Center Tapped Delta Transformer Resources

[rockman7892]

Does anyone have any good references or resources for center tapped delta 120/240V transformers? I understand the voltages and associated phasors that are derived from this arrangement but what I am better trying to understand is the limitation on the center-tap "neutral" which I believe is due to circulating currents in the delta primary? Any explanations or external references would be greatly appreciated.

PS. The application above refers to a Delta-Delta transformer but also interested in understanding how this limitation is reduced with potentially using a wye-delta center tap.

Thank You

 

[FacEngrPE]

Tutorials

https://youtu.be/xuRQWe3xEq0

https://youtu.be/VTVN4YuZy3Y

High Leg Delta woes 195 volts Bad transformer in bank

https://www.youtube.com/watch?v=ER9CDYO8YhE

Does this help?

 

[rockman7892]

FacEngrPE - Thanks those videos are good references. I understand the different voltage references and phasor relationships shown in those videos, but what I'm not clear on is what is the cause for the limitation on the neutral (5% of transformer kVA? From what I understand it has to do with circulating currents that are setup in the delta primary?

 

[waross]

I spent over 15 years dealing with the issues caused by the grounded wye/delta connection.
There are a number of serious issues.
I never encountered an issue with unbalanced loading on one side of a 120/240 Volt winding.
Actually, the allowable 120 Volt load on a single phase transformer is 1/2 the KVA rating of the transformer, but when the transformer is part of a three phase delta bank, the allowable 120 Volt load is the same as the transformer capacity.

eg: A single phase 50 KVA transformer, the maximum 120 Volt load is 25 KVA.
When that transformer is part of a four wire delta transformer bank, the maximum 120 Volt load is 50 KVA.

Also, with three 50 KVA transformers in a 120/240 Volt delta bank, the maximum single phase load is 100 KVA.

A three phase transformer with a three lagged core has issues with unbalanced loads and unbalanced primary voltages due to the phantom delta effect.
This may be the context of the 5% limit that you have seen.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[rockman7892]

Waross

Perhaps the 5% limitation does come from the unbalanced loads and the phantom delta effect in delta transformers. Can you explain that more?

And yes this limitation that I have seen is with 3-legged cores. Esentially manufacturers state that the neutral loading on the center-tap (or unbalanced kVA) cannot be more than 5% of overall transformer rating.

 

[waross]

The grounded wye/delta has issues with individual transformers, 5 legged cores and 3 legged cores.
The phantom delta applies to a wye/wye transformer as well as a delta transformer.
Consider a grounded wye/delta, 240 Volt transformer bank:
Now remove one of the transformers.
We will have 240 usable volts across the open delta.
Now consider that the primary voltage on the phase corresponding to the open delta is 5% low (or high).
The voltage across the single phase transformer is now 228 Volts or 12 Volts low.
Now consider closing the delta. You will have 12 volts driving a circulating current around the delta.
How much current?
The current is limited by 3 times the impedance.
Consider a transformer bank with 1.5% impedance. the circulating current will be 5%/(3 x 1.5%) = 133% of the transformer rated current, with no load on the transformer.
During 15 Years in an area where the connection of choice was the grounded wye/delta. and large industrial users commonly used 240 Volt delta power transformers and single phase 120/240 volt lighting panels I never encountered a problem due to unbalanced 120 Volt loading.
The 5% limitation applies to grounded wye delta transformers and grounded wye/wye, three legged cores.
In addition to the 5% limitation on primary voltage unbalance, the grounded wye/delta connections causes serious issues for all single phase users on the distribution circuit.

Bill
--------------------
"Why not the best?"
Jimmy Carter