this true, and if it's true is this a safe assumption? Also bonus points if anyone has any code sections you can pull from.

 

[Besoeker3]

I've heard from many experienced electricians and engineers that transformers are designed to handle 125% of their rated load. IE) A 112.5 KVA transformer which is normally good for 312A's @ 208V, 3PH can actually handle 1.25x this load or 390A's. Is this true, and if it's true is this a safe assumption? Also bonus points if anyone has any code sections you can pull from.

I can't answer for the USA but we did have one in UK. We had to add forced air cooling fans on the transformers You might have to do the same. The original transformer was nominally 1.5 MVA.

 

[Jraef]

To Besoeker's point, transformer "ratings" are about the transformer operating within its design limits in terns of heat rise, so if you can decrease the heat rise, you can often increase the capacity of the transformer. But that only works for transformers DESIGNED to have added cooling, i.e. OA vs FA cooling on OIL FILLED transformers. Rather than explain it all again, you can read this older thread.

What does OA stand for on older transformers

Just curious to find out what OA stands for on older transformers, this isn't a homework assignment its 1st to answer the right answer in my class gets $5 bucks lol. I been googling and been getting closer to the right answer but I need to know what it stands for.

forums.mikeholt.com

But for "dry type" transformers that we typically see in distribution equipment, the rating is the rating. So unless it specifically STATES a rating higher than it does, you would be in violation of 110.3(B) if you tried to use it that way. However, transformers have a TEMPORARY OVERLOAD capability without being damaged based on NEMA design criteria as follows:

• 200% nameplate load for one-half hour
• 150% load for one hour
• 125% load for four hours

NEMA ALSO requires that there is a 50% load preceding and following the overload. So it's not a good idea to COUNT ON this overload capability unless you can control ALL aspects of the load.

Utilities have their own set of rules and design decisions, so they will often over-burden a pole pig transformer well beyond what we are allowed to do.

 

[kingpb]

There are many factors that affect the rating or capability of the a transformer KVA/MVA rating, with temperature being probably the biggest factor.
Transformers designed and built per IEEE/ANSI C57 standards; there are different C57 standards dependent on dry, oil immersed, pad mount, instrument, etc.
Adding fans can increase the KVA rating, also one example of allowing the temperature to rise from 55deg C to 65 deg C (oil filled) can increase KVA by up to 113%; i.e. 125KVA becomes 141KVA. Operating in a lower than rated temperature can achieve a higher KVA output than nameplate.
The utility companies are notorious for operating a transformer overloaded because they know it may just not last as long; which is the outcome of too high a temperature.
Bottom line there are a lot of ways to play with the rating based on temperature, whether it be ambient or operating, by cooling or overheating.

 

[Julius Right]

According to mine IEEE Std C57.96-1999 LOADING DRY-TYPE DISTRIBUTION AND POWER TRANSFORMERS [the latest edition is 2013] Tab. Table 4—Daily loads above rating to give normal life expectancy in 20 °C average ambient for transformers with a 30 min time constant
if the 150 °C insulation system following and followed by a constant load of 90% then for 1.27 times the rating the peak load time in hours is 2 hrs.

 

[drktmplr12]

They aren't designed to handle 125% of their nameplate rating. They are designed to operate up to a specific temperature before the insulation begins to break down, which will lead to failure of the "boom" variety. Usually, distribution dry-type indoor units are rated 220 C. In 40 C ambient, that allows for 40 + 150 C rise = 190 C with a 30 C allowance for unavoidable hotspots. Generally, engineers will rerate 1% for each 1 C below/above 40 C ambient. There are some guidelines in IEEE if you are so inclined to read further. If you overload a transformer, its temperature rise will be higher than the nominal 150 C, but it is difficult to predict how much more temperature rise will be observed for operating above nameplate ratings.

Likely the NEMA design criteria referenced in Post #3 is a good nugget of info. I would never rely on that in a formal capacity or in a technical report, but it is certainly worth mentioning while sitting around the table and talking through the specific problem. Otherwise, my approach is to consult with the manufacturer to see what they recommend, understanding that operating outside those boundaries will open me to liability if I make a recommendation to do so. If that makes my approach conservative, so be it.

 

[David Castor]

Transformer life and overheating depend on the temperature of the windings. Heat is the enemy. If you're asking if it is safe to assume that a transformer can operate continuously at 125% of its rating at the rated ambient temperature, the answer is no.

 

[Julius Right]

In order to keep the normal transformer life ,for dry-type transformer there are calculation way in

IEEE Std C57.96 or IEC 60076-12 [ and for oil-immersed transformer in IEC 60067-7] as loading guide.