Open Delta Connection - 208V loads

[PnCEngineer]

I am dealing with a three phase, open delta set of station service transformers that was (for whatever reason) set up to deliver 208V potential to a set of open delta auxiliary VTs. The transformers are configured as shown below. One of the open delta auxiliary VTs is fed from phase C to neutral (X2 of the first transformer and X3 of the second transformer)- I understand clearly from the diagram how this yields 208V. The second open delta aux VT is fed from phase A to the ungrounded center tap of the other transformer (X1 of the first transformer and X2 of the second transformer, which is shown untapped on the diagram). I can't seem to wrap my head around this one. Won't this voltage float? I know that it must work as this is existing equipment that has been connected like this for 50+ years. Just trying to understand it better.

 

[itsmoked]

Welcome to Eng-Tips PnCEngineer.

I agree.. That's pretty twisted. Took a devious mind to come up with that and wye would anyone need to too?

Nice question/post. I await an answer also.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[stevenal]

No the voltage does not float. It is fixed midway between c and b by the equal number of turns to each side of this tap.

 

[waross]

That's a fairly common connection.
The left hand transformers feeds normal 120:240 Volt loads. The neutral is grounded. Common connection all over North America.
The second transformer supplies three phase 240 volts for small motor loads.
The connection is also used in rural areas to power larger pumps and agricultural equipment. The use of open delta may save miles of line cable and insulators.
Many times residential loads are tapped off the first transformer. It may feed a secondary rack and power a group of homes.
A location needing three phase power may use either a three wire delta connection for three phase 240 Volts only, or they may use a four wire delta connection to supply both 120:240 Volt single phase loads and 240 volt three phase loads.
Years ago large air conditioners were not available in single phase. Large homes in the south could have a four wire delta service fed from such an arrangement to power large air conditioners.
The 208 Volts to neutral on the third phase is called the wild leg or the high leg. It is not generally used and the use of the 208 Volts is prohibited by code in some locations.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[davidbeach]

the use of the 208 Volts is prohibited by code in some locations

and never a good idea anywhere.

 

[PnCEngineer]

Stevenal, can you expand upon that? Intuitively I know that's how this circuit works, because I know 208V is being delivered to the set of aux VTs. Are the two 208 voltages 120 degrees apart? If you call the first voltage VC-neutral, what is the 2nd? Va-(0.5Vbc)?

 

[davidbeach]

That's just a simple geometry (trig?) problem.

 

[waross]

As I understand your description of the second set of transformers, you are taking a tap from the center point of the teaser transformer to derive a second source of 208 Volts. This voltage will be 120 degrees displaced from the normally available 208 Volts. Neither end of the second 208 Volts will be at ground potential but the voltage will not float. It will have a fixed relation to ground potential.
What is the purpose of the second set of transformers?
It is possible that this arrangement was used to feed the correct voltage to 208 Volt rated three phase equipment.
Four wire delta or open delta is a common connection. This sounds like five wire delta.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[PnCEngineer]

The aux VTs in use are 208/120v step downs and the resulting 120v voltages are then used for relaying and SCADA purposes. I don't know why the station was configured this way, there aren't presently any 208v loads being fed. I find a lot of strange connections like this at very old locations.

One other question- if one were to switch the ground connection to the other transformer (i.e., ground X2 of the second transformer rather than the first), and then bring out the same voltages, they would have the same phase relationship, correct? That is, voltage Va-neutral in this new scenario would be in phase with Va-(0.5Vbc) of the old scenario, and Vc-(0.5Vab) in the new scenario would be in phase with Vc-neutral of the old scenario?

 

[stevenal]

Correct. Ground location has no effect on the phase relationship. Not sure why you would use such a connection for relaying. Are the 208/120 transformers configured in a way to cause another phase shift?

 

[PHovnanian]

Wierd. It would be interesting to see the VT open delta connections. With all the H and X terminals labeled. I can see how the VT setup will produce a delta shifted w.r.t. the station's primary. But I can also see that, for a loss of one phase on the station primary, there are going to be some strange voltage magnitude and phases seen on the VT seondaries.