Determining Transformer kVA ratings

[MoreThanMeetstheEye]

If I have a customer who provides me with their peak kW demand (say, 1000 kW) and wants to know if it is sufficient for them to downgrade from a 2500 kVA unit to a 2250 kVA unit.

Is this determination made by simply using the formula kVA=PF/kW? I don't have a power factor from them - yet. Also, is it the Transformer's PF I use or is it the load's power factor? Is there any guidance/code/recommendation for how much "room" should be left based on peak demand?

 

[magoo2]

kVA = kW/PF

Once they give you the power factor, you can see what kVA you need.

 

[Gr8blu]

I have to ask: what possible reason would a user have to change from a (presumably installed and working) 2500 kVA transformer to a (new, replacement) 2250 kVA version? Is their continuous (and peak) loading NEVER going to increase from what they tell you it is right now?

Converting energy to motion for more than half a century

 

[bacon4life]

When I looked at changing out some oversized transformers, it did not make economic sense on my system unless the transformer was loaded at well under 10%. Perhaps the customer has some other reason for wanting the transformer changed out? If the transformer has to be replaced based on age, right-sizing makes sense.

Transformers that size tend to have relatively high impedance. It is important to check for excessive stead state voltage drop, particularly if the power factor is low. If there are any large motors, it is also important to check voltage drop during motor starting.

For energy efficiency purposes the duty cycle also matters. For a factory that runs 24x7 at a constant load, the transformer should be loaded to much less than the nameplate to optimize series (I^2*R) losses. For equipment that rarely hits peak loads (i.e. stadiums, residential) the transformer size should much smaller to reduced no-load (excitation) losses.

Harmonics can cause additional losses inside the transformer, which can increase the size of the transformer or require a transformer with a higher k-factor rating.

The seasonal loading pattern also matters. During cold ambient conditions transformers can often be loaded more than during hot ambient conditions.

Utilities often have very different rules of thumb compared to customer owned equipment. The cost difference between the two sizes is so small that some utilities do bother stocking 2250 kVA transformers.

 

[jghrist]

See IEEE C57.91-2011, IEEE Guide for Loading Mineral-Oil-Immersed Transformers and Step-Voltage Regulators. Allowable loading depends on the transformer characteristics, peak kVA load, and the loading pattern. It is the load pf that you use to determine load kVA from load kW. The economic loading depends on the transformer losses, energy cost per kWh, demand cost per kW and transformer capital cost as well.

 

[waross]

Ask your customer for the KVA of the load. Transformers are rated by KVA, not by kW.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[prc]

The following may be of interest:

https://www.linkedin.com/pulse/rated-power-transformers-p-ramachandran