3 phase delta/delta with center tap

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SAWMILL TECH

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We have just recently purchased a new plant where I have run across a situation where I'm not comfortable or knowledgeable with. The plant has a 480 volt 1200 amp service feeding it. One of the branches feeds a 75KVA, three phase, delta to delta, 480 volt to 240 volt with a center tap on the secondary for 120 volts. There is no equipment in the plant that uses three phase 240, everything has been converted to 480 volt over the years. The only thing the transformer feeds is some 240/120 single phase panels using the center tap as neutral. The transformer's primary is protected, the center tap is grounded. I'm not sure of how to rate and protect the secondary with the center tap (5% of total KVA comes to mind but I'm not sure). Transformer is old and it's been impossible to get in contact with Square D support. Also I am having trouble finding any reference to this situation in the NEC. I believe the secondary is not protected right or I can even use this transformer in this application.
 
My wife just purchased a new plant too. It's in the garden now. :D

Actually check out 2008 NEC - Table 450.3(B) - if you have primary-only protection sized correctly for your transformer then you might not necessarily need secondary protection.
 
240.21(C)(1) seems to say that the OPs situation is one where secondary protection is NOT required, provided that the primary protection is sized correctly.
 
You do need overcurrent protection on the secondary conductors per 240.21 as you are feeding a 4 wire secondary. The panel(s) probably provide this protection
A 5% "neutral restriction" is common on these transformers but I don't know that I have ever seen the 5% addressed as far as overcurrent.
From the sounds of things, if you don't need the 240, you might be better off with a 208Y/120 transformer.
 
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SAWMILL TECH said:
I'm not sure of how to rate and protect the secondary with the center tap (5% of total KVA comes to mind but I'm not sure). Transformer is old and it's been impossible to get in contact with Square D support.
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If this dry type three phase transformer from Square D, it was rated as 5% maximum single phase loading.

The actual loading is dependent on the load balance (due to circulating currents in the delta-delta transformer windings). In an ideal world with perfectly balanced 3-phase voltages the actual output, of the center-tapped winding, would be KVA=2.S+T, working back wards and with 0 three phase loading; the single phase KVA = 75/2.5 = kVA. The 5% load restrictions was because the circulating currents are hard to predict, and the intent of the transformer was to actually feed 3-phase loads.
 
jim dungar said:
If this dry type three phase transformer from Square D, it was rated as 5% maximum single phase loading.

The actual loading is dependent on the load balance (due to circulating currents in the delta-delta transformer windings). In an ideal world with perfectly balanced 3-phase voltages the actual output, of the center-tapped winding, would be KVA=2.S+T, working back wards and with 0 three phase loading; the single phase KVA = 75/2.5 = kVA. The 5% load restrictions was because the circulating currents are hard to predict, and the intent of the transformer was to actually feed 3-phase loads.
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Since the winding impedances are the same, 2/3 of the single-phase load is on the center-tapped winding and 1/3 of the single phase load is on the other two windings.
 
mivey said:
Since the winding impedances are the same, 2/3 of the single-phase load is on the center-tapped winding and 1/3 of the single phase load is on the other two windings.
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Which is why the effective KVA is greater than the KVA of the single winding (30 vs. 25). But the issue is still the circulating currents due to the unbalanced 3-phase loading.
 
My thoughts:

I've never heard that load imbalance was an issue with Delta-Delta transformers. Don't they behave like three 1ph units?

A high-leg Delta often has a lower high-leg load, and is often supplied from an open-Delta primary. (2/3 of a Y, actually)

The 120/240v section can be treated like any ordinary 1-ph service, and you don't have to have the high leg supplied.

However, you can place any 240v-only (no neutral required) loads between the high leg and either other phase.

If economy permits, you can have a 1ph panel for 120v and 120/240v loads, and a 3ph panel for 240v line-to-line-only loads.
 
Larry,

You don't normally hear of issues with unbalanced loads on delta-delta transformers because they normally are used to feed 3-phase loads.

Many 240/120V 3PH 4W systems are fed from open-delta connections because of the issues surrounding circulating currents in closed-delta transformers. Circulating currents do not occur if the primary connection is a wye or an open-wye.
 
jim dungar said:
Which is why the effective KVA is greater than the KVA of the single winding (30 vs. 25). But the issue is still the circulating currents due to the unbalanced 3-phase loading.
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The center-tap winding sees 1/3 of the three phase load and 2/3 of the single phase load. There are no three phase loads so we are only seeing 2/3 of the single phase load in the center-tapped winding.

So why not 3/2*25 kVA = 37.5 kVA? Is there a rule of thumb you are using that is giving you 80% of this value?
 
The references I am using are based on determining the largest 120V L-N unbalance that can exist. This would occur when 1/2 of the winding is fully loaded and the other half is at 0% load. The result is a severely unbalanced transformer can not handle the same loading as a balanced one

This is why a generally accepted formula for sizing center tapped delta secondary transformer (with wye primary) is KVA = 2.5S + T
 
jim dungar said:
The references I am using are based on determining the largest 120V L-N unbalance that can exist. This would occur when 1/2 of the winding is fully loaded and the other half is at 0% load. The result is a severely unbalanced transformer can not handle the same loading as a balanced one

This is why a generally accepted formula for sizing center tapped delta secondary transformer (with wye primary) is KVA = 2.5S + T
Click to expand...
Thanks. I will add this to my sizing notes.
 
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