Transformer kVA at Transformer Tappings 1

[nightfox1925]

I would like to inquire whether there is a direct or indirect effect of changing transformer taps on the transformer kVA rating. Is the transformer kVA rating varies as the transformer taps varies?

GO PLACIDLY, AMIDST THE NOISE AND HASTE-Desiderata

 

[tinfoil]

The kVA rating of a transformer is a function of its ability to get rid of heat, either overall, or at its worst-case 'hot-spot'.

The amount of heat generated at this hot-spot is almost directly proportional to the current.

Therefore, the kVA rating WILL vary as the taps are changed.
If you look at the nameplate of some power xmfrs, it actually itemizes what the rating is for each tap.

 

[waross]

To add to tinfoil's excellent advice;
The heat generated in a transformer is a combination of mainly:
1> Eddy current losses, usually quite small, but vary with current.
2> Hysteresis loses, a small proportion of the full load losses.
3> I squared R losses. The main losses in a loaded transformer and the losses that will most affect the KVA rating when the taps are changed.
The I in the I squared R losses is a combination of load current and magnetizing current which are acting at 90 deg. to each other.
The result is that the KVA as a function of current is not linear, but for practical purposes can be assumed to be linear at full load.
If the primary taps are used to maintain the secondary voltage then the voltage and the losses in the secondary winding will remain unchanged.
If the primary voltage is low, the taps will remove some turns from the circuit. Therefore the R will be less. However, to maintain the same current at the secondary with a lower primary voltage and a reduced ratio, the primary current will be increased. A simplified formula for a 2% change will have the resistance of the primary winding drop 2% while the primary current will rise 102% squared or 4.04%.
A rise in primary voltage and a tap setting that inserts more turns into the winding will have the opposite effect.
BUT, this is not the total loss of the transformer, just the effect on the primary winding.
There are a number of things that we still don't know.
The factor that is difficult to determine from nameplate data is the R in the I squared R.
The transformer nameplate gives the percent impedance, but that impedance is distributed between the resistance and the reactance.
We often don't know the ratio. Even if we know the ratio, we don't know how the resistance or the reactance is distributed between the primary winding and the secondary winding.
When we estimate the additional percentage heat generated in the primary winding, we have no idea what percentage this is of the total heat generated.
As tinfoil said, "The KVA rating of a transformer is a function of its ability to get rid of heat, either overall, or at its worst-case 'hot-spot'."
The KVA rating is more closely related to the transformers ability to limit the maximum temperature, which it does by removing the heat.
Increasing the load on a transformer will increase the heat generated in both windings. Increasing the current in one winding by changing the taps changes the heating in only one winding, and we don't know the ratio of heat contribution between the windings.
Another possible factor is the efficiency of cooling. If the physical arrangement of the windings favors heat removal from one winding a further non-linear factor has been added.
If the transformer designer has compensated for poorer heat removal from the primary winding by using a lower effective current density in the primary winding, we have another factor to confuse us.

What to do in the field?
For distribution and new construction design. I would ignore the change in KVA rating unless the calculated load was over
95%. Then I would calculate the primary load current at the reduced voltage and compare it to the rated primary voltage.
I would consider that the 5% extra heating in the primary may be only 2% or 3% of the total transformer heating, Probably less.
In cases where the KVA is slightly higher, I would use the rated KVA.
In no case would I exceed the rated current of either winding.
If I were that close to full load I would probably increase the transformer size regardless of the slightly reduced KVA.

If I absolutely had to know the exact rating, I would look at the nameplate. If the information was not included on the nameplate, I would then contact the manufacturer.
The last resort would be to measure both the primary and secondary resistance of the transformer and calculate the losses at full load in each winding. I would then measure the resistance with the taps changed and calculate the current required to generate the same calculated losses in the winding. This current will result in the same heating in the primary and slightly less heating in the secondary. It is conservative but it will not give rise to unexpected "Hotter Hot Spots".
Remember, unless the alternate KVA figures are supplied by the manufacturer you may have warranty issues.
respectfully

 

[itsmoked]

waross sez: "To add to tinfoil's excellent advice;"

Then follows with 756 words, 3596 characters!!

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[waross]

Hi Keith
Did I leave anything out?
Resepctfully

 

[itsmoked]

hahaha No! I think you covered the heck out of it. Nice synopsis. Oh, that didn't include spaces . And yes space characters have ASCII value 20h.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com