Ground Potential Rise > Surface Voltage Potential - possible? 3

[jpatricksss]

Hi all,

Is it possible to have a surface voltage potential (surface voltage profile) which exceeds the grid potential (GPR)?

Refer to Figure B.8 of IEEE (image attached). Is this what it is showing us?

Thanks to all.

JP

 

[jpatricksss]

IEEE 80:2007

 

[jpatricksss]

IEEE 80:2000!

 

[jghrist]

No, it is not possible (unless you have negative soil resistivity).

 

[jpatricksss]

What is the attached image (see original post) from IEEE80 showing me then? Is that not showing surface potential greater than GPR. This occurs for Unequally spaced grid?

This may seem like a silly question but I have done some modelling which shows surface potential greater than GPR.

 

[jpatricksss]

Please refer to the attached image from IEEE80.

 

[alehman]

I think it's showing you a plotting error.

Alan
“The engineer's first problem in any design situation is to discover what the problem really is.” Unk.

 

[jpatricksss]

The image is an extract from IEEE80...

Can anybody explain it?

 

[waross]

Taken out of context, no.
We don't all enjoy the luxury of access to IEEE documents.
Is this homework?

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

 

[stevenal]

Not much context provided. Graph is from the EPRI program, and the point made is that the maximum touch potential does not occur at the corner so it must be evaluated over the entire grid.

Maybe some round off error in the program? Doesn't really matter since the areas of concern are not at the wire locations, but in between where the difference is greatest.

 

[jghrist]

I doubt that Figure B.8 is directly from the EPRI program. It was probably created by some graphics program using the EPRI program output. I suspect the user of the graphics program just didn't get the lines quite right.

Are you sure that your model is showing surface potential? The IEEE std 80 definition of GPR is "The maximum electrical potential that a substation grounding grid may attain relative to a distant grounding point assumed to be at the potential of remote earth." If the fault is to an object connected to the ground grid, then the maximum grounding grid potential will higher than any point in the soil at a distance from the grid (even directly above it). The voltage difference between the grid conductor and any point in the soil is caused by the fault current flowing through the soil resistance. This will always be a voltage drop, not a voltage rise. If your modelling shows differently, it is wrong.

 

[stevenal]

"Using the computer program of EPRI TR-100622 [B63], a square grid with unequally spaced conductors
was modeled as shown in Figure B.7.
The computer output included the grid resistance, a surface voltage profile, the step voltage, and the corner
mesh voltage."

 

[UtilityGy]

Jpatrick,

Read this thread:

http://www.eng-tips.com/viewthread.cfm?qid=167438

Jghrist replies about grounding are one of the best posts on this website.

 

[jpatricksss]

Thank you all for your replies. This is a good forum. I shall enjoy contributing.

Conclusions:
1. It is theoretically not possible to have a surface potential which exceeds the GPR. Practically, through modelling inaccuracies, it is.

2. Figure B.8 in IEEE80 shows an error with the EPRI program. It was caused by inaccurate current ditribution representation (particularly at the connection nodes) due to a lack of segmentation (division) of the conductors.

 

[cuky2000]

Hello all,

I concur with the comment associated with fig 8.

However; we should be aware that there are special conditions were induced voltages in metallic return conductors such as shield wires, telephone cables, pipelines, fences, etc. transfering voltages that produce touch potentials larger than the GPR.

For additional details in this subject check the IEEE Std 80.

 

[jghrist]

However; we should be aware that there are special conditions were induced voltages in metallic return conductors such as shield wires, telephone cables, pipelines, fences, etc. transfering voltages that produce touch potentials larger than the GPR.

The problem was that the earth surface potential was shown greater than GPR, not touch potential.

I'm curious how the touch potential could exceed GPR. This would require that you were touching (or standing on) something that was at lower potential than remote earth while touching something connected to the ground grid, which is at GPR potential with relation to remote earth.

 

[cuky2000]

Enclosed are three pages with the following information:
Page 1: Excerpt from the IEEE Std 80 describing cases for touch potential exceeding GPR.

Page 2: Alternate calc for Example of IEEE Std 80- for Figure B.7 confirming that Vtouch < GPR.

Page 3: Fig B.7 edited with remarks.

I hope this could help clarifying this post.