240V Open Delta

[Shaneyj]

I've got a facility with a 240V open delta service.
This is my first personal experience with an open delta service.
I don't know his service size but utility transformers are 50 and 100kVA.
My reading has led me to understand that because only 2 transformers are supplying all 3 phases - of the 150kVA - only 130k is available (86%). Is this correct?

The customer needs to power some 3 phase 460V equipment, so I'll be sourcing a transformer: 240V delta primary to 480Y secondary.

He's got 2 machines with a possible maximum load of 51.2kW. One of those loads is a 40HP motor operating a vacuum pump - the other is a CNC, so a couple small motors and some electronics.
After the 86% from the open delta is applied (assuming that is correct) do I treat the rest of the calculations using typical 3-phase math? Do I need to figure for some other loss or "open delta factor" at the step up from 240V to 480V?

Proposed Topology: Existing 240V paneld board --> Fused disconnect for XF Primary --> 240D-480Y/277 step up XF --> 480V panel --> branch circuits to respective loads

Anything else to look out for?

 

[augie47]

408.36 on your 480v panel and I would notify POCO of the added load as they may want to address their transformers.

 

[Shaneyj]

408.36 on your 480v panel and I would notify POCO of the added load as they may want to address their transformers.

Thank you sir.
I failed to mention his building is the only thing the utility transformers are feeding.


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[electrofelon]

The transformers are not your concern. You don't even know if the utility is using an open Delta arrangement . The theoretical answer is you always let the power company know of an addition of load and let them do what they want!

 

[Shaneyj]

The transformers are not your concern. You don't even know if the utility is using an open Delta arrangement . The theoretical answer is you always let the power company know of an addition of load and let them do what they want!

I get the point on giving info to poco, but what do you mean by not knowing the arrangement?
Pole mounted - 2 single phase transformers primaries fed from 2 separate phases with secondary connected in an open delta.
What am I missing?

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[LarryFine]

I get the point on giving info to poco, but what do you mean by not knowing the arrangement?
Pole mounted - 2 single phase transformers primaries fed from 2 separate phases with secondary connected in an open delta.
What am I missing?

He's saying it's not really your concern how the POCO provides your power demand.

 

[electrofelon]

I get the point on giving info to poco, but what do you mean by not knowing the arrangement?
Pole mounted - 2 single phase transformers primaries fed from 2 separate phases with secondary connected in an open delta.
What am I missing?

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You are missing that the POCO transformers are not your concern and it is pointless to apply NEC rules to them. If you think you are overloading them, you probably are not

 

[topgone]

The PoCo is a different animal. They have their own method of sizing transformers.
In my green years, I was so worried about seeing the results of the recording ammeter that I strap into a transformer for a day showing 50% overload at peak hours. The engineering department's decision was to upsize the secondary conductors to resolve a customer complaint on low voltage! I was thinking that the right thing to do was to replace the pole transformer pig. End of complaint.

 

[jim dungar]

I've got a facility with a 240V open delta service.
This is my first personal experience with an open delta service.
I don't know his service size but utility transformers are 50 and 100kVA.
My reading has led me to understand that because only 2 transformers are supplying all 3 phases - of the 150kVA - only 130k is available (86%). Is this correct?

The customer needs to power some 3 phase 460V equipment, so I'll be sourcing a transformer: 240V delta primary to 480Y secondary.

An 'open delta' is slang. You actually have a 240/120V 3-phase 4-wire delta system.
This 2 transformer connection can also be used to create a 3-phase 3-wire system.

Effectively you have two systems combined into one transformer bank. A typical formula for sizing the two transformers is .58T + S, where T is the three phase load and S is the single phase load. Working backwards your transformers would be able to handle a maximum 87kVA 3-phase load and then have 50kVA for 1-phase loads.

Utilities may use a different loading formula.

 

[topgone]

Let me share an old calculation that I was taught in my green days. It's been so long since I worked in a PoCo way back in the 80s. That open-delta connection allows for different-size transformers to be banked to supply large single-phase loads and a few three-phase loads.
The bigger transformer with the center tap (240-0-240V) supplies the single-phase loads while the smaller size transformer serves the three-phase loads.
Given your details: ( I have to shake some cobwebs in my mind)
Three-phase amps (from table) = 96A (40 HP, 460V, 3600 rpm)
Single- phase load = 51.2 kW ~ 51,200W/480 =106.67A
Total expected current on the bigger transformer = single-phase amps + three-phase line amps = 106.67 amps + 96 = 202A. The bigger transformer's secondary winding will carry the total single-phase current plus one line current of the three-phase load, that's why we add it.
So, the bigger transformer kVA = 480V X 202A = 97.28 kVA ~ since there is no single-phase transformer of that size, choose 100 kVA
The smaller transformer accompanying the bigger one forms the open-delta = 40HP X 0.746 / 0.80PF = 37.3 kVA, there's no available transformer of that size, so choose a 50 kVA single-phase transformer!
That's how my mentor, an old lineman taught me and it works!
If you want, you can compute the ratio of load served versus the sum of the single-phase transformer capacities = (51.2 + 30 kW)/0.8PF)/(100+50) = 101.5 kVA/ 150 kVA = 0.67, even lesser than if the open-delta bank were of the same size (86%)!

 

[Shaneyj]

Let me share an old calculation that I was taught in my green days. It's been so long since I worked in a PoCo way back in the 80s. That open-delta connection allows for different-size transformers to be banked to supply large single-phase loads and a few three-phase loads.
The bigger transformer with the center tap (240-0-240V) supplies the single-phase loads while the smaller size transformer serves the three-phase loads.
Given your details: ( I have to shake some cobwebs in my mind)
Three-phase amps (from table) = 96A (40 HP, 460V, 3600 rpm)
Single- phase load = 51.2 kW ~ 51,200W/480 =106.67A
Total expected current on the bigger transformer = single-phase amps + three-phase line amps = 106.67 amps + 96 = 202A. The bigger transformer's secondary winding will carry the total single-phase current plus one line current of the three-phase load, that's why we add it.
So, the bigger transformer kVA = 480V X 202A = 97.28 kVA ~ since there is no single-phase transformer of that size, choose 100 kVA
The smaller transformer accompanying the bigger one forms the open-delta = 40HP X 0.746 / 0.80PF = 37.3 kVA, there's no available transformer of that size, so choose a 50 kVA single-phase transformer!
That's how my mentor, an old lineman taught me and it works!
If you want, you can compute the ratio of load served versus the sum of the single-phase transformer capacities = (51.2 + 30 kW)/0.8PF)/(100+50) = 101.5 kVA/ 150 kVA = 0.67, even lesser than if the open-delta bank were of the same size (86%)!

So the 86% I read about is only valid if both transformers are same size?
This is the info I was interested in. Appreciate it.


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[synchro]

Effectively you have two systems combined into one transformer bank. A typical formula for sizing the two transformers is .58T + S, where T is the three phase load and S is the single phase load. Working backwards your transformers would be able to handle a maximum 87kVA 3-phase load and then have 50kVA for 1-phase loads.

Just taking the OP's situation as an example, the high leg current will be limited to 50kVA / 240V = 208.3A because there's only one 50kVA transformer connected to the high leg. And so the kVA available for 3-phase loads is 1.732 x 208.3A x 240V = 86.6kVA, which is consistent with the formula that Jim provided.

Utilities may have access to software that addresses specific customer requirements such as the following:
http://www.dstar.org/research/project-desc/GSVIC/
But I'm not sure that they'd bother with this unless it was a larger customer. But it's still worth asking about what voltage balance they can provide with your loads.

Because of the unsymmetric nature of the open wye, the impedances it presents are not balanced between the phases. And so when the 3-phase loading changes the amount of voltage imbalance will change, at least to some extent.

 

[topgone]

So the 86% I read about is only valid if both transformers are same size?
This is the info I was interested in. Appreciate it.


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That is 86% of the sum of the 2 transformer capacities or 57.7% of the original 3-phase bank capacity.

 

[jim dungar]

That is 86% of the sum of the 2 transformer capacities or 57.7% of the original 3-phase bank capacity.

But you still need to consider the 3-phase vs 1-loading. It is 86% of the sum of the 2 transformers minus the amount used for single phase and the limitation of not exceeding twice the minimum transformer size. For example: if the OP transformers were an open delta 50kVA and 75kVA the result is not equivalent to 86% of a 125kVA 3-phase transformer.

 

[kwired]

So the 86% I read about is only valid if both transformers are same size?
This is the info I was interested in. Appreciate it.


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Jim sort of addressed it in post 14 - you could supply 100 KVA of single phase load directly from the 100 kVA transformer.

That 86% comes in if you had same sized units and was supplying balanced load on all three output phases. You would only have 86% the capacity of the combined ratings of the two transformers before you start to overload things. Where a full delta would have full (balanced load) capacity of the sum of the transformer ratings.

Again POCO's may tend to allow heavier loading on them they we would with NEC calculations, especially for loads that do have sufficient "cool down time" between cycles.

 

[topgone]

But you still need to consider the 3-phase vs 1-loading. It is 86% of the sum of the 2 transformers minus the amount used for single phase and the limitation of not exceeding twice the minimum transformer size. For example: if the OP transformers were an open delta 50kVA and 75kVA the result is not equivalent to 86% of a 125kVA 3-phase transformer.

He was asking if 86% holds true if the transformers were sized the same. I replied in teh affirmative. If not, the calculations I shared should work. The said calculations are based on the single-phase supply (center-tapped, bigger) transformer and the smaller transformer connected in an open-delta connection with the bigger transformer to be able to provide 3-phase power at the same time.
Take note, the computed ratio of the total load served and the total installed transformer capacity was only 67 percent compared to 86% if the transformers were of the same size, meaning, the necessary derating was already part of the computations.