Calculating Equivelant Power of 3 Phase Delta system

[7JLAman4]

On a 450/120V delta system where on windings A, B & C, 2.25, 2.4 & 2.2 kVA are connected respectively. What is the equivelant 3 phase kVA or kW that is seen. Based on these numbers, (3) 3kVA transformers were selected (loading not more than 80% of winding rating). Ratings of the single phase equipment connected to the transformer create this unbalance. The loads are connected to the transformer to provide the closest possible loading. Is it possible to sum single phase kVA loads and 3 phase kVA loads to acquire a system total kVA. The attempt is how to calculate the loading at the 450V panel supplying mulitple transformer delta loads.

 

[waross]

Hello jlamann
Yes and no.
It is possible to add the singlephase loads.
The load on your panel may not be all single phase.
I suggest you reconsider your delta secondary connection.
If you lose on primary phase, expect to trip the preaker feeding the transformer bank.
If the primary voltage is slightly unbalanced, about 1% unbalanced, expect circulating currents in the delat which will add to the load on the panel.
If the primary voltage unbalance reaches 4% or 5% (the exact amount is related to the transformers impedances) expect the transformers to fail.
Corner grounding a delta is the least desirable grounding scheme. Any other grounding scheme is unrealistic at 12 KVA.
Consider a 120/208 volt wye secondary connection with a grounded centerpoint or neutral.
yours

 

[mpparent]

kVA is kVA, it doesn't matter what voltage you are at. You can add up kVA's to get a total kVA of the system. Think of it this way:

P(total) = P(phase A) + P(phase B) + P(phase C)

If you have a balanced system, then P(total)=3*P(phase)

Mike

 

[waross]

If you take the primary KVA as KVAp and the secondary KVA as KVAs, then KVAp=KVAs+losses.
If you use a solid neutral primary and a delta secondary, the losses can easily exceed the transformer rating with only a small primary voltage unbalance. I have seen 3 50K VA transformers at two locations lost this way in the past 8 months. The loading on two of them was about 50% and on the third transformer the loading was about 10%. The primary KVA was quite a bit more.
respectfully

 

[ScottyUK]

mpparent,

That is true if you assume that all reactive loads are lagging (or all leading). If there is a mixture then arithmetic addition will give an incorrect result.

kVA also includes harmonic currents which will add or cancel to some degreee but produce no flow of active power (unless there is harmonic voltage source).

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I don't suffer from insanity. I enjoy it...

 

[mpparent]

scotty..

Agreed.

Mike

 

[7JLAman4]

Not to be a pain, but then how is line or phase currents calculated for an unbalanced polyphase circuit delta or wye?