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Old 02-20-2015, 05:29 PM   #21
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In a Wye secondary, as you know you have three voltages out of phase by 120 deg.If you measure to the neutral from any phase you get 120 volts. If you measure phase to phase you get 208 volts. Now if you remove one of the three transformers; you still get 120 volts line to neutral on two phases. You can still read 208 volts across the two lines that are left. if you reverse the secondary connection on one of the transformers, you move the phase angle through an angle of 180 deg. The phase displacement on a wye circuit is 120 deg. if you rotate that line 180 degrees (By reversing the connections) you end up with 300 degrees. The difference between 300 deg. and 60 deg. is whether you measure the inside angle or the outside angle. Now you still measure 120 volts from line to neutral from either line, but from line to line where you previously read 208 volts, you now read 120 volts because the phase angle has been changed. You now read 120 volts line to line as well as line to neutral. You now have a corner grounded 120 Volt Open*Delta*bank.There is not much call for 120 Volts*Delta*three phase so change the internal transformer connections back to 120/240 volts. Now you have a corner grounded 240 Volt Three Wire Open*Delta. Much more usable. Do you have single phase loads?Move the ground connection from the corner to the center tap of one of the transformers and from there run a neutral to the service head. You now have a four wire Open*Delta*with 120/240 Single Phase as well as 240 Volt three phase.
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Old 02-20-2015, 06:28 PM   #22
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Originally Posted by hardworkingstiff View Post
TWN, if you look at figure 21 on page 12, I think that is the drawing that addresses what I think you are asking about.

I think you get the 3-phase output because the 2 "hot" phases are not 180 out when referencing to the neutral. I believe that is what you are asking about.
YES!!! This is exactly what I am talking about. Two one bushing cans connected to only TWO phases on the primary. There is somehow a manufactured third phase, and it says a small 3phase load and primarily a 120/240 single phase load. As this seems like just a can added to a simple resi system.

Seems this would be cheaper to do for the POCO but it is stated that there is a significant load on the neutral as it's an inherently unbalanced system.

Aparently it's a y-delta system. About half of the 3pahse services I see around here are done this way, even when there is a third phase on the pole, baffling. Lots of closed delta high leg systems and the 120/208Y that we are all used to.

I'd like to see a waveform of this under load. Doesent seem like the manufactured phase would be 120 degrees out. Seems like two would be at 180 and the third at some obscure angle.


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Old 02-20-2015, 06:41 PM   #23
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That page is GREAT!!! Figure 16 on page 9 is what I expect to see, but that would require two bushing cans on the pole. The POCO here can make the "Y-Delta one unit missing" with two of their standard resi cans, not using the center tap on the small one. I just seems wonky to do that where all three phases are present, with 2 two bushing cans they can make an oped delta-delta and not have high neutral current.

TWN
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Old 02-20-2015, 11:20 PM   #24
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The primaries is where the confusion is, they only use TWO phases on the pole, hence the corner ground, open delta configuration. How in the world do you put TWO phases in and get THREE out? ...
The directions everyone seem to be splintering off to actually don't address the question AS ASKED, but I think I may see the problem here.

Open Delta is very common.

Corner Grounded Delta is less common, but common.

BOTH systems require 3 phases to be present on the primary of the transformers. You can have only 2 transformer and get 3 phases out from the open delta version, but there must me 3 phases IN to the primaries of that transformer bank. So it cannot* be only two PHASES feeding 2 transformers. That I think is where your confusion lies, you were asking how can you get 3 out of 2? You don't. It's 3 out of 3, but you are only using 2 transformers to get it done. That's different.

* IN THEORY however, Corner Grounded Open Delta is possible, in the same way an SWER system is used. For those who don't know, SWER = Single Wire Earth Return, a technique used by utilities to feed far flung rural customers with SINGLE phase power, in which they only run ONE wire for the primary of the pole transformer (to save cost of the wire) and ground the other side of that transformer primary and ground the system at the source end as well. This means the EARTH is the return path for the primary voltage. Done all the time, perfectly acceptable.

If I wanted to use 2 wires out to two transformers and get 3 phase, I would have to begin with a 3 phase source at the sub station, then GROUND one phase (i.e. corner grounded delta). I could run the OTHER 2 wires out to the remote pole transformer primary to make an open delta primary, but instead of connecting the return paths of the 2 legs to the 3rd phase, I connect them to ground, just like on an SWER single phase system. Now I have an Open Delta primary with a Corner Ground, earth return. I actually found an image that shows this, albeit NOT from utility transformer connections:


In this diagram, they are actually showing an open delta primary and an open delta secondary, and the SECONDARY is corner grounded (this is for feeding a meter). But if you look at it from right to left with the utility substation in place of "EPM7000", then imagine only 2 hot wires going out along the poles to the transformer to A1 and C1 and B1 is grounded at the site, then the user can get 3 phase open delta at the pole, with the utility running only 2 wires, but using the earth return for the 3rd phase (to B at the substation).

Story time:
This entire concept came up after trying to figure out what kind of service we had in our old old shop building in Seattle when I lived there. We had 120/240 3 phase 4 wire as measured on our panelboard. But one day we lost a transformer and when Seattle City Light came out to investigate, their lineman was standing around scratching his head trying to figure out what was going on. My partner was a PE, I'm an EE, we decided to "help" him (poor guy) and got on the roof with him.

The queer thing was, this is exactly what we saw: 2 primary wires from the utility going to 2 transformers and a ground rod going down the pole from the cross arm holding the transformers, with 2 of the wires from the transformer primaries tied together and attached to that ground rod. Yet we got 3 phases from it. The lineman had no idea what he was looking at, my PE partner had no idea either. But I had a theoretical discussion in a class I took in college years earlier where we thought of how it could work, but as far as anyone knew, it was not done. Yet there it was, big as life, on an electrical service of a building built in 1893, PRIOR to there even BEING electrical power in Seattle, which took place around 1906, so we figured that was how old this might have been. So the best we could figure, someone had thought of this way back when this facility had been a foundry and they wanted to electrify it, but probably back then, wire to do it was hellaciously expensive so they did an earth return for one phase!

There are several layers of known problems with SWER systems with single phase, those would get exponentially worse if doing this with 3 phase. First and foremost, any slight difference in ground resistance will immediately result in unbalanced voltages, which create lots of problems from the get go. We had all of those problems! SCL freaked out a little when they discovered it, we had a parade of their Engineers traipse through to come and see it that day. But we needed our power back on, so they replaced it with a modern conventional 480Y277 setup (all 3 phases were actually at the pole, likely put in after this original service was done and nobody had upgraded it). They made US redo our interior wiring because of the change from 240V, but that's par for the course.

Anyway, I just thought I put it out there that there MIGHT STILL BE such thing as "Corner Grounded Open Delta" in some old legacy systems like that, but it would be exceedingly rare because of the problems it creates. It's going to be either Open Delta, or Corner Grounded Delta, pick one.

As already explained by others, if you have a "neutral" on an Open Delta system, that is NOT "Corner Grounded". That is a grounded center tap on one of the transformers to get single phase lighting and power.
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Old 02-21-2015, 03:19 AM   #25
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Originally Posted by TheWireNut View Post
YES!!! This is exactly what I am talking about. Two one bushing cans connected to only TWO phases on the primary. There is somehow a manufactured third phase, and it says a small 3phase load and primarily a 120/240 single phase load. As this seems like just a can added to a simple resi system.

Seems this would be cheaper to do for the POCO but it is stated that there is a significant load on the neutral as it's an inherently unbalanced system.

Aparently it's a y-delta system. About half of the 3pahse services I see around here are done this way, even when there is a third phase on the pole, baffling. Lots of closed delta high leg systems and the 120/208Y that we are all used to.

I'd like to see a waveform of this under load. Doesent seem like the manufactured phase would be 120 degrees out. Seems like two would be at 180 and the third at some obscure angle.


TWN

Most common connection for an open delta. Grounded wye open delta is a poco favorite where large amounts of single phase are used with a little bit of 3 phase tagging along. Eliminating one can is cheaper in addition being able to use standard single bushing cans without Ferroresonance.

I typically see these serving gas stations and auto body shops. POCO will set down a 50kva "master pot" to serve 120/240 single phase while setting down a 10 or 25kva "booster/ kicker pot" next to it.


For those who may not have them in their POCO:
Attached Thumbnails
Corner grounded open delta-attachment.jpg   Corner grounded open delta-5805860919_5e851bcaeb_z.jpg  
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Old 02-21-2015, 03:25 AM   #26
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Originally Posted by TheWireNut View Post
That page is GREAT!!! Figure 16 on page 9 is what I expect to see, but that would require two bushing cans on the pole. The POCO here can make the "Y-Delta one unit missing" with two of their standard resi cans, not using the center tap on the small one. I just seems wonky to do that where all three phases are present, with 2 two bushing cans they can make an oped delta-delta and not have high neutral current.

TWN
They have customer water pipe bonds that take care of that

A single bank doesn't contribute that much current because of the relative size to the rest of the system, but if you "roll" the phases between every bank most current starts to cancel out down the line.
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Old 02-21-2015, 03:47 AM   #27
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The directions everyone seem to be splintering off to actually don't address the question AS ASKED, but I think I may see the problem here.

Open Delta is very common.

Corner Grounded Delta is less common, but common.

BOTH systems require 3 phases to be present on the primary of the transformers. You can have only 2 transformer and get 3 phases out from the open delta version, but there must me 3 phases IN to the primaries of that transformer bank. So it cannot* be only two PHASES feeding 2 transformers. That I think is where your confusion lies, you were asking how can you get 3 out of 2? You don't. It's 3 out of 3, but you are only using 2 transformers to get it done. That's different.

* IN THEORY however, Corner Grounded Open Delta is possible, in the same way an SWER system is used. For those who don't know, SWER = Single Wire Earth Return, a technique used by utilities to feed far flung rural customers with SINGLE phase power, in which they only run ONE wire for the primary of the pole transformer (to save cost of the wire) and ground the other side of that transformer primary and ground the system at the source end as well. This means the EARTH is the return path for the primary voltage. Done all the time, perfectly acceptable.

If I wanted to use 2 wires out to two transformers and get 3 phase, I would have to begin with a 3 phase source at the sub station, then GROUND one phase (i.e. corner grounded delta). I could run the OTHER 2 wires out to the remote pole transformer primary to make an open delta primary, but instead of connecting the return paths of the 2 legs to the 3rd phase, I connect them to ground, just like on an SWER single phase system. Now I have an Open Delta primary with a Corner Ground, earth return. I actually found an image that shows this, albeit NOT from utility transformer connections:


In this diagram, they are actually showing an open delta primary and an open delta secondary, and the SECONDARY is corner grounded (this is for feeding a meter). But if you look at it from right to left with the utility substation in place of "EPM7000", then imagine only 2 hot wires going out along the poles to the transformer to A1 and C1 and B1 is grounded at the site, then the user can get 3 phase open delta at the pole, with the utility running only 2 wires, but using the earth return for the 3rd phase (to B at the substation).

Story time:
This entire concept came up after trying to figure out what kind of service we had in our old old shop building in Seattle when I lived there. We had 120/240 3 phase 4 wire as measured on our panelboard. But one day we lost a transformer and when Seattle City Light came out to investigate, their lineman was standing around scratching his head trying to figure out what was going on. My partner was a PE, I'm an EE, we decided to "help" him (poor guy) and got on the roof with him.

The queer thing was, this is exactly what we saw: 2 primary wires from the utility going to 2 transformers and a ground rod going down the pole from the cross arm holding the transformers, with 2 of the wires from the transformer primaries tied together and attached to that ground rod. Yet we got 3 phases from it. The lineman had no idea what he was looking at, my PE partner had no idea either. But I had a theoretical discussion in a class I took in college years earlier where we thought of how it could work, but as far as anyone knew, it was not done. Yet there it was, big as life, on an electrical service of a building built in 1893, PRIOR to there even BEING electrical power in Seattle, which took place around 1906, so we figured that was how old this might have been. So the best we could figure, someone had thought of this way back when this facility had been a foundry and they wanted to electrify it, but probably back then, wire to do it was hellaciously expensive so they did an earth return for one phase!

There are several layers of known problems with SWER systems with single phase, those would get exponentially worse if doing this with 3 phase. First and foremost, any slight difference in ground resistance will immediately result in unbalanced voltages, which create lots of problems from the get go. We had all of those problems! SCL freaked out a little when they discovered it, we had a parade of their Engineers traipse through to come and see it that day. But we needed our power back on, so they replaced it with a modern conventional 480Y277 setup (all 3 phases were actually at the pole, likely put in after this original service was done and nobody had upgraded it). They made US redo our interior wiring because of the change from 240V, but that's par for the course.

Anyway, I just thought I put it out there that there MIGHT STILL BE such thing as "Corner Grounded Open Delta" in some old legacy systems like that, but it would be exceedingly rare because of the problems it creates. It's going to be either Open Delta, or Corner Grounded Delta, pick one.

As already explained by others, if you have a "neutral" on an Open Delta system, that is NOT "Corner Grounded". That is a grounded center tap on one of the transformers to get single phase lighting and power.
Wait, no neutral on the pole primary? If there was one then its open wye, very common to feeding farms and rural lines, POCOs intentionally drop one phase (see pic) knowing the bank only needs two plus a grounded neutral.


Maybe Im not picturing it right... but I can see this happening if no primary MGN was present to start with. Chances are the line used to be single phase 2 wire (with a single phase service to the shop) as most older lines were ungrounded delta with no MGN. The trunk line was upgraded from say 2,400 delta to 4160Y and an MGN added. However the tap line was not, back then people didn't always understand risk, so another pig was set down and the drop neutral with water bond became the MGN back to the main line. In theory such a system will work. ITs not uncommon for MGN breaks to be masked by municipal water lines.

Note, the pic below is NOT a corner grounded delta primary, rather one phase missing from a 3 phase 4 wire multi grounded wye circuit.
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Corner grounded open delta-photo3.jpg  
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Old 02-21-2015, 07:52 AM   #28
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Since we have so many very smart people opining here, I thought I'd try to ask a question that I'm not sure how to even properly word. 1st, please remember that I have no formal training on electrical work or power distribution. Everything I've learned has been by reading or watching.

Let's assume we are talking about a Wye PoCo distribution. If you use all 3 "hots" on the primary side of a 2 transformer high leg secondary, all of the phases are 60° to the other 2 phases (both primary and secondary sides).

If you connect the primary side with 2 "hots" and the neutral, the secondary output still has the "hots" being 60° in reference to the other 2 "hots", but the primary is connected to 2 phases that are 30° as reference to the other 2 conductors and the neutral is 120° referencing to the 2 phase conductors.

How is it that these difference in angles of the line side conductors does not change the output of this transformer setup?

Please remember I've had no actual formal training on this, so if you can keep it on a 1st year college level explanation, well that would be great.

Thanks.
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Old 02-21-2015, 08:09 AM   #29
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In theory (I know this will never be done, just theory), if you had the proper transformer ratings (so the different primary voltage gave the same secondary voltages), could you connect the line side of the 2 transformer high leg secondary L1,L2 on transformer one and L1,N on transformer two? In essence using L1 as the common between the 2 transformers instead of the N.

In mind, yes you could (and yes I know there is no reason to do it this way and reasons to not doing it this way), but I'm not sure because of the phase angles of the primary.

Thanks
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Old 02-21-2015, 10:44 AM   #30
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Most common connection for an open delta. Grounded wye open delta is a poco favorite where large amounts of single phase are used with a little bit of 3 phase tagging along. Eliminating one can is cheaper in addition being able to use standard single bushing cans without Ferroresonance.

I typically see these serving gas stations and auto body shops. POCO will set down a 50kva "master pot" to serve 120/240 single phase while setting down a 10 or 25kva "booster/ kicker pot" next to it.


For those who may not have them in their POCO:
Yes and Yes! This is so common around here it has had me looking at every pole with more than one can on it.

The need for only TWO primary phases and a grounded neutral (must have anyway) if why it's referred to as farmer's three phase or poor man's three phase.

If I'm reading all of these posts right (well the ones that actually address what has been pictured/ my actual question) the grounded conductor is used as a third phase and cheap one bushing cans can be used.

TWN
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Old 02-21-2015, 10:51 AM   #31
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The three pictures posted by meadow show very clearly what I'm speaking of. If I could figure out how to post pictures on this forum I'd have posted some in my OP. There are clearly only TWO phases used from the primaries and a typical wild leg service, which comes from any delta system with a center tap for 120V loads.

TWN
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Old 02-21-2015, 11:42 AM   #32
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Ok so this has had me confused for quite sometime now. Or POCO has a lot of these around, works like a typical wild leg as far as panel/distribution is concerned. Call it poor-mans three phase, or farmers three phase. Due to the lack of a third primary wire needed.

The primaries is where the confusion is, they only use TWO phases on the pole, hence the corner ground, open delta configuration. How in the world do you put TWO phases in and get THREE out?

I'm completely fine with the secondary side of the cans as they are the same as in a closed delta 240 3phase typically used in a majority motor load facility with minimal need for the 120 side. Just one less can, it's this corner ground primary 2phase into 3 that has me all


TWN

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Originally Posted by TheWireNut View Post
The three pictures posted by meadow show very clearly what I'm speaking of. If I could figure out how to post pictures on this forum I'd have posted some in my OP. There are clearly only TWO phases used from the primaries and a typical wild leg service, which comes from any delta system with a center tap for 120V loads.

TWN
No problem regarding pics, took me a while to figure that out too. I was going to ask you for pictures, but you explained why, plus you say meadow's pictures are what you are seeing in your area.

Open delta, plus the wild leg system used interests me, that's why I am following this thread. Which wild leg system do you have for your service? If you don't mind me asking.

Borgi
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Old 02-21-2015, 11:51 AM   #33
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These systems are a 240 three phase system with two phases at 120 to ground and the "wild" one is usually around 208 to ground. All three are 240v to each other. Same as a 240 closed delta as far as voltages go, it was just the use of only TWO primary phases like the ones pictured that had me confused.

Wild leg is always the B phase and is always marked orange per code.

TWN
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Old 02-21-2015, 12:09 PM   #34
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These systems are a 240 three phase system with two phases at 120 to ground and the "wild" one is usually around 208 to ground. All three are 240v to each other. Same as a 240 closed delta as far as voltages go, it was just the use of only TWO primary phases like the ones pictured that had me confused.

Wild leg is always the B phase and is always marked orange per code.

TWN
Ok, got ya, thanks. I thought you had one on your property.

In Canada the wild leg is phase " A ", ( normal colour of red ), and we have special rules for that type of service.

I am always looking up at poles too. Excellent thread!

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Old 02-21-2015, 09:07 PM   #35
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How is it that these difference in angles of the line side conductors does not change the output of this transformer setup?

Please remember I've had no actual formal training on this, so if you can keep it on a 1st year college level explanation, well that would be great.

Thanks.
2nd What the hardworking one said.....

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