Transformer Impedance

[Grouch1980]

Hey guys,
I'm running short circuit calculations for a feeder that has a step up 112.5 KVA transformer. The transformer has a temperature rise of 150 deg C.

The nameplate of the transformer states 6.3% for the transformer impedance, at 170 deg C. When I plug in the impedance value to determine the short circuit current on the transformer secondary, do I just use this value of 6.3%, or does one have to convert this value of 6.3% to something else, since this impedance is specifically at 170 deg C.

 

[jim dungar]

Hey guys,
I'm running short circuit calculations for a feeder that has a step up 112.5 KVA transformer. The transformer has a temperature rise of 150 deg C.

The nameplate of the transformer states 6.3% for the transformer impedance, at 170 deg C. When I plug in the impedance value to determine the short circuit current on the transformer secondary, do I just use this value of 6.3%, or does one have to convert this value of 6.3% to something else, since this impedance is specifically at 170 deg C.

I have never made a temperature correction for transformer impedance.

It's importance might depend on the X/R ratio for the transformer.

 

[winnie]

Not my area of expertise, but I expect that temperature change makes a difference that is usually ignored.

The winding temperature alters the R but not the X of the transformer, and the impedance Z depends upon both.

With a transformer that has a very low X/R of 1.0, the winding temperature change from 180C to 20C results in an 10% drop in Z.

The higher the X/R, the smaller the temperature effect.

 

[Grouch1980]

Thanks guys. I'm assuming then I just use the 6.3%, with no correction. I've come across several publications on short circuit calculations, and not one of them mentions it.

 

[winnie]

Do you know the X/R of your transformer

 

[David Castor]

Don't worry about it. The 150 deg C rating is a temperature RISE above ambient at full output. The 170 C value is the actual conductor at which the impedance was measured. At noted, the temperature only impacts the resistance, not the reactance.

 

[Grouch1980]

Don't worry about it. The 150 deg C rating is a temperature RISE above ambient at full output. The 170 C value is the actual conductor at which the impedance was measured. At noted, the temperature only impacts the resistance, not the reactance.

I'm trying to follow. So assuming the ambient is 40 C... 150 C on top of that gives you 190 C as the temperature on the coils at full load.

So the 6.3% impedance was measured at 20 C less, at 170 C. Why was 170 C chosen and not 190 C?

 

[Grouch1980]

Also, what is the ballpark temperature the coils reach during a short circuit? Let's say the short circuit is 100KA. Why wouldn't we use the impedance value during a short circuit, if impedance is affected by the temperature?

 

[David Castor]

If the ambient is 30 deg C and the temperature rise is at its maximum allowable of 150 deg C, then the winding temperature would be 180 C. (There's actually a hotspot allowance, so actual temperature in the hottest region could be higher.)

US transformer ratings are based on a maximum AVERAGE ambient (over 24 hours) of 30 deg C. If the average ambient exceeds 30 deg C, it has to be de-rated.

The temperature of the windings won't be constant during a short circuit. If the ambient is 0 deg C and you close into a fault, the fault will likely clear before the temperature of the windings gets close to the rated maximum. It takes a while to raise the temperature of all that steel and copper.

 

[Grouch1980]

US transformer ratings are based on a maximum AVERAGE ambient (over 24 hours) of 30 deg C. If the average ambient exceeds 30 deg C, it has to be de-rated.

A lot of online articles mention 40 deg C as the ambient temperature. OR.. is this the high, and the average as you mention is 30?

The temperature of the windings won't be constant during a short circuit. If the ambient is 0 deg C and you close into a fault, the fault will likely clear before the temperature of the windings gets close to the rated maximum. It takes a while to raise the temperature of all that steel and copper.

Nice... you always learn something new.

 

[Carultch]

A lot of online articles mention 40 deg C as the ambient temperature. OR.. is this the high, and the average as you mention is 30?

Nice... you always learn something new.

40C is high for most of the continental US. Most temperatures I see, are between 30C and 35C for the Ashrae high. Here's a site where you can look this up:

Solar Reference Map - FSEC®

[Last updated: 4/28/2023]

energyresearch.ucf.edu

ASHRAE publishes the data for the range of ambient temperatures you expect at a representative city of your location. Usually, the data is available from airport weather stations. It is not necessarily the record temperatures you are expected to use. I know I've experienced temperatures colder than -21C and hotter than +32C where I live, but nevertheless, that's my design temperature range. It is the statistical "middle 96%" of the temperatures you expect, that you use for meeting the NEC.

It's common that you may have equipment such as breakers and fuses tested in laboratory conditions at 40C, just to work with a conservative number for the background temperature. This doesn't necessarily mean they expect your ambient temperature to be 40C.

 

[David Castor]

Some electrical equipment has a maximum temperature rating of 40 C (104 F). But transformer ratings (in the US) are based on a 24-hour maximum temperature of 30 deg C (86 F). They can operate in higher ambient, but not up to their full kVA rating. You can specify operation at a higher ambient, and they will just provide a larger transformer with lower nameplate at that temperature. The flip side is that if the ambient is lower than 30 deg C, they can operate at higher kVA ratings (but not allowed by NEC). Utilities typically have summer and winter ratings for transformers. In larger transformers, the winding temperature and oil temperature are monitored to allow additional loading.

 

[jim dungar]

The flip side is that if the ambient is lower than 30 deg C, they can operate at higher kVA ratings (but not allowed by NEC).

The NEC provides no guidance on determining the kVA output of transformers. Fan cooling, with its associated increased kVA, is common place in NEC compliant installations.

I do agree rerating a transformer based on a cooler ambient temperature is definitely not as common as rerating an OCPD or conductors, which is allowed by the NEC, and in general is discouraged as a design practice.