about line-ground fault

[bulancak]

We have a DELTA-seconder transformer on a post which supplies electricity over a transmission line and occurs an line-ground fault in one phase
i have two question

1-what is the voltage on the three phases and the point where the faulted phase touches the ground?(i wonder if you can give a detailed explanation)
2- Since there is no a current to return to transformer there will be a step voltage between feet of a cow or man??

and finally i have s third question.

3- if we had a transformer wye secondered and directly grounded to earth with an electrode , so where would be maximum voltages to create step-voltage?? i think on the fault point and on the transformer post ( i mean transformer earthing electrode) there would be maximum voltages because the current enter and exit the earthi am i right??

as you see i wanna understand where the maximum voltages exist during a fault and the voltage distribution in case a fault occurs on a transmission line....

 

[alehman]

If the secondary is not grounded at the source or elsewhere, then
1. The grounded phase will be at roughly earth potential near the point of contact. The other two phases will be at line-to-line voltage relative to earth.
2. Step potential will be relatively low, depending on soil conditions.

There will still be some ground current due to charging capacitance.

3. Yes. That is why you need a proper grounding system. IEEE 80 and IEEE Green Book are good references.

 

[waross]

If I understand your questions;
1> The voltage from the grounded phase to ground will be zero Volts.
2> The voltages of the unfaulted phases to ground will be phase to phase voltage. There should be no touch or step voltage but don't trust it.
The resistance and capacitance to earth of the two unfaulted phases will result in a current to ground. Although this current will be low, there may still be a shock hazard in the location of the ground fault. There are a lot of variables involved, but some combinations of variables may result in dangerous voltages on some exposed conducting surfaces.
If another phase is grounded in another location fault current will flow between the two ground faults in the ground and touch and step potentials are more likely.
3> Yes there will be a danger of touch and step potentials.

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

 

[WhiteyWhitey]

If I understand Correctly from question 2, you are saying that if a single phase is grounded then no step / touch potentials will be present at the fault location.

Can someone explain this to me?

If there is a grounded HV conductor then at the point of grounding it will act like a fault taking earth conductor untill protection kicks in.

If you could point out what I am missing that would be great.

Thanks,
Andrew

 

[waross]

Andrew; Ungrounded system. Only charging current will flow and that will generally not develop touch or step voltages except under unusual conditions. Grounding one phase results in an equivalent circuit to a corner grounded delta.

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

 

[bulancak]

can we calculate using a fault current value the voltage at a distance from the fault point? I guess there is a formula like this
V= I.s.S./2 .pi.(X.X)
here s = earth resistance
S= step distance (0,6 -1 meter)
x: distancve from the point
I: fault current

Do you know this formula ? if so make an analyse please...

 

[jghrist]

It's not that simple. I suggest that you get a copy of IEEE Std 80-2000 Safety in AC Substation Grounding. Member price for .pdf version - $99 at

https://sbwsweb.ieee.org/ecustomerc...tomer&SWEHo=sbwsweb.ieee.org&SWETS=1192713657