2 phases of 3 phase service to 3 phase.

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JohnRoss

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Hello,

My name is John Ross. I am new to the forum. I found the forum through
Google, looking for info on using two phases of a three phase electrical
service, to develop three phase 240 volt service. This is a common system
to broadcast transmission plants that I service. I would like to get a diagram
with phase notation. I suppose that this is done to save the cost of a third
transformer, because in every case I have seen the primary third phase
is available, but not used. I understand the available KVA is a portion of
the KVA ratings of the transformers involved. I also welcome your comments
on the advisability of this method. I am concerned that most broadcast transmitter manufactuers reccommend a closed Delta for the most safe
power feed connection. They have concerns about harmonics.

Thanks for your comments,

John Ross
 
An open delta is a 3-phase service so you would not be developing a 3-phase service from a "2-phase" service. Perhaps you can give us some more details on your service configuration, voltage levels, and power requirements.
 
[quote = JohnRoss]... looking for info on using two phases of a three phase electrical service, to develop three phase 240 volt service. This is a common system to broadcast transmission plants that I service. I would like to get a diagram with phase notation. I suppose that this is done to save the cost of a third transformer, because in every case I have seen the primary third phase is available, but not used. I understand the available KVA is a portion of the KVA ratings of the transformers involved. I also welcome your comments on the advisability of this method. I am concerned that most broadcast transmitter manufactuers reccommend a closed Delta for the most safe power feed connection. They have concerns about harmonics.[/quote]

It sounds like you are describing a service fed from an Open-Delta, which uses two utility transformers. The output of an open-delta is usually, but definitely not always, a 240/120 3-phase 4-wire service.

I would be surprised that a utility would not provide you with a closed delta system if you are willing to pay for it.

The only major performance issue with open-delta systems is when the loading is severely imbalanced. Other than that they pretty much behave the same as a closed-delta system.
 
Not that this is the OP's situation, but a 2-phase 5-wire system has the two windings connected in the middle. The ends of the windings make 4 wires and the wire from the connection point makes the 5th.
 
JohnRoss said:
... looking for info on using two phases of a three phase electrical service, to develop three phase 240 volt service.
... I suppose that this is done to save the cost of a third
transformer, because in every case I have seen the primary third phase is available, but not used.
.... I am concerned that most broadcast transmitter manufactuers reccommend a closed Delta for the most safe power feed connection. They have concerns about harmonics. ...
Click to expand...
John -
The way you phrased this, it sounds like you have two phases of a three phase, 208V and a neutral. The third distribution phase is not connected. I say this because, if you had an open delta, the third distribution phase would be connected.

So, if you have two phases of a 208V and a neutral, yes you can get 240V Delta. There are a couple of ways to do it, one a bit trickier than the other. And it matters if you want the 240V delta to be corner grounded, highleg, or ungrounded.

If however, if you have 240V open delta on the pole, you can still get 240V closed delta.

Either way, it's fairly easy - just takes money.

Need to know which you have.

cf
 
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Cold Fusion said:
I say this because, if you had an open delta, the third distribution phase would be connected.
Click to expand...
Not necessarily. You can have an open delta using only two of the distribution phases.

add: and the neutral, of course
 
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mivey said:
Not necessarily. You can have an open delta using only two of the distribution phases.
Click to expand...
Within the constrainst the OP gave us, I don't see how.
However, none of this conversation will help the OP. May I suggest you start another thread on this topic and educate us all.

cf
 
Cold Fusion said:
Within the constrainst the OP gave us, I don't see how.
However, none of this conversation will help the OP. May I suggest you start another thread on this topic and educate us all.

cf
Click to expand...
An open delta on a distribution system having only two phases is very common using open-wye open-delta. If that is what he has, then he already has 240 volt 3-phase. I don't see how starting another thread helps the OP. We can't just assume he has a 208 volt service.

If he doesn't, his options are easy. If he does, then it is more complicated to get 240 delta, or trickier as you put it.
 
The reason that I asked where he was located was because it would have been a good indicator of if 2ph was even available where was located. Some people refer to 120/240, 1ph3w as 2ph because you have 240 between lines.
You will note that there was no reply.
We're still shooting in the dark trying to figure what the guy actually has.
 
templdl said:
The reason that I asked where he was located was because it would have been a good indicator of if 2ph was even available where was located. Some people refer to 120/240, 1ph3w as 2ph because you have 240 between lines.
You will note that there was no reply.
We're still shooting in the dark trying to figure what the guy actually has.
Click to expand...
Good point. Maybe he just has a single pot with the primary connected line-line ("two phases") and he only has a single-phase 120/240 service and is calling the 3-wire "3-phase".

Maybe we will get an answer.
 
templdl said:
The reason that I asked where he was located was because it would have been a good indicator of if 2ph was even available where was located. Some people refer to 120/240, 1ph3w as 2ph because you have 240 between lines.
You will note that there was no reply.
We're still shooting in the dark trying to figure what the guy actually has.
Click to expand...

mivey said:
Good point. Maybe he just has a single pot with the primary connected line-line ("two phases") and he only has a single-phase 120/240 service and is calling the 3-wire "3-phase".

Maybe we will get an answer.
Click to expand...

Being he mentioned saving the cost of a third transformer leads me to believe there are two pots, and he's seen three pot services... and the likelihood of being 2? 5W ??? I'm gonna lean towards an open-wye primary, open-delta secondary, also known as a Wye-Delta with one unit missing ;)
 
JohnRoss said:
Hello,

...
Google, looking for info on using two phases of a three phase electrical
service, to develop three phase 240 volt service. This is a common system
to broadcast transmission plants that I service. I would like to get a diagram
with phase notation. I suppose that this is done to save the cost of a third
transformer, because in every case I have seen the primary third phase
is available, but not used.

...They have concerns about harmonics.
Click to expand...

Agree with smart $

open wye - open delta, since he says that it would safe the cost of a third transformer, but the thing is that in open wye - open delta you would need the three transformers (for the connection) and you would only get 2/3 of the power rated on that bank of transformers.

Harmonics. Non linear load analysis.
 
jim dungar said:
[quote = JohnRoss]... looking for info on using two phases of a three phase electrical service, to develop three phase 240 volt service. This is a common system to broadcast transmission plants that I service. I would like to get a diagram with phase notation. I suppose that this is done to save the cost of a third transformer, because in every case I have seen the primary third phase is available, but not used. I understand the available KVA is a portion of the KVA ratings of the transformers involved. I also welcome your comments on the advisability of this method. I am concerned that most broadcast transmitter manufactuers reccommend a closed Delta for the most safe power feed connection. They have concerns about harmonics.
Click to expand...

It sounds like you are describing a service fed from an Open-Delta, which uses two utility transformers. The output of an open-delta is usually, but definitely not always, a 240/120 3-phase 4-wire service.

Jim, This is exactly the case. I have a 240/120 3 phase 4 wire service,
The phase to phase is 240, the phase to ground of my service is 120/120/and about 200. (the high leg). I am have at the service pole behind my transmitter building only two transformers. A 15 KVA and a 37.5 KVA.
The are only two primary high voltage wires feeding these two transformers.
These high voltage wires come from a main service line about 750 feet
from from my building. This main feed has three phases. The power company only brings two of these three phases on to my property. They obtain
my three phase service described above from two primary phases and
a nuetral. My questions is can anyone supply or direct me to a connection drawing of for this service, with the drawing having the phase notation and
an explaination of the generation of the third phase. I think I know how this
works, but I want to confirm. I also would like to know the effective KVA of
this service. My goal is to change the service to a three transformer,
closed Delta service, fed by all three primary phases.

Thanks,

JohnRoss


I would be surprised that a utility would not provide you with a closed delta system if you are willing to pay for it.

The only major performance issue with open-delta systems is when the loading is severely imbalanced. Other than that they pretty much behave the same as a closed-delta system.[/QUOTE]
 
Mayimbe said:
... but the thing is that in open wye - open delta you would need the three transformers (for the connection)...
Click to expand...
Not in this part of the world.

Y-Delta2pot.gif
 
oh wooww!!!

One question. Say each transformers have a power rate of 1 p.u. The maximum power output that you can get from this two (as the picture shows) transformers together will be 2 p.u??
 
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