transformer loading

[jcraft]

We have a unit substation in our manufacturing facility. The transformer section is a 5oo Kva 4.16Kv to 480/277v. It is connected to an 800 amp main breaker which feeds a large panelboard. The low votage amp rating of the transformer is 602 amps. the unit is about 5 years old and installed in a clean environment which is climate controlled year round. It has a Siemens panelview meter on it that reads 450 amps. We want to add another piece of machinery onto this sub that is 140 amps total bringing this sub to almost max.However the air conditioning load on this sub is starting to drop due to colder weather so the sub will drop about 25% in the next few weeks. So this sub will be maxed out about 4 months out of the year when the Ac load is on then will drop off. The load is on this sub 24/7 and it is supervised. It looks like there are provisions to mount a exhaust fan in the transformer section but I am not sure if it is common practice to overdrive a transformer a little bit and put a fan on it. Thanks for any input

 

[rbulsara]

Its not a problem to run a tansformer at full load indefinitely or slight overload (less than 10% overload) for short duration. You need to watch the ternial voltage.

It is common and standard practice to increase the capacity of a trasformer by 25% to 40% (typically 33%) by use of forced fan cooling. The unit windings needs to be fitted with temperature sensors to automatically control the fan. Check with the mfrs.

Not recommended, but if you know what you are doing and by keeping a watch on the temerature rise of the transformer, you can set up external cooling by fans. The idea is to disspate the heat generated in the tranformer so that the winding temperature does not exceed the rating of its insulation.

 

[jcraft]

what exactly does the temperature rise mean. The one we have says 115 deg. c . Does that mean the temp. inside the windings is capable of 115 deg. c over ambient temp.. Could you just take an infrared test periodically on the windings and make sure they don't exceed this value.

 

[cuky2000]

………the unit is about 5 years old and installed in a clean environment which is climate controlled year round

The actual equivalent transformer capacity varies with the ambient temperature. For instance, if the ambient temperature is 20oC (68 oF), the transformer rated for 500 kVA at 30 oC is able to be continually loaded up to 500x110% = 550 kVA according with ANSI Std C57 (see enclose figure below).

This means that the ampacity available at 480 V side is 661A. If the unit is suitable to accomodate fan coolers, it is possible to add extra capacity as described in the previous post.

It is good idea to double check the protective devices setting and rating to make sure they can handle the additional load.

 

[cuky2000]

Transformers are designed to a maximum temperature rise value. This value is usually 80° C, 115° C, or 150° C. These values are usually based on a maximum ambient temperature of 40° C, thus a transformer with a 115° C maximum temperature rise will not reach a temperature above 155° C (the total temperature is equal to the temperature rise plus the ambient temperature).
_{The insulation of the conductors also have a maximum temperature value. This value is usually 200° C or 220° C. This is the maximum temperature that the insulation can reach without degrading. Once this temperature is exceeded, the life expectancy of a transformer will be cut inhalf for every 10° C to a point where the insulation degrades, destroying the transformer and usually anything connected to it. Transformers with a low efficiency emit a lot of heat and need cooling devices to get rid of the excess heat. These devices are costly and usually need constant maintenance. Also, if one of these cooling methods fail, the transformer runs the chance of being destroyed by the excess heat.
A desirable indoor dry type transformer is convection (self) cooled and has a maximum temperature rise of 115° C with an insulation temperature of 200° C.}

[red]Beware of the 80° C rise transformers. They are often a 115° C rise transformer that has been de-rated, providing a costly approach.[/red]

 

[rbulsara]

I believe the tranformer winding insulation are rated for its max. temperature. For example, 115 deg c would have average temperature rise of 65 degC above 40 deg C ambient and hot spot temperature would be 75 deg c. (40+75=115 max)

 

[davidbeach]

rbulsara, transformer ratings are temperature rises over an ambient as indicated in cuky2000's post. The OP doesn't say, but given the ratings it appears to be a dry type transformer. At least in the US, dry type transformers typically have a 220C insulation system. For 150C rise, 40C ambient + 150C rise + 30C hot spot = 220C. Most 80C and 115C rise transformers also have the capability of 30% and 15% continuous overload capability without exceeding the 220C rating of the insulation.

jcraft, no, you can't monitor the transformer with an IR scanner, the hot spot of interest in buried deep within one of the cores.

 

[rbulsara]

davoidbach:

I agree with you, it depends what the tranformer nameplate indicates. It it says temperature "rise" then what you say is true.

However tranformers are specified with insulation "class" and there are insulation classes other than 220 (185 dec C I beleive for 115 deg rise). The insulation "class" indicates maximum temperature permitted at any spot.

185 is for smaller units dont know where the break is.

 

[davidbeach]

rbulsara, true, but the break is for small transformers. I don't have a reference at hand, but it is somewhere in the 9 or 15 kVA range (if not lower).