Single phase loads on Open Delta

On Tue, 19 Feb 2008 00:51:13 -0800 (PST) snipped-for-privacy@gmail.com wrote: | Hi everyone, | | Im an electrical engineer in europe, | But now I have to design an Electrical installation where they have a | so called "Open Delta" connection. | | I believe it is the same as our 3-phase delta system, exept that one | leg of the transormer is 'missing". | Theoretical I understand the principle, but how to use it in practice? | | In the installation there are several pumps (3-phase electrical | motors) wich should not be any problem, | | But there are also a lot of single phase consumers | Computers 100% duty cycle, | 24VDC generators 100% duty cycle | Heaters (On/Off switching depending of temp demand) | coils | eetc. | | How should I devide these consumers between the 3 phases. | a) equaly over all 3 phases | or | b) so that all single phase loads are wired the the phisical windings | (not using the missing leg of the open delta connection) | | Can onyone help me with this situation,

The open delta does have one side of the delta triangle missing. What that means for loading is that any loads connected across the open side will use current through both of the other two secondary windings that are present.

For a single phase load connected on the open end, you have to derate the capacity of the transformer to one half because each winding can have only that original current maximum, but you are only getting the regular voltage while using two windings in "series".

For a three phase balanced load, you have to calculate what the current is through the windings based on the vector sum of the three phases of load.

Normally, you would connect the single phase loads to only the sides that have windings, and not to the open side. Three phase loads connect as if the configuration were a closed delta. Now you just need to add up all of the currents, using the correct vector formulas, for the two windings that are present. Once you have the current in each winding, select the one that is larger and double it to use two identical windings, and multiply by the voltage (230 or 400 or whatever it is you will be using). Maybe you will want to go back and rebalance the single phase loads to reduce the current on the winding with the most.

You will also need to calculate the current on the common corner wire of the system, which would be the vector difference of the current in the two windings (which, because the vector angles are quite different, will be a lot). With all three phase loads it will be the same as the other two wires. With all single phase loads in balance (and not connected across the open end), it will be twice as much as each end wire. The mix of single phase and three phase loads will be somewhere in between.

Connecting single phase loads across the open end would normally not be done. But if you have more transformer capacity and less neutral/common wire capacity, connecting single phase loads across the open end would be a way to lessen the loading on that wire, at the expense of excess loading on the transformer.

Open delta will be grounded on the centertaop of one of the transformers, and this is usually larger than the other transformer. All of your 120v single phase loads need to go on the center tapped transformer. The "wild leg" will be 208v to ground/neutral. It is also bad practice to connect L/L loads to the wild leg on open delta.

On Tue, 19 Feb 2008 12:36:59 -0500 snipped-for-privacy@aol.com wrote: | On Tue, 19 Feb 2008 00:51:13 -0800 (PST), snipped-for-privacy@gmail.com | wrote: | |>Hi everyone, |>

|>Im an electrical engineer in europe, |>But now I have to design an Electrical installation where they have a |>so called "Open Delta" connection. |>

|>I believe it is the same as our 3-phase delta system, exept that one |>leg of the transormer is 'missing". |>Theoretical I understand the principle, but how to use it in practice? |>

|>In the installation there are several pumps (3-phase electrical |>motors) wich should not be any problem, |>

|>But there are also a lot of single phase consumers |>Computers 100% duty cycle, |>24VDC generators 100% duty cycle |>Heaters (On/Off switching depending of temp demand) |>coils |>eetc. |>

|> How should I devide these consumers between the 3 phases. |>a) equaly over all 3 phases |>or |>b) so that all single phase loads are wired the the phisical windings |> (not using the missing leg of the open delta connection) |>

|>Can onyone help me with this situation, |>

|>Thanks alot in advance, |>

| | Open delta will be grounded on the centertaop of one of the | transformers, and this is usually larger than the other transformer. | All of your 120v single phase loads need to go on the center tapped | transformer. The "wild leg" will be 208v to ground/neutral. | It is also bad practice to connect L/L loads to the wild leg on open | delta.

Given the OP is in Europe, it is unlikely to be a 208/120 setup. Maybe it will be a 400/230 setup. But maybe it will just be 230 delta with the corner grounded. The OP should have provided more details.

. Depending the current rating of the phases, I would think you would want to balance the loads on different phases. Is it more acceptable to connect L-L to the high leg with closed delta?

If you have 3 transformers it is just delta and all phases are basically the same for L/L loads. The only place I have seen it is when it "grew" from open delta but these are utility decisions.

Note that for a single phase load across the open leg, you are actually loading both of the actual legs rather than a single leg. You will have the same current magnitude in both of the actual legs. If you carry single phase loads on only the actual legs- then you can handle twice the total load. Draw a few diagrams and look at the currents.

This problem doesn't occur with closed delta.

Open delta screws up the power factor of the supply transformers so they have to be derated which I think is what you refer to. My recolleciton is the derating is 87%, but that would be for equal loading all the way around.

. The loading on the transformers is the problem of the utility. From the users standpoint, I would think you would also want to load the high leg within the rating of the service.

My recollection is that Xcel only provides open delta and voltages can be further out of balance than Y. Heavier loading on the 120/240 side tends to unbalance the delta. SquareD used to have a quite low percentage of load on its delta transformers that could be used for

120V. I suspect that was also because of imbalance that could cause circulating currents in the delta secondary.

| Open delta screws up the power factor of the supply transformers so they | have to be derated which I think is what you refer to. My recolleciton | is the derating is 87%, but that would be for equal loading all the way | around.

Yes, that is the derating factor for a balanced three phase load. Actually closer to 86.6%. For single phase loads on the 2 real legs, no derating of the transformer is needed. However, the neutral wire needs to be derated.

. It is even closer to 86.60254%. Obviously it was rounded.

Because?

On Sat, 23 Feb 2008 02:22:22 -0600 bud-- wrote: | snipped-for-privacy@ipal.net wrote: |> On Fri, 22 Feb 2008 14:16:38 -0600 bud-- wrote: |> |> | Open delta screws up the power factor of the supply transformers so they |> | have to be derated which I think is what you refer to. My recolleciton |> | is the derating is 87%, but that would be for equal loading all the way |> | around. |> |> Yes, that is the derating factor for a balanced three phase load. Actually |> closer to 86.6%. | . | It is even closer to 86.60254%. Obviously it was rounded. | |> For single phase loads on the 2 real legs, no derating of |> the transformer is needed. | |> However, the neutral wire needs to be derated. |> | Because?

OK, I finally did the math and it needs to be derated 57.735%.

Consider a 3-wire circuit with a variable phase angle. The line to neutral voltage is the same on each. The loads are purely resistive and equal. This results in an expected specific current on each line wire. Consider a load and voltage such that the line current on each phase is 1.0. The current on the neutral at an angle of 180 degrees is 0.0. The current on the neutral at an angle of 0 degrees (that's equivalent to two circuits on the same phase trying to share a neutral) is 2.0. At angles from 120 to 0, the neutral current will at or more than the line currents. See the 60 degree difference angle line below:

difference neutral current angle amps angle

180 0.000000000 --- 175 0.087238775 87.5 170 0.174311485 85.0 165 0.261052384 82.5 160 0.347296355 80.0 155 0.432879228 77.5 150 0.517638090 75.0 145 0.601411599 72.5 140 0.684040287 70.0 135 0.765366865 67.5 130 0.845236523 65.0 125 0.923497226 62.5 120 1.000000000 60.0 see NEC rule 310.15(B)(4)(b) 115 1.074599217 57.5 110 1.147152873 55.0 105 1.217522858 52.5 100 1.285575219 50.0 95 1.351180415 47.5 90 1.414213562 45.0 85 1.474554674 42.5 80 1.532088886 40.0 75 1.586706681 37.5 70 1.638304089 35.0 65 1.686782892 32.5 60 1.732050808 30.0 derate 57.735% 55 1.774021666 27.5 50 1.812615574 25.0 45 1.847759065 22.5 40 1.879385242 20.0 35 1.907433901 17.5 30 1.931851653 15.0 25 1.952592014 12.5 20 1.969615506 10.0 15 1.982889723 7.5 10 1.992389396 5.0 5 1.998096443 2.5 0 2.000000000 0.0

For extreme precision trivia:

57.735026918962576450914878050195745564760175127012687601860232648397767 % 86.602540378443864676372317075293618347140262690519031402790348972596650 %

What are you talking about. This is high leg delta. The neutral is with a 120/240 single phase circuit and caries the unbalanced hot load. Or are you trying to feed a 208V load to the high leg?

While this isn't an exact response to the question asked, the link below leads to a short paper that might be of some interest and application toward the subject of open delta vs. closed delta and WYE configurations.

It was written by a former colleague of mine at Harris Broadcast Division (USA).

RF

On Sun, 24 Feb 2008 02:10:17 -0600 bud-- wrote: | snipped-for-privacy@ipal.net wrote: |> On Sat, 23 Feb 2008 02:22:22 -0600 bud-- wrote: |> | snipped-for-privacy@ipal.net wrote: |> |> On Fri, 22 Feb 2008 14:16:38 -0600 bud-- wrote: |> |> |> |> | Open delta screws up the power factor of the supply transformers so they |> |> | have to be derated which I think is what you refer to. My recolleciton |> |> | is the derating is 87%, but that would be for equal loading all the way |> |> | around. |> |> |> |> Yes, that is the derating factor for a balanced three phase load. Actually |> |> closer to 86.6%. |> | . |> | It is even closer to 86.60254%. Obviously it was rounded. |> | |> |> For single phase loads on the 2 real legs, no derating of |> |> the transformer is needed. |> | |> |> However, the neutral wire needs to be derated. |> |> |> | Because? |> |> OK, I finally did the math and it needs to be derated 57.735%. |> |> Consider a 3-wire circuit with a variable phase angle. | | What are you talking about. This is high leg delta. The neutral is with | a 120/240 single phase circuit and caries the unbalanced hot load. Or | are you trying to feed a 208V load to the high leg?

I thought it had shifted topic to a 2 leg delta.

I should have said open delta (2 leg) in a high leg delta configuration (neutral at the centertap of one of the transformers). I don't think there is properly a neutral in a corner grounded delta. I don't know what else you are talking about.

On Mon, 25 Feb 2008 00:02:39 -0600 bud-- wrote: | snipped-for-privacy@ipal.net wrote: |> On Sun, 24 Feb 2008 02:10:17 -0600 bud-- wrote: |> | snipped-for-privacy@ipal.net wrote: |> |> On Sat, 23 Feb 2008 02:22:22 -0600 bud-- wrote: |> |> | snipped-for-privacy@ipal.net wrote: |> |> |> On Fri, 22 Feb 2008 14:16:38 -0600 bud-- wrote: |> |> |> |> |> |> | Open delta screws up the power factor of the supply transformers so they |> |> |> | have to be derated which I think is what you refer to. My recolleciton |> |> |> | is the derating is 87%, but that would be for equal loading all the way |> |> |> | around. |> |> |> |> |> |> Yes, that is the derating factor for a balanced three phase load. Actually |> |> |> closer to 86.6%. |> |> | . |> |> | It is even closer to 86.60254%. Obviously it was rounded. |> |> | |> |> |> For single phase loads on the 2 real legs, no derating of |> |> |> the transformer is needed. |> |> | |> |> |> However, the neutral wire needs to be derated. |> |> |> |> |> | Because? |> |> |> |> OK, I finally did the math and it needs to be derated 57.735%. |> |> |> |> Consider a 3-wire circuit with a variable phase angle. |> | |> | What are you talking about. This is high leg delta. The neutral is with |> | a 120/240 single phase circuit and caries the unbalanced hot load. Or |> | are you trying to feed a 208V load to the high leg? |> |> I thought it had shifted topic to a 2 leg delta. |> | I should have said open delta (2 leg) in a high leg delta configuration | (neutral at the centertap of one of the transformers). I don't think | there is properly a neutral in a corner grounded delta. I don't know | what else you are talking about.

A corner grounded delta identifies the grounded corner at least as the grounded conductor. That can therefore be termed the neutral, as well, even though the meaning of "neutrality" is less applicable. The list of conductor currents I gave applies to that configuration, regardless of whether the transformer arrangement is open delta or closed delta. It applies to the corner conductor even if it is not the grounded one. So if you have a 240 delta with a center tap grounded, and put single phase 240 volt loads on just TWO of the sides (for example, A-B and B-C) then the table applies to the corner conductor (B for this example). Since the delta has 60 degree corners, you use the 60 degree line. If the angle is different, the calculation is different. For example, for

2 wires and center of wye, it is 120 degrees, so use the 120 degree line. If you have an old 2-phase system at 90 degrees, use the 90 degree line. Single phase would use the 180 degree or 0 degree line depending on how you connect the loads.

. I don't see how grounding a phase conductor makes it a neutral.

310.15(B)(4)(b)applies to wye systems.

I haven't seen where a neutral wire has to be derated.

True, this is one if not the only time when the grounded conductor is not a neutral (carrying the unbalanced current) although "neutral" is never really defined in the NEC.

Corner delta is not seen a lot but it does show up in sewer lift stations and other places where you only need 3 phase. It has the advantage of being able to use simpler equipment although, to the initiated it might trick you into thinking you were looking at single phase. You will see 2 ungrounded conductors, a 2 pole breaker and a white one going to the ground bus. The first tip off would be no bare grounding conductor from the utility.

On Mon, 25 Feb 2008 11:56:22 -0600 bud-- wrote: | snipped-for-privacy@ipal.net wrote: |> On Mon, 25 Feb 2008 00:02:39 -0600 bud-- wrote: |> | snipped-for-privacy@ipal.net wrote: |> |> On Sun, 24 Feb 2008 02:10:17 -0600 bud-- wrote: |> |> | snipped-for-privacy@ipal.net wrote: |> |> |> On Sat, 23 Feb 2008 02:22:22 -0600 bud-- wrote: |> |> |> | snipped-for-privacy@ipal.net wrote: |> |> |> |> On Fri, 22 Feb 2008 14:16:38 -0600 bud-- wrote: |> |> |> |> |> |> |> |> | Open delta screws up the power factor of the supply transformers so they |> |> |> |> | have to be derated which I think is what you refer to. My recolleciton |> |> |> |> | is the derating is 87%, but that would be for equal loading all the way |> |> |> |> | around. |> |> |> |> |> |> |> |> Yes, that is the derating factor for a balanced three phase load. Actually |> |> |> |> closer to 86.6%. |> |> |> | . |> |> |> | It is even closer to 86.60254%. Obviously it was rounded. |> |> |> | |> |> |> |> For single phase loads on the 2 real legs, no derating of |> |> |> |> the transformer is needed. |> |> |> | |> |> |> |> However, the neutral wire needs to be derated. |> |> |> |> |> |> |> | Because? |> |> |> |> |> |> OK, I finally did the math and it needs to be derated 57.735%. |> |> |> |> |> |> Consider a 3-wire circuit with a variable phase angle. |> |> | |> |> | What are you talking about. This is high leg delta. The neutral is with |> |> | a 120/240 single phase circuit and caries the unbalanced hot load. Or |> |> | are you trying to feed a 208V load to the high leg? |> |> |> |> I thought it had shifted topic to a 2 leg delta. |> |> |> | I should have said open delta (2 leg) in a high leg delta configuration |> | (neutral at the centertap of one of the transformers). I don't think |> | there is properly a neutral in a corner grounded delta. I don't know |> | what else you are talking about. |> |> A corner grounded delta identifies the grounded corner at least as the |> grounded conductor. That can therefore be termed the neutral, as well, |> even though the meaning of "neutrality" is less applicable. | . | I don't see how grounding a phase conductor makes it a neutral. | | 310.15(B)(4)(b)applies to wye systems. | | I haven't seen where a neutral wire has to be derated.

You will when it burns up. I've explained the theory. I don't see why I should re-explain it.

| On Mon, 25 Feb 2008 11:56:22 -0600, bud-- | wrote: | |>I don't see how grounding a phase conductor makes it a neutral. | | True, this is one if not the only time when the grounded conductor is | not a neutral (carrying the unbalanced current) although "neutral" is | never really defined in the NEC. | | Corner delta is not seen a lot but it does show up in sewer lift | stations and other places where you only need 3 phase. | It has the advantage of being able to use simpler equipment although, | to the initiated it might trick you into thinking you were looking at | single phase. You will see 2 ungrounded conductors, a 2 pole breaker | and a white one going to the ground bus. The first tip off would be no | bare grounding conductor from the utility.

I've noted that special breakers are required, even though 2-pole, for corner grounded delta. I suppose that is probably because of 240 volts between line and ground. NEC 240.85 spells this out without reasons. I also note that the short circuit interruption ratings are lower for CGD. It is not specified whether it matters if the system is open delta or closed delta.

As long as the load is balanced three phase pulling the same current on each wire, there is no need for a derating. But if the load is simply two single phase loads, each pulling 100 amps, the conductor in common will have a current of 173.2 amps. It sure isn't very neutral and any labeling as such would be misleading. But it is still common to label the grounded conductor also as the neutral.

I could argue that in a system based on 2 phases connected from a 3 phase wye system (as one might see for single phase services to apartments and condos in a large residential building), the grounded conductor there is also not validly labeled a neutral. NEC 310.15(B)(4)(b) refers to the "common conductor" and to "line-to-neutral loads", suggesting that they don't want to call it a neutral in this case, but acknowledge that the term gets used. A 100 amp load on each of the two phases in that system results in 100 amps of current on the common conductor, requiring that it be counted in multi conductor derating where a neutral in a true single phase system would not need to be.

There is not a lot of code in the NEC related to a corner grounded delta or even delta in general, other than mid-point grounded delta. None even considers single phase loads at all. Presumably no one would conenct such loads? But you never know. Someone could be fooled by CGD being wired through what is essentially a 2-pole panel with 2-pole breakers. Even if they knew it was CGD, they might assume it can't be worse than the 2"-phase wye" situation (even though it is worse).