transformer question

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Copperclad

Copperclad

Member
Location
Merritt Island, FL USA
Hi all,
Can anyone help me to understand this scenario?
An existing 15kva transformer. 480 3ph. primary to 240 3ph. secondary.
The old name plate was very hard to read, but Im pretty sure the secondary showed a "y" connection with the "XO" tap coming off the middle of one winding, like a on a delta?
Totally confused me. I thought a 240 volt 3ph. secondary would constitute a high leg (none present)
Also, we were using a solenoid type tester like a wiggy, and from any one of the secondary hots to ground, we get nothing? We confirmed that the ECG was intact, but nothing.
A digital meter would read 120 phase to ground, but the solenoid tester would not read anything...........any help would be much appreciated.
 
Copperclad

Copperclad

Member
Location
Merritt Island, FL USA
Thats what I dont get,
Phase to X0 reads 130 volts ( all three legs)
phase to phase reads 240 ( any two legs)

I wish I could draw a picture of the secondary schematic
I swear it shows a Y connection with the center of ONE winding bonded and labled X0..... like on a delta connection with a high leg, kinda weird I know
 
W

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
If the secondary is ungrounded, then phase to ground voltage readings will be unstable and almost meaningless. A low impedance meter (such as a wiggy) will read no voltage, and a high impedance meter (such as a digital meter) could read anything from zero to more than the line to line voltage.

If the system is well balanced, and there are no faults, then a digital meter would likely read approximately the 'wye' voltage (line-line/1.732), although this reading is not because of any sort of wye connection, but because of capacitive coupling.

It is possible that you are looking at a transformer that internally uses a 'scott' connection. Scott connections are rare, and most commonly used for 3 phase to 2 phase conversion, but I believe that the scott connection can be used for 3 phase to 3 phase transformation, using 2 coils. See http://en.wikipedia.org/wiki/Scott-T_transformer to get the idea.

-Jon
 
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philly

Senior Member
winnie said:
If the secondary is ungrounded, then phase to ground voltage readings will be unstable and almost meaningless. A low impedance meter (such as a wiggy) will read no voltage, and a high impedance meter (such as a digital meter) could read anything from zero to more than the line to line voltage.
Click to expand...

I would venture to guess that this is because with the low impedance wiggy, the low impedance in parallel with the high impedance of the capacitance coupling will cause an overall low impedance and all of the voltage would appear across the higher impedance in series somewhere although I'm not sure where?

And with the digital meter the imput impedance will be high thus causing the coupling impedance / meter combination to stay high and thus all the voltage will drop across this combination as opposed to the lower impedance part of the series circuit measurement although I'm not sure where the other portion of the voltage divider would be?
 
W

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
Philly,

Here is a diagram of an ungrounded system showing the parasitic capacitors. Essentially each phase has capacitance to ground and to the other phases.

http://www.littelfuse.com/protectio...ction/system-capacitive-charging-current.html

The voltage divider that you are looking for goes from one phase, to ground, and back to the other two phases.

With a low impedance meter, the meter essentially shorts one capacitor, and the other two capacitors drop the full line voltage.

With a high impedance meter, the meter is in parallel with one capacitor, and the voltage gets divided in a wye fashion by all three phases.

Copperclad,

Sounds like a system intentionally designed to be confusing. Just combine a non-standard transformer configuration with an ungrounded system....

-Jon
 
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philly

Senior Member
winnie said:
Philly,

Here is a diagram of an ungrounded system showing the parasitic capacitors. Essentially each phase has capacitance to ground and to the other phases.

http://www.littelfuse.com/protectio...ction/system-capacitive-charging-current.html

The voltage divider that you are looking for goes from one phase, to ground, and back to the other two phases.

With a low impedance meter, the meter essentially shorts one capacitor, and the other two capacitors drop the full line voltage.

With a high impedance meter, the meter is in parallel with one capacitor, and the voltage gets divided in a wye fashion by all three phases.
Click to expand...

O.K. winnie I see the voltage divider now.

So for the sake of making things interesting. what if you put a low impedance meter from L-G on one phase and put a high impedance digital meter from L-G on another phase?

I would expect to see 0V on the low impedance meter and see 240V on the high impedance digital meter. Reason being the low impedance meter shorts one cap as you mentioned and forces the 480V L-L voltage to be dropped across the other two capacitors now in series with ground being the midpoint. So if we put a high impedance meter between L-G on the other phase we will be reading across only 1 capacitor or1/2 of the L-L voltage dropped across the other two caps and thus read 240V.
 
J

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
The ANSI convention for a center tapped delta is X4 not X0.

It sounds like you have a 480:240Y/136 delta to wye transformer with an ungrounded secondary.
 
J

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
winnie said:
It is possible that you are looking at a transformer that internally uses a 'scott' connection. Scott connections are rare, and most commonly used for 3 phase to 2 phase conversion, but I believe that the scott connection can be used for 3 phase to 3 phase transformation, using 2 coils. See http://en.wikipedia.org/wiki/Scott-T_transformer to get the idea.
Click to expand...
A Scott is a special Tee connection that goes between 2-Phase and 3-Phase.
A standard Tee connection is used for 3-phase to 3-phase.
 
P

philly

Senior Member
philly said:
O.K. winnie I see the voltage divider now.

So for the sake of making things interesting. what if you put a low impedance meter from L-G on one phase and put a high impedance digital meter from L-G on another phase?

I would expect to see 0V on the low impedance meter and see 240V on the high impedance digital meter. Reason being the low impedance meter shorts one cap as you mentioned and forces the 480V L-L voltage to be dropped across the other two capacitors now in series with ground being the midpoint. So if we put a high impedance meter between L-G on the other phase we will be reading across only 1 capacitor or1/2 of the L-L voltage dropped across the other two caps and thus read 240V.
Click to expand...

After re-reading, I want to clarify that the additional example I gave was based off of a 480V delta secondary.

For the OP's transformer I would think that with 1 high impedance meter that he would see 136V to ground and not 120V, and if he were to use a low impedance meter on one leg and a high impedance meter on the other, then he would see 120V on the high impedance meter. This does not matter if the transformer is a secondary delta or wye connection.
 
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