Transformer capacity

[bassett]

I need to clear up a question

When using three single phase transformers arranged in a Delta - Delta configuration i.e.

3 - 15KVA 480/240 transformers connected into a three phase delta arrangement - what is the maxium current that can be safety drawn across each phase i.e. X1 - X2, X1- X3, X2 - X3. Each transformer is rated at 62 amps if used a a single transfomer which would indicate that 62 amps is the maximum that can be pulled across any phase. A three phase transformer rated at 45KVA would provide 108 amps at 240 volts.

My customer insists that the standard capacity chart shows in a three phase arrangement each 15KVA unit would be capable of providing 36 amps with a combined total of 108 amps.

What is correct?

 

[dydt]

The delta-delta transformer connection requires that the transformer primary winding be rated as a full line to line voltage...the primaries and secondaries both see their respective individual winding currents and are reduced from the line currents by root of 3...or 1.732

on your case the total kva load will be 15x3 or 45kva..
the primary phase current will be 31 amps and secondary phase current will be 62 amps...the primary line current will be 45000/(1.732x480) = 54 amps....and the secondary line current will be 45000/(1.732x240) = 108 amps..

'hope this helps

dydt

 

[busbar]

The phrase “combined total of 108 amps” is meaningless, as would be calling a 3-pole 50-amp circuit breaker “150 amp” or referring to a 2-wire 100-amp circuit as a “200-amp” anything. It is incorrect to sum individual currents that are not of the same phase {as in conductors that have a potential difference between them.}

So, each 15kVA transformer has a 240-volt current of 62.5 amperes, and intuitively each 2-wire secondary winding (if on its own/isolated) could serve a 15kW 240V load such as a heater. Three-phase currents are a bit less straightforward, but their use and definition are universally accepted in the trade. If the three 15kVA 240V transformer secondaries (and the three 15kW 240V heaters) were connected in a delta configuration, each 240V winding (individual transformer coil) and each heater would still operate OK and could be measured with 62.5 amperes circulating in each individual 1ø heater and each individual transformer winding. What has changed is that the COMBINED current in each phase—where each phase conductor is connected to two transformer coils on one end and the other end is connected to two 1ø heaters. Now though, the PHASE currents {i.e., in single cables, tied to X1, X2, X3, serving the combined pairs of heaters by pairs of combined transformer coils} acts a bit differently. It does not measure 62.5x2 or ~125 amperes in each phase cable. This is where the Magic of Three Phase steps in; for each phase current will instead measure 62.5x1.73 or ~108 amperes. In this example the heaters are also connected in a 3ø-delta arrangement.

It is acceptable to sum powers, like transformer kVA or heater kW in a 3ø circuit—e.g., 15kVA+15kVA+15kVA = 3x15kVA = 45kVA. It may that be the customer mistakenly DIVIDED 62.5 by 1.73, yielding the erroneous “36” answer.

A check is to use rules of thumb on power and current relationships—for 480V 3ø, per-phase current is ~1.2 amps/kVA. For 240V 3ø, per-phase current is ~2.4 amps/kVA. The 45kVA bank matches the 45kW heater. And, you {and—your customer} find that after 24 hours of operation there's no telltale burnt-enamel aroma.