Voltage in a three phase system.

[wchild1520]

Hope i can get an explanation I can understand. Heres the situation 3ph, 480v . 495v a-b, 502 a-c, 497 b-c. Here's where I get confused 511v a-gnd, 46v b-gnd, 536v c-gnd. Can someone give me the "I,m dumber than a fifth grader explanation?:dunce:

 

[iceworm]

wc -

Just guessing:
the system is ungrounded 480D. And there is a pretty good ground on B phase.

OR

The system is corner grounded 480D and B phase is intentionally grounded

ice

 

[iwire]

Is it supposed to be a grounded or ungrounded system?

 

[iceworm]

And there is one other that it can be:

Impedance grounded 480Y witha ground on B phase.

ice

 

[kwired]

And there is one other that it can be:

Impedance grounded 480Y witha ground on B phase.

ice

I don't think that is as likely as a corner ground or ungrounded system. The voltages to ground on the high impedance system should be closer to 277 than what they are, even if there is a fault on one of them.

 

[iceworm]

I don't think that is as likely as a corner ground or ungrounded system. The voltages to ground on the high impedance system should be closer to 277 than what they are, even if there is a fault on one of them.

If a ground develops on one leg of an impedance grounded 480Y - that leg goes to 0V, the neutral geos to 277V above ground and the other two phases go to 480V above ground.

Sketch it out. Ground one phase. Put in the currents. I've watched your posts - it will go "click" in seconds or less:happyyes:

ice

 

[kwired]

If a ground develops on one leg of an impedance grounded 480Y - that leg goes to 0V, the neutral geos to 277V above ground and the other two phases go to 480V above ground.

Sketch it out. Ground one phase. Put in the currents. I've watched your posts - it will go "click" in seconds or less:happyyes:

ice

Just "click"? Transformers are supposed to hum

 

[iceworm]

Just "click"? Transformers are supposed to hum

You're right, "Click - Hummmmmm ...."
unless it is a big transformer, then it's "Click - Thump - Hummmmmmmmm" :roll:

ice

 

[beanland]

Neutral Inversion

Neutral Inversion

Hope i can get an explanation I can understand. Heres the situation 3ph, 480v . 495v a-b, 502 a-c, 497 b-c. Here's where I get confused 511v a-gnd, 46v b-gnd, 536v c-gnd. Can someone give me the "I,m dumber than a fifth grader explanation?:dunce:

What nobody has explained is why the phase to ground voltage is greater than the phase to phase voltage. What has happened is that this is an ungrounded 480V service with capacitance in the secondary cables and a high impedance volt meter. The neutral, rather than being inside the triangle made of the three phases has moved outside the triangle. This can happen on an ungrounded feed when the combination of reactance (transformer) and capacitance (cables) is right.

 

[kwired]

What nobody has explained is why the phase to ground voltage is greater than the phase to phase voltage. What has happened is that this is an ungrounded 480V service with capacitance in the secondary cables and a high impedance volt meter. The neutral, rather than being inside the triangle made of the three phases has moved outside the triangle. This can happen on an ungrounded feed when the combination of reactance (transformer) and capacitance (cables) is right.

Every fith grader should know that

Originally Posted by wchild1520
Hope i can get an explanation I can understand. Heres the situation 3ph, 480v . 495v a-b, 502 a-c, 497 b-c. Here's where I get confused 511v a-gnd, 46v b-gnd, 536v c-gnd. Can someone give me the "I,m dumber than a fifth grader explanation?:dunce:

 

[wchild1520]

Was supposed to meet with poco today . They postponed until tomorrow. He said phase to phase was good , but there was a problem phase to ground.(DUH) He said he needs the disco opened before he can tell if its his side or mine.

 

[kwired]

Was supposed to meet with poco today . They postponed until tomorrow. He said phase to phase was good , but there was a problem phase to ground.(DUH) He said he needs the disco opened before he can tell if its his side or mine.

I think you had an incompetent techician when it comes to troubleshooting from the POCO. If this is pole top transformer bank he should be able to tell just looking at the connections whether or not it is wye, delta, and which conductor if any is grounded. He won't know how good connections are, unless a wire pulled out of a termination and is just hanging there - I have seen that. He should have been able to measure voltage on his side someplace and have a good idea where to proceed. If he did not know before going out he also should have been able to contact his operations people and they should have been able to tell him what system voltage and configuration the customer is supposed to have.

 

[Hv&Lv]

I think you had an incompetent techician when it comes to troubleshooting from the POCO. If this is pole top transformer bank he should be able to tell just looking at the connections whether or not it is wye, delta, and which conductor if any is grounded. He won't know how good connections are, unless a wire pulled out of a termination and is just hanging there - I have seen that. He should have been able to measure voltage on his side someplace and have a good idea where to proceed. If he did not know before going out he also should have been able to contact his operations people and they should have been able to tell him what system voltage and configuration the customer is supposed to have.

If you open the disconnect, the voltages could "correct" themselves simply by having the load removed, and the POCO guy will probably say the problem is on your side. Don't believe him. Have him check the voltages at the transformers, not on the ground somewhere. I absolutely hate it when one of our guys tell the consumer that the problem is on their side because the voltages straightened out by pulling the meter or the disconnect. All that will prove is that the disconnect is working... And in order to correctly check the transformers, they need to be "unbanked" and checked individually. I have seen (and changed :ashamed1: ) the wrong XF out because I was "sure" and trying to finish in a hurry.

 

[kwired]

If you open the disconnect, the voltages could "correct" themselves simply by having the load removed, and the POCO guy will probably say the problem is on your side. Don't believe him. Have him check the voltages at the transformers, not on the ground somewhere. I absolutely hate it when one of our guys tell the consumer that the problem is on their side because the voltages straightened out by pulling the meter or the disconnect. All that will prove is that the disconnect is working... And in order to correctly check the transformers, they need to be "unbanked" and checked individually. I have seen (and changed :ashamed1: ) the wrong XF out because I was "sure" and trying to finish in a hurry.

Exactly - a bad connection has no voltage drop across it if there is no load.

 

[wchild1520]

There was a small transformer on our side . Once it was shut down voltages went to 480 - 0 - 480 to ground. Tech said all there transformer are grounded . This is a delta .

 

[kwired]

There was a small transformer on our side . Once it was shut down voltages went to 480 - 0 - 480 to ground. Tech said all there transformer are grounded . This is a delta .

If I understand what you are saying - the service is 480 volt corner grounded delta. You apparently have a transformer within the facility that is supposed to be a separately derived system that likely has improper grounding issues.