High-Leg, Open Delta - Large 3-phase load

[Shoe]

I have a client who wants to add a large 3-phase load to their exisiting 120/240V, 3-phase high-leg, open delta service. The service is rated for 400 amps fed from (2) pole-mounted transformers, hence the open-delta. Not sure of the capacities, as I've only seen pictures from afar at this point, but it's expectedly a "large-can, small-can" type arrangement.

They want to add a large (relative to the service) 3-phase air conditioning unit to the facility, which will add approximately 14 KVA per phase of load. I'm looking to understand the limitations of adding significant load to the open delta phase and whether or not this should be modified to a standard delta configuration.

Thanks.

 

[Jraef]

I have a client who wants to add a large 3-phase load to their exisiting 120/240V, 3-phase high-leg, open delta service. The service is rated for 400 amps fed from (2) pole-mounted transformers, hence the open-delta. Not sure of the capacities, as I've only seen pictures from afar at this point, but it's expectedly a "large-can, small-can" type arrangement.

They want to add a large (relative to the service) 3-phase air conditioning unit to the facility, which will add approximately 14 KVA per phase of load. I'm looking to understand the limitations of adding significant load to the open delta phase and whether or not this should be modified to a standard delta configuration.

Thanks.

Shouldn't make a difference if the A/C is ALL 3 phase. It's adding too much SINGLE phase load to it that can cause a problem with imbalance. The more 3 phase the better. Just be careful though that the A/C unit is not splitting up the 3 phase feed you give it into separate 1 phase loads.

 

[Shoe]

Makes sense. Thank you!

 

[Jraef]

I should have said though, within the capacity of the transformer bank...

I know that seems obvious, it was just bugging me that I didn't mention it.

 

[GoldDigger]

Makes sense. Thank you!

To me the biggest issue is whether the small pot is large enough for the load Anthe third phase wire.
If not, then POCO can choose between adding a third pot and upsizing the small one. They need to minimize their cost but keep the primary side balanced.

 

[Shoe]

Yes, that is a good point and one the POCO will need to address. A nice illustration of this is in the link below:

http://forums.mikeholt.com/showthread.php?t=124662&page=4

 

[Shoe]

A slight twist: it appears the original design reduced the size of the B-phase (high-leg) conductor by 50%. A, C, and neutral are all 500kcmil and B-phase conductor is 3/0.

I presume, pending final load calculations, that this may need to be upsized to 500kcmil to accommodate the additional 100 amps of 3-phase load. Not a lot of 3-phase load in this facility, so it also may be OK.

 

[mbrooke]

A slight twist: it appears the original design reduced the size of the B-phase (high-leg) conductor by 50%. A, C, and neutral are all 500kcmil and B-phase conductor is 3/0.

I presume, pending final load calculations, that this may need to be upsized to 500kcmil to accommodate the additional 100 amps of 3-phase load. Not a lot of 3-phase load in this facility, so it also may be OK.

The high leg will need to be upsized.

Have you considered converting to a wye service? I ask because not all pocos like to upgrade delta services considering that they are considered non standard in some areas.

In any case there is a good chance either poco needs to go to a closed Delta or increase the size of the "kicker" pot. Common KVAs are 25 or 50 for the single phase pot while 10 or 25 go for the smaller pot. Usually done with large amounts of single phase mixed with a small amount of 3 phase.

Pics of the bank will help here though.

 

[Shoe]

This is, unfortunatley, the best picture I have right now. It's at a remote location, and I'm getting information sent over to me piece-meal.

 

[JoeStillman]

I would just notify the POCO of the need for increased load and let them decide what to do about their transformer. The B-phase leg must be increased, though. Was there a smaller fuse on that leg in the main?

BTW, the building in that picture doesn't look like it needs much air conditioning.

 

[Shoe]

Yes, 200 amp fuse on that leg versus 400 amp on the remaining legs.

Thank you all for your insights.

Ha! Yes, it is a project up north where air conditioning is not a priority. The service is actually feeding a building across the alley (not pictured).

 

[mivey]

Shouldn't make a difference if the A/C is ALL 3 phase. It's adding too much SINGLE phase load to it that can cause a problem with imbalance. The more 3 phase the better. Just be careful though that the A/C unit is not splitting up the 3 phase feed you give it into separate 1 phase loads.

I should have said though, within the capacity of the transformer bank...

I know that seems obvious, it was just bugging me that I didn't mention it.

Even within the capacity of the bank (generally taken as 86.6% of the rating of the two units in the open configuration or 57.7% of the rating of the closed bank with three units), a balanced load will cause imbalance issues because the voltage regulation is different for the three phases.

SOPs include de-rating the attached load and over-sizing the open-delta bank (i.e. the two units would be sized larger than 115% of the load). I have seen situations where if the equipment is not de-rated, this could push the bank size to 200-250% of the load size. Normally the equipment should allow for de-rating because balanced voltages are not a guarantee even with closed banks and the customer should be prepared for as much as a 2-3% voltage unbalance (requiring approximately a 4% to 11% de-rating).