Substation Grounding
Substation Grounding
(OP)
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?






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