More fallacious grounding

mivey

Senior Member
I'm trying to find something good, but nothing on the physics of why that is.


If you look at lightning arrestor sizing, uni grounded has its own category like ungrounded.


I'd PM Mivey, he would know what that is.
Any good book or course or white papers or application guides on distribution/transmission modeling will do and there are tons of them.

Westinghouse T&D, Kersting's Distribution System Modeling, Meliopoulos' Power System Grounding, EE Handbook, Cooper Distribution System Protection, to name a few off the top of my head.
 

mbrooke

Batteries Not Included
Location
United States
Occupation
*
Any good book or course or white papers or application guides on distribution/transmission modeling will do and there are tons of them.

Westinghouse T&D, Kersting's Distribution System Modeling, Meliopoulos' Power System Grounding, EE Handbook, Cooper Distribution System Protection, to name a few off the top of my head.
I know, have to find the pages. I've seen design builds for distribution / along with k1/ground compensation for MHO distance elements all modeling the lines that way.


However, I do not understand the theory behind it. Gold Digger makes sense in every way, yet at the same time the real world shows otherwise when it comes to 50/60Hz power.
 

mivey

Senior Member
I know, have to find the pages. I've seen design builds for distribution / along with k1/ground compensation for MHO distance elements all modeling the lines that way.


However, I do not understand the theory behind it. Gold Digger makes sense in every way, yet at the same time the real world shows otherwise when it comes to 50/60Hz power.
You mentioned a unigrounded system. Grounding it again as he supposed is not unigrounded.
 

mbrooke

Batteries Not Included
Location
United States
Occupation
*
You mentioned a unigrounded system. Grounding it again as he supposed is not unigrounded.
I did and thats what I'm talking about.


California is loaded with 3 wire unigrounded distribution systems and there are some utilities like National Grid in Massachusetts that have 3 wire 69kv transmission lines without a lightning guard or static wire.



As you move down line the 12kv distribution line insulators, bushing and lightning arrestors are sized higher and fault currents decrease the further a fault happens from the substation.
 

mbrooke

Batteries Not Included
Location
United States
Occupation
*
what is 'uniground'....? :?~RJ~
A distribution system where the secondary neutral is connected (bonded) to the substation ground grid, but no multi grounded neutral is brought out of the substation and run along the poles. Hence uni, only one location. Only 3 wires (Phase A, B, C) are up on the pole.


In some 2400/4160Y system out in cali there is a 4th neutral run along the poles but its 100% insulated from earth. So technically that too would count as uni grounded.


Here is a pic of a substation "ground grid"






Page 9 of what it does, mainly to reduce voltage potential during faults :


https://ccaps.umn.edu/documents/CPE-Conferences/MIPSYCON-PowerPoints/2017/TutIIISubstationGroundingTutorial.pdf






..............................................................................................


The thing is, and where I must admit my knowledge ends as I can not explain it-


If I have a 7.2/12kv fault to a 25 ohm ground rod 1/4 of a mile from the substation the current will be higher then if I had a fault on the same line 20 miles from the substation on an identical 8 foot 25 ohm ground rod.


However, grounding theory (among others) contradicts this by saying only the resistance of the ground rod (25 ohms) and the substation ground grid (1 ohm) would matter as the earth itself has a resistance of 0 ohms given all the infinite parallel paths which exist.


Theory says that at both 1/4 of a mile and 20 miles the current should always be 276 amps (excluding the impedance of the phase wire of course)


Yet in reality 1/4 of a mile may produce 200amps, while 20 miles down line only 5 amps.


:?:dunce::?


As such a fault at the 20 mile point will cause the phase to ground voltage to rise requiring phase-phase rated insulators and lightning arrestors.
 
Last edited:

Wire-Smith

Senior Member
Location
United States



..............................................................................................


The thing is, and where I must admit my knowledge ends as I can not explain it-


If I have a 7.2/12kv fault to a 25 ohm ground rod 1/4 of a mile from the substation the current will be higher then if I had a fault on the same line 20 miles from the substation on an identical 8 foot 25 ohm ground rod.


However, grounding theory (among others) contradicts this by saying only the resistance of the ground rod (25 ohms) and the substation ground grid (1 ohm) would matter as the earth itself has a resistance of 0 ohms given all the infinite parallel paths which exist.


Theory says that at both 1/4 of a mile and 20 miles the current should always be 276 amps (excluding the impedance of the phase wire of course)


Yet in reality 1/4 of a mile may produce 200amps, while 20 miles down line only 5 amps.


:?:dunce::?


As such a fault at the 20 mile point will cause the phase to ground voltage to rise requiring phase-phase rated insulators and lightning arrestors.

I don't have anything to provide as a reference and i'm likely completely wrong, but would the long length along with the distance between the earth and the wires be the problem. while if you had the physically close static line that would compensate with capacitance, without the static line the distance between the ground and conductors possibly wouldn't create enough capacitance? kind of like running an egc separate from the circuit conductors?
 

mbrooke

Batteries Not Included
Location
United States
Occupation
*
I don't have anything to provide as a reference and i'm likely completely wrong, but would the long length along with the distance between the earth and the wires be the problem. while if you had the physically close static line that would compensate with capacitance, without the static line the distance between the ground and conductors possibly wouldn't create enough capacitance? kind of like running an egc separate from the circuit conductors?
Not sure what you are specifically referring to, but the capacitance is negligible relative to resistance and inductance of a line. Often ignored...
 

Wire-Smith

Senior Member
Location
United States
Not sure what you are specifically referring to, but the capacitance is negligible relative to resistance and inductance of a line. Often ignored...
i was just thinking there would be a difference of capacitance between MGN and Uniground, and in turn fault current characteristics, like routing an EGC separate from the circuit conductors, ground fault current would be different.
 

Wire-Smith

Senior Member
Location
United States
the physical distance between the conductors and earth compared to a static line ran on the pole near the circuit conductors(and frequently grounded).
 
Last edited:

GoldDigger

Moderator
Staff member
To keep it simple, picture the neutral and egc at a panel as being at the same potential then diverging as you move down the feeder. An Earth bonded wye point at a substation puts Earth at the neural point of the phases. As we move well away from that location, the neutral point begins to float away and eventually Earth is practically unbonded from the neutral point.
Absolutely. It just has no relation to the effectiveness of Earth as a current carrying conductor.

Sent from my XT1585 using Tapatalk
 

GoldDigger

Moderator
Staff member
A distribution system where the secondary neutral is connected (bonded) to the substation ground grid, but no multi grounded neutral is brought out of the substation and run along the poles. Hence uni, only one location. Only 3 wires (Phase A, B, C) are up on the pole.


In some 2400/4160Y system out in cali there is a 4th neutral run along the poles but its 100% insulated from earth. So technically that too would count as uni grounded.


Here is a pic of a substation "ground grid"






Page 9 of what it does, mainly to reduce voltage potential during faults :


https://ccaps.umn.edu/documents/CPE-Conferences/MIPSYCON-PowerPoints/2017/TutIIISubstationGroundingTutorial.pdf






..............................................................................................


The thing is, and where I must admit my knowledge ends as I can not explain it-


If I have a 7.2/12kv fault to a 25 ohm ground rod 1/4 of a mile from the substation the current will be higher then if I had a fault on the same line 20 miles from the substation on an identical 8 foot 25 ohm ground rod.


However, grounding theory (among others) contradicts this by saying only the resistance of the ground rod (25 ohms) and the substation ground grid (1 ohm) would matter as the earth itself has a resistance of 0 ohms given all the infinite parallel paths which exist.


Theory says that at both 1/4 of a mile and 20 miles the current should always be 276 amps (excluding the impedance of the phase wire of course)


Yet in reality 1/4 of a mile may produce 200amps, while 20 miles down line only 5 amps.


:?:dunce::?


As such a fault at the 20 mile point will cause the phase to ground voltage to rise requiring phase-phase rated insulators and lightning arrestors.
Once again, the difference comes from the impedance of the ungrounded conductors. The earth impedance plays no role in the calculation except to the extent that it allows capacitive coupling from all lines to the earth surface.

Sent from my XT1585 using Tapatalk
 

mbrooke

Batteries Not Included
Location
United States
Occupation
*
Once again, the difference comes from the impedance of the ungrounded conductors. The earth impedance plays no role in the calculation except to the extent that it allows capacitive coupling from all lines to the earth surface.

Sent from my XT1585 using Tapatalk
But why does a uni grounded system go from behaving essentially solidly grounded to ungrounded? Why would a ground rod further from the substation have a higher ohm value to the substation ground grid?
 

mbrooke

Batteries Not Included
Location
United States
Occupation
*
i was just thinking there would be a difference of capacitance between MGN and Uniground, and in turn fault current characteristics, like routing an EGC separate from the circuit conductors, ground fault current would be different.
You're actually right, there is. Good point :)


But from my understanding its not enough to so profoundly effect fault current or impedance such that you see ungrounded system behavior end-of-line.
 

Wire-Smith

Senior Member
Location
United States
You're actually right, there is. Good point :)


But from my understanding its not enough to so profoundly effect fault current or impedance such that you see ungrounded system behavior end-of-line.
https://en.wikipedia.org/wiki/Electrical_reactance
In electric power systems, inductive reactance (and capacitive reactance, however inductive reactance is more common) can limit the power capacity of an AC transmission line, because power is not completely transferred when voltage and current are out-of-phase (detailed above). That is, current will flow for an out-of-phase system, however real power at certain times will not be transferred, because there will be points during which instantaneous current is positive while instantaneous voltage is negative, or vice versa, implying negative power transfer. Hence, real work is not performed when power transfer is "negative". However, current still flows even when a system is out-of-phase, which causes transmission lines to heat up due to current flow. Consequently, transmission lines can only heat up so much (or else they would physically sag too much, due to the heat expanding the metal transmission lines), so transmission line operators have a "ceiling" on the amount of current that can flow through a given line, and excessive inductive reactance can limit the power capacity of a line. Power providers utilize capacitors to shift the phase and minimize the losses, based on usage patterns
i know pretty much nothing about distribution design or theory, but i'm visualizing CEMF and no to little cancelling effect due to the distance between the wires way up on the pole and the ground(during ground fault). i keep thinking of how you are supposed to keep circuit conductors routed with each other and close in a cabinet to mitigate any inductive heating and needing the egc with the circuit conductors, like i stated before. that's the only difference i'm seeing in the schemes. it doesn't change the resistance of earth from zero.
 
Last edited:

GoldDigger

Moderator
Staff member
But why does a uni grounded system go from behaving essentially solidly grounded to ungrounded? Why would a ground rod further from the substation have a higher ohm value to the substation ground grid?
A ground rod farther from the substation does not have a higher ohm value to the substation ground. A unigrounded system does not have a ground rod distant from the substation. So the transient impedance from each individual transmission line to ground will be essentially the wire impedance of that transmission line, as part of the multiphase set, all the way back to the substation.
 

Wire-Smith

Senior Member
Location
United States
A ground rod farther from the substation does not have a higher ohm value to the substation ground. A unigrounded system does not have a ground rod distant from the substation. So the transient impedance from each individual transmission line to ground will be essentially the wire impedance of that transmission line, as part of the multiphase set, all the way back to the substation.
so are you saying the ground fault current would be the same on a uniground as it would a multi grounded neutral system at say 20 miles(long distances)?


thanks, just interested, not arguing
 
Last edited:
Top