5% Single Phase Capacity on Center Tapped Delta Transformer secondary

[jim dungar]

The 5% loading limit is using the high leg to neutral (208V lighting).

Not quite. The 5% limit is on 120V L-N loads.

A 208V L-N does actually involve one of the 120V windings, so it does count as part of the 5%.

 

[mull982]

Mull...

If you list your transformer's parameters, I'll run it thru my model!

Regards, Phil Corso

By transformer parameters I'm assuming you are refereeing to voltage, kVA, %Z etc... The transformer being considered is a 75kVA unit with a 480V Delta primary and a 240V center-tapped Delta secondary. Transformer %Z is 4.42%

As I mentioned previously transformer load is approx. 45kVA with 35kVA consisting of three phase loading and 9.6kVA consisting of single phase L-N loads (4.8kVA balanced on each of the C1 and C2 windings). We are allowing about 30kVA or future spare capacity most of which will likely be L-N loads.

Not quite. The 5% limit is on 120V L-N loads.

A 208V L-N does actually involve one of the 120V windings, so it does count as part of the 5%.

I seem to be a bit confused on weather the 5% actually refers to the total connected L-N loads or just the unbalance? When reading other information it comes across as you stated that the limit is on L-N loads in general and doesn't specifically talk about the L-N unbalance.

So with the 5% limit does that mean that my total 9.6kVA L-N load (although it may be balanced on neutral) can only be 5%, or does it mean that just the neutral unbalance resulting from this 9.6kVA has to be limited to less than 5%?

 

[jim dungar]

I seem to be a bit confused on weather the 5% actually refers to the total connected L-N loads or just the unbalance? When reading other information it comes across as you stated that the limit is on L-N loads in general and doesn't specifically talk about the L-N unbalance.

The 5% limit is the amount of current on the 'neutral conductor'.
A-N and N-C loads that result in zero amps on the neutral, would be treated as any other L-L load.
This means that turning off some L-N loads could result in an overload of the transformer.
So most(?) designers play it safe and refer to the 5% limit as applying to the total L-N loads.

 

[mull982]

The 5% limit is the amount of current on the 'neutral conductor'.
A-N and N-C loads that result in zero amps on the neutral, would be treated as any other L-L load.
This means that turning off some L-N loads could result in an overload of the transformer.
So most(?) designers play it safe and refer to the 5% limit as applying to the total L-N loads.

Thanks I understand it now. I see the concern that although L-N loads may be balanced on paper they may become unbalanced in field leading to neutral current.

So in my case with 9.6kVA of L-N loads balanced on the neutral I can look at 4.8kVA as being the worst case unbalance (assuming all single phase loads on one of the windings are turned off). So looking at this as the worst unbalance and using the 5% rule this would tell me to use a 96kVA transformer with the next size up being a 112.5 kVA transformer.

The thing I need to consider however would be any additional future L-N loads that would be added that would result in a worse unbalance. Not knowing what these are and having to assume that they could possibly be grossly unbalanced doesn't seem to justify putting a largely oversized transformer there. One has to assume that the loads will be somewhat balanced and that an extreme with the loads only on one side of the neutral may be to extreme of an assumption to justify a larger transformer?

 

[mull982]

Thanks I understand it now. I see the concern that although L-N loads may be balanced on paper they may become unbalanced in field leading to neutral current.

So in my case with 9.6kVA of L-N loads balanced on the neutral I can look at 4.8kVA as being the worst case unbalance (assuming all single phase loads on one of the windings are turned off). So looking at this as the worst unbalance and using the 5% rule this would tell me to use a 96kVA transformer with the next size up being a 112.5 kVA transformer.

The thing I need to consider however would be any additional future L-N loads that would be added that would result in a worse unbalance. Not knowing what these are and having to assume that they could possibly be grossly unbalanced doesn't seem to justify putting a largely oversized transformer there. One has to assume that the loads will be somewhat balanced and that an extreme with the loads only on one side of the neutral may be to extreme of an assumption to justify a larger transformer?

I'm thinking about going with a 150kVA transformer just to cover any future L-N loads, and not have to worry about the possibility of overheating transformer?

 

[Smart $]

I'm thinking about going with a 150kVA transformer just to cover any future L-N loads, and not have to worry about the possibility of overheating transformer?

Don't forget that your 1Ø kVA loading will not be 1 for 1 in sizing a 3Ø delta secondary transformer. For quick [very minimum] estimates, you can multiply the 120/240V load by 2. So if you're powering 35kVA 3Ø, plus 9.6kVA of 120/240V, and want an additional 35kVA 120/240V to spare, you'd need at the very least a 125kVA transformer by my calculation... and that's not making any considerations for neutral loading.

Do you have or plan any 240V 1Ø loads that you can connect A-B or B-C?

You do realize you can make the neutral loading issue less significant if not moot if you went with a different configuration, right?

 

[gar]

151218-0046 EST

I do not understand the problem.

Use one single phase center tapped transformer for the single phase loads, and one single phase transformer to supply the delta wild leg. You should not put any load from the wild leg to the center tap. Size as needed for the loads. This is probably the lowest cost approach, and the best from an availability of transformers.

.

 

[Smart $]

...
You do realize you can make the neutral loading issue less significant if not moot if you went with a different configuration, right?

151218-0046 EST

I do not understand the problem.

Use one single phase center tapped transformer for the single phase loads, and one single phase transformer to supply the delta wild leg. You should not put any load from the wild leg to the center tap. Size as needed for the loads. This is probably the lowest cost approach, and the best from an availability of transformers.

.

This is the "moot" configuration.

 

[mull982]

You do realize you can make the neutral loading issue less significant if not moot if you went with a different configuration, right?

Hmm... ok maybe I have had such tunnel vision trying to figure out the center tap issue that I overlooked other obvious solutions.

What are other configurations that would allow the three phase 240V as well as 120V L-N loads? Customer does not want to run the 240V rated motors on 208V. There are also heaters rated at 240V which would put out less heat at 208V.

I'd need a winding configuration that would be commercially available from one of the major manufacturers.

151218-0046 EST

I do not understand the problem.

Use one single phase center tapped transformer for the single phase loads, and one single phase transformer to supply the delta wild leg. You should not put any load from the wild leg to the center tap. Size as needed for the loads. This is probably the lowest cost approach, and the best from an availability of transformers.

.

I'm not sure I follow this. Can you explain further. I don't need necessarily need the wild leg since there are no 208V single phase loads. Just 240V three phase, and 120V L-N loads. I understand the use of the center-tapped single phase transformer for the single phase loads but what about the three phase loads?

Does it make sense to just have a three phase delta transformer to fed the three phase loads and another small single phase transformer for L-N loads? This may be difficult b/c we are feeding an enclosure that has the individual circuits broken out internally so I'm not sure how the internal wiring is broken out via the voltage connection.

I appreciate the help!

 

[gar]

151318-1020 EST

What I described is mostly called an open delta. I don't have time now. Others can describe the circuit. It uses two transformers.

.

 

[kwired]

Gar is stating you can build a open delta bank out of two single phase transformers.

If majority of load is on the 120/240 single phase portion of the bank you still have similar unbalance on the three phase primary but do not have any 5% of the neutral issues.

Another option is three phase delta (high leg or corner grounded doesn't really matter) for three phase loads and separate 120/240 single phase transformer. Still can have balance issues on the primary - if that is a concern maybe a 240 delta transformer for anything that must have 240 volts and a 208/120 wye to supply 120 volt loads and even some three phase loads if you wanted. Many motors work fine either voltage they just draw a little more current on 208, heating elements definitely do put out less heat on 208 though.

 

[Smart $]

...What are other configurations that would allow the three phase 240V as well as 120V L-N loads? Customer does not want to run the 240V rated motors on 208V.

480V wye primary, 240 delta secondary. This option increases neutral loading to 30%.

The other option is two single-phase transformers configured as open delta (explanation being handled by kwired and gar). This option makes neutral loading moot. The transformers must be overrated to compensate for the missing third winding.

There are also heaters rated at 240V which would put out less heat at 208V.

Can these 240V heaters be connected to 1Ø circuits. If yes, did you include in 120V load or put in the 3Ø load? If the former, please state kVA of heaters... i.e. break total load down into 3Ø, 1Ø L-L, and 1Ø L-N.

I'd need a winding configuration that would be commercially available from one of the major manufacturers.

480 wye, 240 delta is a commercially available dry-type transformer.

For example, see page 8-9: http://download.schneider-electric...._File_Id=643500471&p_File_Name=7400CT0601.pdf

As I understand your needs at present, looks like you'd need the 150kVA model: EE150T151HCT.

 

[gar]

151221-0949 EST

mull982:

Recently there was a drawing of an open delta on this forum, I believe. But a quick search did not produce a result.

However, I found the following that should be useful.
http://forums.mikeholt.com/showthread.php?t=141629

.

 

[mull982]

The other option is two single-phase transformers configured as open delta (explanation being handled by kwired and gar). This option makes neutral loading moot. The transformers must be overrated to compensate for the missing third winding.

Can you purchase this open delta transformer configuration already connected in a single enclosure, or must you purchase two single phase transformers (one with a center tapped secondary) and connect them open delta in the field. My concern is that these two single phase transformers may cause confusion with the personnel installing them if they are two single phase units.

Can these 240V heaters be connected to 1Ø circuits. If yes, did you include in 120V load or put in the 3Ø load? If the former, please state kVA of heaters... i.e. break total load down into 3Ø, 1Ø L-L, and 1Ø L-N.

The heaters are 3 phase 15-20kW units rated 240-277V.

Total breakdown of loads are as follows:

3 Phase load (motors and heaters) - 35kVA
1-Phase 120V load existing (lighting and receptacles) - 9.6kVA
1-Phase load 120V (future lighting and receptacles) - allowing for about 35kVA capacity.

 

[kwired]

Can you purchase this open delta transformer configuration already connected in a single enclosure, or must you purchase two single phase transformers (one with a center tapped secondary) and connect them open delta in the field. My concern is that these two single phase transformers may cause confusion with the personnel installing them if they are two single phase units.

All kinds of things you could throw in there that may cause confusion with installers or future installers/troubleshooters. A high leg delta (full or open) is just one of those things, so is corner grounded systems, ungrounded systems, in some cases just having three phase or more then 120 volts to ground is confusion for some. It all comes down to having qualified people working on it in the first place, or at least someone that will do some research if they find something they don't know much about.

 

[Smart $]

Can you purchase this open delta transformer configuration already connected in a single enclosure, or must you purchase two single phase transformers (one with a center tapped secondary) and connect them open delta in the field. My concern is that these two single phase transformers may cause confusion with the personnel installing them if they are two single phase units. ...

Finding a non-custom, one-unit, open-delta transformer is quite doubtful.

FWIW, being able to use two different size transformers in the bank is one of the advantages of going open delta.

 

[GoldDigger]

Finding a non-custom, one-unit, open-delta transformer is quite doubtful.

FWIW, being able to use two different size transformers in the bank is one of the advantages of going open delta.

FWIW, you could also mix sizes in a closed delta with the center tapped pot being larger, or even configure an asymmetric single core three phase pot.

 

[Tony S]

FWIW, you could also mix sizes in a closed delta with the center tapped pot being larger, or even configure an asymmetric single core three phase pot.

Makes sense to me