Substation Grounding

[StarmanJack]

I am designing a grounding grid and have had no problems meeting step and touch potentials. I am having a hard time meeting the ground potential rise requirement of 5000 V. It is a small substation that is roughly 300x700 feet. The designs fault level is 25 kVA and I have a ground resistance currently of .25 ohms. The soil has a value of 80 ohm-m and is pretty consistant even as you go deeper. How can I reduce my GPR without making the grid overly big and without having to lay a ridiculous amount of conductor into the ground? Do some substations have grounding grids that extend far beyond the substation? If so, how much? I had one of our guys model the substation laying I think it was 2 inches of GEM material around the conductor but I didn't see that help much. When he put in grounding wells, I saw some improvement but not that much and I beleive thats due to teh soil not getting any better the deeper you go. Are there no nice solutions and you end up just laying a buttload of conductor into the ground?

 

[magoo2]

Did you mean 25 kA?

 

[jghrist]

Do you have incoming or outgoing lines with grounded neutrals or shield wires? If so, some of the fault current will flow through the wires instead of through the ground grid. This will lower the GPR.

What is the reason for the 5000 V GPR requirement? Communication cables entering the station? Maybe you can replace them with fiber optics.

 

[cuky2000]

In cons tract with Canada and other countries, in the USA there is not common to limit the GPR. Telephone companies serving substations may have thresholds for GPR to use different surge protector for hard copper wires or use FO as indicates in the previous post.

Current split factor (current division factor) as determine in the IEEE Std 80 may be a good starting point.
Underground metallic infrastructures such as waterline, pipelines, conduits, sewer lines etc. may help to divert current away from the substation.

Increasing the grid resistance up to 1 Ohm may also help reducing the GPR since less current is injected into the ground for any ground fault event.

 

[pwrtran]

StarmanJack -
First, the GPR<5000V is NOT a MUST, it is recommended practice. In some difficult soil or site conditions there is no easy way to control the GPR less than 5000V. Things you really need to control are the Step & Touch potentials. Below are something you could do:

1) As jghrist already mentioned, the skywires, system neutral can act as "parallel Z", which can help to split the fault current. 25kA will be multiplied by a splitting factor.
2) Use uneven spacing grid strategy. Use large meshes in the area where a personal has nothing to touch, whereas using fine meshes in the perimeters and around any metallic structures.
3) Use asphalt to control step potential where is practical
4) Install some remote grids if applicable
5) Transformers at the source station ground through impedance, or
6) Install in-line reactors at incoming feeders

80 ohm.m soil resistivity and the 0.25 ohm ground resistance is fairly good, you should be able to find the way out.

Hope above helps!

 

[StarmanJack]

Magoo2 - I meant 25 kA.


pwrtran - I think I am running into problems due to the fact that the design short circuit current is very conservative.

I am close on this substation but I have others that are worst. I am have been trying to increase the size of the station to reduce the grounding resistance, laying down more conductor, and using more grounding rods. It feels like I have to lay down an excessive amount of conductor and rods to reduce the GRP but maybe this is normal if you are including GPR requirements. If you have GPR requirements, is it typical that you need a much stronger grounding system than if you were just trying to meet step and touch limits? The grid that i had to meet touch and step requirements was simple in comparison because the soil is fairly conductive.


The incoming and outgoing lines are included in the spit factor gotten from IEEE 80. I don't have anything near to connect to to reduce it further.

Are some of my problems just the result of the soil not improving as you go deeper? Won't rods not really help that much if this is the case?

 

[pwrtran]

Grounding design is one of the things that it can be easily over engineered or poorly engineered. If you consider everything in the worst case scenario then it will lead over design. Potential is not the key criteria but the potential difference! Yes, we have GPR requirement but GPR is not the only determine factor. If you can ideally bring everything at the same potential then why GPR matters? For argument saying the minimum ground grid resistance you can get is to install a solid metal plate same size as the property, however, it is not practical and economical, that is why you install a mesh system. Vertical ground rod is not as effective as the horizontal grounding conductor in terms of dissipating the 60HZ fault current, it is more effective for dissipating lightning current. So, focus on the potential difference. What if the station is in a rocky area? don't feel freak to say I can't make GRP < 5000V.

 

[jghrist]

cuky suggested increasing the grid resistance. This is counter-intuitive, but might work. The higher the grid resistance, the less current flows through the ground grid and more flows through shield wires and neutrals. Of course the current that does flow through the grid will produce more GPR per amp, so it might not work. Conversely, adding a lot of ground rods or grid area to reduce the grid resistance may be counterproductive. A fixed, approximate split factor won't show this; you might need a more sophisticated calculation of the current split.

 

[pwrtran]

Increasing the grid resistance does NOT work as the product of Ig*Rg = GPR will not reduce. However, adding a grounding resistor or reactance between the ground and the neutral point at the remote source station will work, or by inserting in-line reactor will work.

 

[7anoter4]

IEEE Std 141
ch.7.2.3 Reactance-grounded system
"Much greater reduction in fault current
value is permissible with resistance grounding without risk of overvoltage"
As pwrtran said a resistor of 0.15-0.2 ohm [1000 A for 10 sec] inserted between neutral point and the grounding connection
point could do the job.

 

[7anoter4]

Correction:
As rated voltage of the resistor has to be VL-L/sqrt(3) if I=1000 A R=VL-L/sqrt(3)/1000 Volt/Ampere [OHM] or approximate
R=VL-L/sqrt(3) if VL-L is in kV.

 

[cuky2000]

IEEE Std 80: _{There are no universal rules for the determination of the
worst fault location. The following discussion relates to some, but by no means all, possibilities. For distribution substations with the transformer grounded only on the distribution side, the maximum grid current IG usually occurs for a ground fault on the high-side terminals of the transformer. However, if the source of ground fault current on the high side is weak, or if a parallel operation of several transformers results in a strong ground fault current source on the low side, the maximum grid current may occur for a ground fault somewhere on the distribution circuit.....}

Grounding resistance may reduce the GPR if the worst fault occur on the Y side of the transformer where the grounding resistor is connected. The maximum fault often happen along the transmission line outside the substation as shown in the enclosed example.

Reducing below 1 Ohm the substation resistance may not provide the optimal design. For most application 1 Ohm grid resistance is acceptable and this may help to reduce the injected current into the ground withing a specific range usually below 1 Ohm.

 

[7anoter4]

I agree with you ,cooky.Nevertheless, I think it is Y neutral solid grounded since 25 kA through Ground it is difficult to achieve in Phase-to-Ground-to-Phase short-circuit case.