grounding of MV distribution systems (4-wire multi-grounded neutral
grounding of MV distribution systems (4-wire multi-grounded neutral
(OP)
This refers to the grounding requirements for medium voltage distribution systems outside the substation, that is, along the line. These systems typically operate at 12.47 kV, 24.94 kV or 34.5 kV, although other voltages are still around.
In providing grounds for equipment (distribution transformers, reclosers, capacitor banks, arresters, etc.), our company has a target of 25 ohms. In our area, this usually requires driving 2 - 6 or 8 ft rods. These are stacked with a coupling between them.
In some cases, we can't even reach 50 or 100 ohms with as much as 60 ft of driven rod! This leads me to my questions:
What is a reasonable target for grounding impedance?
Can you relate any experience-base for this selection or is it an opinion?
Does a high grounding impedance have an adverse effect on the performance of the surge arrester? If so, at what value?
Thanks in advance.
In providing grounds for equipment (distribution transformers, reclosers, capacitor banks, arresters, etc.), our company has a target of 25 ohms. In our area, this usually requires driving 2 - 6 or 8 ft rods. These are stacked with a coupling between them.
In some cases, we can't even reach 50 or 100 ohms with as much as 60 ft of driven rod! This leads me to my questions:
What is a reasonable target for grounding impedance?
Can you relate any experience-base for this selection or is it an opinion?
Does a high grounding impedance have an adverse effect on the performance of the surge arrester? If so, at what value?
Thanks in advance.






RE: grounding of MV distribution systems (4-wire multi-grounded neutral
RE: grounding of MV distribution systems (4-wire multi-grounded neutral
Also, I don't hold OC Seevers' anecdotes as experimental evidence. He provides more of a 10,000-ft view of grounding. There's not much details and data to lead one to adopt his suggestions.
RE: grounding of MV distribution systems (4-wire multi-grounded neutral
divert lightning?
RE: grounding of MV distribution systems (4-wire multi-grounded neutral
There is not uncommon for utility to target 25 Ohms and 4 ground connections per miles since low overall grounding help to reduce overvoltage, stray voltage and failure rated of equipment.
It is important to notice that the number of ground connection per miles plays an important role as the footing resistance in the reduction of overvoltage. Study on distribution systems shows that 25 Ohms footing resistance with 8 ground connection per miles limit the overvoltage at the end of line near to 1.3 pu.
RE: grounding of MV distribution systems (4-wire multi-grounded neutral
Utility distribution systems often include what is called a common neutral, that is a neutral connected to multiple grounds. A local utility by experimenting determined the common neutral to be low impedance ground path, drawing something like 80% of the ground fault current with earth in parallel drawing the remaining 20% *. (This finding is consistent with magnetic coupling theory and also supports NEC requirements that grounding conductors be with the supply conductors)
If your lightning arrestors are tied to the common neutral, all will work well.
(* this work was published locally by IEEE in the NorthWest United States, not sure if it made international or even national publication)
RE: grounding of MV distribution systems (4-wire multi-grounded neutral
To respond to another comment, the main interest with the ohmic value of ground resistance is in lightning performance. The NESC dictates the minimum 4 grounds per mile and the 25-ohm reference only applies to an isolated ground, so a multi-grounded neutral system doesn't have a specific ground resistance target for each ground rod location.
Thanks.
RE: grounding of MV distribution systems (4-wire multi-grounded neutral
As cuky stated, it is not possible to give a specific answer to "how good is good enough?" There is no value where the probability of a lightning outage is zero. You can estimate the performance at different levels of grounding and make an engineering judgement based on the cost of driving more rods and the benefits achieved.
RE: grounding of MV distribution systems (4-wire multi-grounded neutral
Go to this site for more info on ground testing.
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RE: grounding of MV distribution systems (4-wire multi-grounded neutral
? Some people believe that 25 Ohm was adopted since is close to an average soil resistivity for most region of the US.
? In 1933 the NEC introduced 25 ohms as the maximum value of resistance for buried or driven ground electrodes. http
? Possible influence from the telegraph industry that use Varley = 25 Ohm as resistance unit. http://www.sizes.com/units/varley_unit.htm
RE: grounding of MV distribution systems (4-wire multi-grounded neutral
On reread of Seevers, he did shoot for a target of 10 ohms, but quit after driving the third rod. 20 ohms and most problems went away. Magoo, your OP asked for experience-base, not experiment, that's why I referred you to Seevers.
RE: grounding of MV distribution systems (4-wire multi-grounded neutral
RE: grounding of MV distribution systems (4-wire multi-grounded neutral
His book dealt with stray voltage. Did his stray voltage problems go away with this grounding impedance range? I would doubt that since there's many other factors to consider.
In some IEEE references, I found the stability of the ground resistance is dependent on the depth of the water table. In many cases, you had to go to a depth of 30 feet or so to be sure you have a stable ground resistance over the year.
We use 8-ft, 5/8-inch diameter ground rods. Applying Dwights formula with a rho of 100 ohm-m, this shows that you need 2 stacked ground rods to achieve 20 ohms. This seems reasonable. By contrast, it would take about 5 ground rods to get below 10 ohms.
In certain portions of our system, rho is closer to 300 ohm-m. In these cases, you need 5 ground rods to meet 20 ohms. 10 rods would get you down to 15 ohms, so 10-ohms would be out of the question with the higher rho conditions.
Based on the stability combined with the range of rho values, we usually employ 4 or 5 ground rods.
I'm not sure I agree with apowerengr in the issue about round potential rise and arrester protective characteristic. The ground potential rise affects the neutral. If the neutral potential increases due to GPR, the protection across the transformer winding shouldn't be affected. Am I missing something?
I agree that the grounding improves as the soil becomes more compacted. I know of a consultant that used bentonite in their ground rod installations. He claims that the bentonite clings to the rod and makes the initial as well as long term reading lower.
I also have used the AEMC/Yokagowa clamp on ground resistance tester. Not only does it permit faster checks with utility grounds, it also gives lower readings that the crank-type meggars. I think the frequency of the clamp-on tester is around 5 kHz, so you're in effect comparing a high frequency reading (clamp-on) to a dc reading (crank-type).
I appreciate all the good comments.
RE: grounding of MV distribution systems (4-wire multi-grounded neutral
RE: grounding of MV distribution systems (4-wire multi-grounded neutral
RE: grounding of MV distribution systems (4-wire multi-grounded neutral
RE: grounding of MV distribution systems (4-wire multi-grounded neutral
Stevenal, I glad you clarified the book. From what you described as the problem he solved, it sounds among other things like the damage due to a direct hit above the arrester capability. Did he offer any conclusions why the lower grounding resistance lowered the failures?
One of the problems often overlooked is that lightning activity can vary considerably from one year to the next. If he initiated some improvement program, the improved performance could be due to decreased lightning activity in that area the following year. As an example, Tom Short in his Electric Power Distribution Handbook relates an example of line protection using arresters at LILCo. As I recall, the control group, the one in which no improvements were made, showed better performance than the ones where they installed arresters. The problem is that he based his evaluation on a single year.
We've found similar conclusions. With the availability of data on flash density today, you can see why a multi-year evaluation is necessary to 1) decide what areas to work on and 2) to properly evaluate the results. A single year or snapshot evaluation can be very misleading.
I'm still having trouble understanding how GPR is a real issue here.
RE: grounding of MV distribution systems (4-wire multi-grounded neutral
RE: grounding of MV distribution systems (4-wire multi-grounded neutral
The utility I previously worked for put lightning arrestors only at underground termination poles and line deadends. Often times the line deadend arrestors would be blown (pieces of porcelain around the base of the pole) by distant lightning strikes, that had travelled down the line. For medium voltage power distribution systems this was ample evidence the system worked sufficiently (though we didn't like the durability of the arrestors) despite high measured individual pole ground resistance.
I would expect such neutral to be near zero volts prior to a lightning strike, and though there may be substantial potential rise during a strike, the impedance was low enough to allow the surge arrestors to valve most of the energy away from the distribution line which was the primary objective.
RE: grounding of MV distribution systems (4-wire multi-grounded neutral
Check with Erico for bentonite (GEM)users:
http://www.erico.com/products/GEM.asp
In a past life, we specified for some utility applications in Southeast Alaska, where they are sitting on big rocks, basically.