Earthing Grid question - Touch potentials for a 1500kVA Transformer 2

[bertbot]

Hi Guys,

I'm a bit confused with touch potentials for a 1500kVA Transformer. Just switched industry so learning from scratch again.

Please reference the attachment (Powerpoint)

I've used a program to calculate the touch and step potentials of an earth grid I created under a 1500KVA Transformer.

Grid resistance = 1.25 ohms
fault current = 1913 Amps

On the first slide (touch voltages), the white area (8x6m around the transformer) shows safe touch potentials (<254.18V)

This is all good if i am touching the transformer under a fault condition.

However the Switchroom (MCC) is 3metres away from the transformer (just outside of the earth grid) and is in the unsafe touch potential zones (>254.18V). Does this mean that if i am touching the Switchroom under a fault condition, than it's unsafe? or is it OK and why? Someone asked me why we don't have an earth grid underneath the switchroom and i'm unsure of the answer.

Also, i would be expecting the touch potentials to lessen as you move away from the transformer, but the results show the touch potentials getting greater as you move away from the transformer.

Thanks for your help guys, looking forward to being enlightened!

 

[7anoter4]

I think the Touch Potential Distribution is logical.
First of all in the ground grid aria around the transformer the current density-if is the same conductor all-around the Grid -that means
the current intensity too- is less in the centre and more at the periphery-similar to the skin effect in a conductor-see IEEE-80
Annex F (informative) Fig. F2.
The earth potential drop from the grounding conductor towards the mash centre approx. V=I/(2*PI()/r where I= near-by conductor current
r=distance from conductor centre line to mash center. So if the mash is same size all-around the grid, touch potential rise to the grid extremity.
For the outside part following Sunde formula Vr=I/(2*PI()*rx) the potential of the point to the infinite [as GPR is] decrease with the distance
from the center of the grid [the current is Ig=total grounding current].So if the touch potential definition is the same[that means the voltage
difference between one grounded point and the earth local point potential] this difference grows with the distance from the grid center.

 

[rcw retired EE]

Touch potential at the MCC will be high. The MCC metal skin will be electrically connected to the transformer ground through the equipment grounding conductors per Codes. The tranformer metal case and ground grid are connected together, so the grid, the transformer and MCC skin are all at the same potential.

Your program assumes fault current is returning to the transformer neutral through the earth and the grid, creating IR voltage drops as the current flows through the earth's resistance. Someone standing at the far end of the MCC will be able to touch the MCC, which is at ground potential, when their feet are at a different potential due to the IR drop. That is what creates the touch potential.

For touch potential we analyze the voltage difference between every point on the ground surface and the grid. We assume there is a metallic object like a fence or enclosure that is electrically connected to the grid hovering over each point. Someone standing on that point and touching the metal will be exposed to that touch potential.

As explained in the previous post, the further you are from the grid, the closer your feet are to the remote earth at 0 Volts so the potential between your feet and the grid is larger.

To make your system better, loop the MCC with a gorund ring so the person's feet and MCC are at the same potential and Vtouch = zero.

 

[7anoter4]

Correction:
Since ro[r] in Greek Alphabet was obsolete in may above post formula the actual Sunde should be:
Vr=ro*Ig/(2*pi()*rx)
where ro =earth resistivity presumed uniform

For potential drop from conductor to mash center [touch potential]:
From EPRI 2682 [for instance]
V(x,y)=Ir/4pl*ln[g(x,y)]
g(x,y)=[sqrt((x+l/2)^2+y^2)+x+l/2]/[sqrt((x+l/2)^2+y^2)+x-l/2]
simplifying:
Vx=I*ro/2/pi()/l*ln(dstref/distdirect)
where distref =distance from the "conductor reflection" above the
Ground
distdirect =distance from actual conductor to mash center
l=length of the conductor segment

 

[7anoter4]

Another publication [I think 1960 ?]instead of Sunde formula
S.Z. HADDAD PAPER FEC-P142/S&L ENGINEERING-GROUND AND CATHODIC PROTECTION
Vex=2*Vg/PI*arcSin(r/(r+dx)
Vg=voltage potential rise[GPR] ,r=equivalent radius of the grounding system
dx=distance to the perimeter of grounding system
A good solution: to finish the Grounding Grid to form a complete quadrilateral.

 

[cuky2000]

Berbot,

Nice slides!!

1.Does this mean that if I am touching the Switchroom under a fault condition,
a. Than it's unsafe? Appear to be unsafe since the calculated voltage in the Switchroom (MCC) is in excess of 1000V (green area >254V max allow). However; this could be because the grounding model may not consider the actual surface material other than the top layer

b.Why? Here are possible explanation of the grounding model of the program:

i. Single Surfacing material: Different allowable voltage should be considered if there are multiple soil surfacing materials (crushed rock, concrete slab, native soil, etc).
ii. Switchroom/MCC Area: Verify if there is concrete slab ~5mx15m approx. (guessing from the layout on slide #3). Concrete slab if any behave as a quasi equipotential surface virtually as the same potential as the MCC metallic cabinet.
iii. Footwear Resistance: Also, there is high probability that the program consider a person with bare foot standing in the area. Shoes resistance could help increasing the allowable voltage particularly in soil with top layer low resistivity as in this case (36.2 Ohm.m unusual low)

2. Someone asked me why we don't have an earth grid underneath the switchroom and I’m unsure of the answer. As suggested by RCWilson, a ground loop around the MCC will ensure better touch potential profile in this area. Bonding the rebar (if any) on MCC foundation could also help to equalize the voltage rise in the MCC and the standing area.

3. Also, I would be expecting the touch potentials to lessen as you move away from the transformer, but the results show the touch potentials getting greater as you move away from the transformer. The program assumes the existence of a grounded object above grade can be touched during a fault at any point on the area analyzed. Voltage decay almost exponentially with the distance to the ground grid. That is why the upper left corner of the area is shown in red (~1700V with respect the metallic grid conductor). By inspection of the site, engineer should determine if this is a concern with possible grounded object in the area.

QUESTION: What surfacing material resistivity and fault clearing time values do you used to determine the allowable touch potential of 254.18V?

I hope that those suggestions could help.

 

[bertbot]

Hi Guys, thanks very much for your input. It’s helped my understanding a lot more. With 7anoter4 referencing IEEE80,
I found and read through IEEE80, and it’s alot better written than the Australian standards I’ve read so far.

From my understanding now, any metallic structure (MCC, Conveyors, buildings etc) that is bonded back to the same earth as the transformer, will require a grounding ring to ensure the potential where I’m standing at is the same as the metallic structure that I’m touching.

The plant I’m working on has combined HV & LV Earthing system, to have a grounding ring below all the conveyors and structures would be too costly to implement.

Could you guys please share your experiences in what is realistically installed? Is it considered safe enough to just have the grounding ring around all the transformers and MCC’s?

Thanks

 

[pwrtran]

Touch potential = GPR - Surface Potential
The Surface Potential depends on:
1) upper layer soil resistivity - the lower the less surface potential
2) buried depth of the grounding electrodes - the deeper the less Surface Potential

Therefore, several things you could consider:
1) if in a middle of no where, a personal has nothing to touch, don't bother
2) bring the surface potential closer to GPR by putting finer meshes around the metallic object perimeter, or putting a grounding mat and tie to the grid to make equipotential
3) make better insulation between foot (bare) and ground by putting high resistivity surface material such as asphalt

To increase your maximum allowable touch potential you could:
1) reduce the fault clearing time
2) send personal only who has heavier body weight > 70 kg work inside the station

Good Luck!