How Does Delta High-Leg get 208 to Neutral?

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
How do we know which is the b phase? Utilities use any phase as the high leg.
Very true on their MV side, however almost all 'service manuals' I have seen include instructions similar to this from Xcel Energies, in regards to metering compartments (not meter sockets.)
2. ?B? phase shall be that phase having the higher voltage to ground on
3-phase, 4-wire, delta-connected systems.
http://www.xcelenergy.com/staticfil...-Energy-Standard-For-Installation-and-Use.pdf
 

Hv&Lv

Senior Member
Location
-
Occupation
Engineer/Technician
Very true on their MV side, however almost all 'service manuals' I have seen include instructions similar to this from Xcel Energies, in regards to metering compartments (not meter sockets.) http://www.xcelenergy.com/staticfil...-Energy-Standard-For-Installation-and-Use.pdf
You are correct. There is a reason the high leg has to be put on the "c" phase position in the meter socket. Otherwise it would be on "B" all the way. Although there was a change in the 2008 that allowed the high leg to be put on "C" if the meter was part of the panel.
 

kwired

Electron manager
Location
NE Nebraska
Jim nailed it. Here is a picture for more help.

View attachment 7057
Now I'm confused. So what is the benefit/difference/reason for corner grounding a delta transformer. Is it just about generating a Neutral? How does the high leg disappear?

Again, sorry for the ignorance but I appreciate you hanging with me on this...
Go back to the drawing in Post #3 which I quoted above. Disregard A,B,C or X1,X2,X3 for now as they are not marked in that drawing according to rules that B phase is supposed to be higher voltage to ground. The main point to understanding it is that the mid point of one pase is grounded and it is the bottom coil in that drawing. You probably already realize that the voltage across each coil is 240 volts. The voltage across half the bottom coil is 120 volts because it is half of 240 right. Typical pole top transformers used to build this system are all single phase 120/240 transformers connected as shown in the drawing, only the center tap of one of them is grounded and the others are left "floating" as though there was no tap there. Voltage from the top corner of the drawing to the grounded point is 208 volts. The simple answer to why is there is more windings between the points than there is between the points that have 120 volts to the grounded point. Get used to that before even try to understand phase angles and things like that. If you draw a triangle just like was mentioned and have volts represented by a unit of length you will find if the triangle is 240 units on each side the distance from one corner to center of opposite side is 208 units, not exactly a coincidence change those units of length to number of turns needed to achieve a certain voltage and they are still in same ratios.

Now to understand a corner ground - leave the center tap that was grounded in the above example to "float" as if it were not even there and ground any one corner of the triangle and you have a corner grounded system. You no longer have 4 conductors to the system you just have three that are all 240 volts to one another and one of them just happens to be at ground potential because it is intentionally connected to ground. Connect more than one point of the system to ground and you will have undesired current flowing between the two points, and if the two points have a low impedance it will be a high current.

Either system is not limited to 240 volts, there are a lot of 480 volt systems both with high leg as well as corner grounded, and in reality can be any voltage you can imagine but those two are most likely to run into in North American power systems anyway.
 
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