corner ground delta

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craig65

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i had a service call last week at a mall parking lot. There was a 120/208 wye/ 480 delta transformer feeding a lighting panel. In this case the delta was the load and the Wye was the line side. the problem is the C phase on the wye side is pulling ov 200 amps and the others almost nothing. I did notice there was no corner ground on the delat side and a neutral was connected in the Wye side. I dont have a lot of experience with delta systems,so I apologize if this is a dumb question.... but it seems to me that there is a ground fault on one phase only so it is not tripping a breaker.
If corner ground the delta side and remove the jumper from xo to ground on the wye side would that at least show me where the fault is? :confused:
Any advice?
thanks
 
Pierre C Belarge

Pierre C Belarge

Senior Member
Location
Westchester County, New York
If one does not ground one of the legs of a corner grounded delta, then an alarm system should be installed.

I do not think that grounding the system to look for a fault condition is a good idea.
I would try troubleshooting the circuits first.
 
brian john

brian john

Senior Member
Location
Leesburg, VA
That picture is on the DELTA side?

The neutral does not need to be connected on the Wye side.

What is the current on the phases on the delta?

What are the phase to phase voltages on the delta?

What is the current on the Wye A, B, C and Neutral.

What are the phase to phase and phase to XO on the wye side?

Is there a neural ground bond on the XO (field or factory installed.)
 
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jim dungar

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PE (Retired) - Power Systems
brian john said:
The neutral does not need to be connected on the Wye side. Is there a neural ground bond on the XO (field or factory installed.)
Click to expand...

With more emphasis. The X0 bushing on the 208V side must not be connected. If this is connection exists there is a high probability of overheating with unbalanced 480V loading.
 
C

craig65

Member
brian john said:
That picture is on the DELTA side?

The neutral does not need to be connected on the Wye side.

What is the current on the phases on the delta?

What are the phase to phase voltages on the delta?

What is the current on the Wye A, B, C and Neutral.

What are the phase to phase and phase to XO on the wye side?

Is there a neural ground bond on the XO (field or factory installed.)
Click to expand...

the picture is on the Wye side, I took the bonding jumper off the XO that went to the ground on the wye side but did not take the neutral from the wye side off XO. The breaker is shot so I couldn't get readings on the delta side, but the wye side was 117 on A and C and 121 on b
the current was 3,0,2 all it feeds is a time clock and a few outlets other than the transformer.
 
C

craig65

Member
mull982 said:
why should the wye connection on the primary side not be grounded?
Click to expand...

It should be. I have a groung from the 240v panel to the frame, but if I disconnect the neutral what good is the jumper from XO to ground


The lighting contractor that takes care of the pole lights has been in there as well as several others who have just moved the breaker around the panel to get the lights working, but no one has taken the time to figure out the cause. I have the transformer off with a lock out / tag out on the panel that feeds it until I can sort out the problem. If some were to corner ground with the jumper in place I didnt know what would happen. can someone post a diagram that I can follow...I'll take it all apart and start from scratch if need be. Like I said I dont deal alot with delta systems,so I'm not 100% sure of what I'm doing. I dont want someone to get hurt...or worse.
 
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jim dungar

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PE (Retired) - Power Systems
mull982 said:
O.K I guess I'm trying to understand why this is?
Click to expand...
With the X0 floating (no neutral or ground connection) the middle point of the wye can move around as needed to accommodate the unbalance on the delta side. Effectively this means the wye windings shift the amount of load they each carry (the voltage drop across each winding changes) in an effort to balance their loading.
 
C

craig65

Member
its not the XO thats getting hot it is the C phase.... on the wye side it has melted the bus bar. No damage on the delta panel that I saw..:-?:-?:-?
 
brian john

brian john

Senior Member
Location
Leesburg, VA
craig65 said:
It should be. I have a groung from the 240v panel to the frame, but if I disconnect the neutral what good is the jumper from XO to ground
Click to expand...

YOU remove high levels of current from the GROUND.

BOND the transformer frame, do not bond the XO


The lighting contractor that takes care of the pole lights has been in there as well as several others who have just moved the breaker around the panel to get the lights working, but no one has taken the time to figure out the cause. I have the transformer off with a lock out / tag out on the panel that feeds it until I can sort out the problem. If some were to corner ground with the jumper in place I didnt know what would happen. can someone post a diagram that I can follow...I'll take it all apart and start from scratch if need be. Like I said I dont deal alot with delta systems,so I'm not 100% sure of what I'm doing. I dont want someone to get hurt...or worse.
Click to expand...

And people wonder why I hate to see a delta wye transformer installed back-fed. Because 97% of electricians have issues with these systems, from improper installation to not knowig what to do when the run into such a system.
 
M

mull982

Senior Member
jim dungar said:
With the X0 floating (no neutral or ground connection) the middle point of the wye can move around as needed to accommodate the unbalance on the delta side. Effectively this means the wye windings shift the amount of load they each carry (the voltage drop across each winding changes) in an effort to balance their loading.
Click to expand...

O.K. so it seems like if you ground the XO and there is unbalanced current on the secondary delta windings then there will be unbalanced current in the primary wye windings. This unbalanced current in the primary wye windings will cause the unbalanced current to travel through the XO and through ground back to the source. This could not only burn up the XO as mentioned but also put current on the ground system. Is this correct?

By not grounding the XO it will allow XO voltage to float as you mentioned and will vary the voltage across the wye windings in order to try to balance current? Is this correct?
 
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jim dungar

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Location
Wisconsin
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PE (Retired) - Power Systems
mull982 said:
By not grounding the XO it will allow XO voltage to float as you mentioned and will vary the voltage across the wye windings in order to try to balance current? Is this correct?
Click to expand...
Yes. Just remember, this is a simplified explanation.
 
C

craig65

Member
brian john said:
YOU remove high levels of current from the GROUND.

BOND the transformer frame, do not bond the XO




And people wonder why I hate to see a delta wye transformer installed back-fed. Because 97% of electricians have issues with these systems, from improper installation to not knowig what to do when the run into such a system.
Click to expand...
I agree, and 99% of that 97% wont ask, they just trow there hands up and walk off and leave it for the next guy. Thats how I got stuck with it.
I didnt wire this I'm just trying to fix it the right way. I may not know it now but when I'm done,I will....;)


the frame is bonded via grounding electrode conductor / grounding conductor to the wye side panel and the delta panel. If I take the neutral off the wye side and corner ground the delta side will that give me fault protection on the circuits feeding the light poles?
 
J

jim dungar

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Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
mull982 said:
Is my first part correct as well regarding ground current on on the primary for unbalanced loads on the secondary?
Click to expand...
Not really.

Yes unbalanced current on the delta will cause unbalanced currents.
Unbalanced currents on the primary may try to leave through the X0 bushing. The entire transformer is subject to overheating, and has more to do with the unbalanced flux distribution and winding currents than it has to do with current flowing on X0. This is not like harmonics adding on X0.

With X0 connected to neutral, the wye side of the transformer will be behaving like any wye connected multi-wire branch circuit. If X0 is bonded to ground as well as to the neutral then you will have current flowing on the ground, the same as you would with any improper downstream neutral-ground connection.
 
augie47

augie47

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Location
Tennessee
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State Electrical Inspector (Retired)
craig65 said:
......................................................

the frame is bonded via grounding electrode conductor / grounding conductor to the wye side panel and the delta panel. If I take the neutral off the wye side and corner ground the delta side will that give me fault protection on the circuits feeding the light poles?
Click to expand...


That's sounding better :) How are your your equipment grounding conductors installed / connected ? Where and how are you going to corner ground ?
 
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