Open delta question

[electrofelon]

We are all familiar with the figures that the capacity of the bank is .577 of what a full delta would be, or .866 of the KVA of the two transformers. Can we increase this value with strategic placement of loads? The "derating" is due to the poor internal power factor of this arrangement, but what Im not clear on is does that change depending on how we load the bank? For example say I connect a single phase L-L load across two of the closed phases. Can utilize the full nameplate KVA of that transformer? Unless I am missing something, It seems that I could, as that second transformer is just sitting there and does not need to be involved with that is going on.

 

[kwired]

We are all familiar with the figures that the capacity of the bank is .577 of what a full delta would be, or .866 of the KVA of the two transformers. Can we increase this value with strategic placement of loads? The "derating" is due to the poor internal power factor of this arrangement, but what Im not clear on is does that change depending on how we load the bank? For example say I connect a single phase L-L load across two of the closed phases. Can utilize the full nameplate KVA of that transformer? Unless I am missing something, It seems that I could, as that second transformer is just sitting there and does not need to be involved with that is going on.

You can fully load the 120/240 pot to it's rated capacity, particularly with single phase loads.

How much combination of single and three phase load you can have I'm not certain

I think that .577 value would be the maximum balanced three phase load you can handle but there possibly is still some capacity left over that can supply single phase load on the 120/240 pot. This be if both units are same VA rating.

If you have larger lighting pot than stinger pot, the .577 value is probably meaningless.

 

[electrofelon]

You can fully load the 120/240 pot to it's rated capacity, particularly with single phase loads.

How much combination of single and three phase load you can have I'm not certain

I think that .577 value would be the maximum balanced three phase load you can handle but there possibly is still some capacity left over that can supply single phase load on the 120/240 pot. This be if both units are same VA rating.

If you have larger lighting pot than stinger pot, the .577 value is probably meaningless.

So if you have just single phase L-L loads, can you utilize the full KVA rating of the two transformers?

 

[tortuga]

Yes Syncro covered it here in this post:

Trouble finding straight 240v breaker vs 120/240 breaker

There is no real derating necessary for using the 'open' phase. Typical formula for sizing the unequal transformers are: Power pot = .58T Lighting pot = .58T + S Where T is the total three phase load and S is the total single phase load. Note how the transformer sizing takes into account the...

forums.mikeholt.com

 

[electrofelon]

Yes Syncro covered it here in this post:

Trouble finding straight 240v breaker vs 120/240 breaker

There is no real derating necessary for using the 'open' phase. Typical formula for sizing the unequal transformers are: Power pot = .58T Lighting pot = .58T + S Where T is the total three phase load and S is the total single phase load. Note how the transformer sizing takes into account the...

forums.mikeholt.com

Thanks, I recalled there was a somewhat recent thread on this but I couldnt find it.

 

[kwired]

So if you have just single phase L-L loads, can you utilize the full KVA rating of the two transformers?

I suppose yes as long as none of the loads are across the open end of the delta.

Three phase loads would have portion of load across the open end though.

 

[electrofelon]

Ok so with the guidance of reading over synchro's post in the previous thread, I can connect singe phase L-L loads across the closed phases and maintain full KVA. So in practice, IF I am thinking about this correctly, then there would be no reason ever to connect a single phase L-L load across the open phases correct? Well that is ignoring conductor ampacity and thinking just about the transformers....

 

[kwired]

Ok so with the guidance of reading over synchro's post in the previous thread, I can connect singe phase L-L loads across the closed phases and maintain full KVA. So in practice, IF I am thinking about this correctly, then there would be no reason ever to connect a single phase L-L load across the open phases correct? Well that is ignoring conductor ampacity and thinking just about the transformers....

I'd say yes. If there is no three phase loads, I'd say you would be better off with higher capacity single phase supply. If there is three phase loads you need to consider how those impact loading if you are otherwise heavily loading the source.

If A -B is the open end of delta ideally you should never place a single phase load between A and B as that current is passing through all windings of the system in addition to A-C and B-C connected loads.

 

[wwhitney]

Ok so with the guidance of reading over synchro's post in the previous thread, I can connect singe phase L-L loads across the closed phases and maintain full KVA.

Sure. If you have no 3 phase loads, then you don't really need an open delta; you can just think of it as two separate single phase services as far as loading. Where you have to pay more careful attention is when you have a mix of 3 phase loads and single phase loads.

Which brings up a related question--say I have an open delta and a mix of 3 phase motor loads and single phase power factor 1 loads. When the motor is drawing (close to) its rated current, what can we say (guarantee) about the phase angle of the current?

If the 3 phase load were resistive, then the math in synchro's post in the other thread applies, and the current that would be provided by the missing coil in a closed delta is at a 60 degree phase shift through the two coils, compared with the current from loads on those coils. Which provides a slight reduction in coil current and increase in capacity. But if the 3 phase load could have an arbitrary phase angle, we'd have to add the currents without any cancellation (making the math easier, at least).

Cheers, Wayne

 

[electrofelon]

Sure. If you have no 3 phase loads, then you don't really need an open delta; you can just think of it as two separate single phase services as far as loading. Where you have to pay more careful attention is when you have a mix of 3 phase loads and single phase loads.

But even with three phase loads (assume balanced) would there ever be a reason to connect a single phase load across the open phases (concerning transformer capacity and ignoring conductor capacity)?

 

[tortuga]

But even with three phase loads (assume balanced) would there ever be a reason to connect a single phase load across the open phases (concerning transformer capacity and ignoring conductor capacity)?

Correct and just note Syncro's post he used B-C as the open phase.

 

[tortuga]

Wayne were you or did you write a PI to require the open phases to be identified somehow?
If say the open jaw were always B-C / Orange - Blue it would be easier to know and thus avoid overloading a bank.

 

[wwhitney]

But even with three phase loads (assume balanced) would there ever be a reason to connect a single phase load across the open phases (concerning transformer capacity and ignoring conductor capacity)?

No.

And so your 3 phase loads should only be loads that really require all 3 phases (are there any such loads besides motors?). If you had something like a 3 phase resistive water heater with 3 elements in a delta configuration, you'd be better off (transformer capacity wise) rewiring it to avoid the open phase. Or depending on the relative size of your two transformers, getting a water heater with 2 or 4 elements to balance it better across the two transformers.

Cheers, Wayne

 

[wwhitney]

Wayne were you or did you write a PI to require the open phases to be identified somehow?

No, but you can do that for the 2029 NEC. : - )

Cheers, Wayne

 

[electrofelon]

No.

And so your 3 phase loads should only be loads that really require all 3 phases (are there any such loads besides motors?). If you had something like a 3 phase resistive water heater with 3 elements in a delta configuration, you'd be better off (transformer capacity wise) rewiring it to avoid the open phase. Or depending on the relative size of your two transformers, getting a water heater with 2 or 4 elements to balance it better across the two transformers.

Cheers, Wayne

We have 3 phase AUX heat on HVAC units. Those would pretty much have to stay three phase as I am not going to go into the unit and rewire a factory duct heater for single phase operation. Also we have some three phase machines that consist of only have single phase loads, but these are rather complicated and I am not going to attempt to turn them into single phase.

 

[wwhitney]

We have 3 phase AUX heat on HVAC units. . .. Also we have some three phase machines that consist of only have single phase loads . . .

OK, it may not be possible for you to reconfigure the existing loads, but my point was that transformer capacity wise, on an open delta you'd be better off with single phase versions of those loads. So you could distribute the single phase loads across the two transformers according to their ratings, and avoid any single phase loads on the "phantom" coil.

Cheers, Wayne

 

[Birken Vogt]

Is this a utility supplied open delta or something like an in-plant 480 -> 240 transformation?

 

[electrofelon]

Is this a utility supplied open delta or something like an in-plant 480 -> 240 transformation?

So this is utility supplied, which makes for an interesting decision process. In these cases we have to decide whether to design around distributing loads for conductor ampacity or transformer capacity. OF course some will say the utility transformers are not our problem, but in practice there are cases when we want to avoid utility hassles, costs, and upgrades, so maybe we want to have some concern for the transformer loading.

 

[tortuga]

Is there a 3rd primary available from the utility ? Perhaps they can close the delta for you? Thats my goto method, but in rural areas that third primary might be miles away.

 

[electrofelon]

Is there a 3rd primary available from the utility ? Perhaps they can close the delta for you? Thats my goto method, but in rural areas that third primary might be miles away.

there is, and i would love it if they would. I may try to get them to do it but I doubt they will. this is for Seattle city Light and they do not offer closed delta in their book, and I have never seen a closed one there. IT would certainly be more economical for them vs upgrading the two existing.

I could just send them a new load sheet and see what they do.