Ground fault- Why doesn't anyone get shocked?

[mbrooke]

A straw-man argument. That is not practice by standards but practice by experience in certain areas. The standard is still 4/mile plus at equipment poles. In today's world of broken grounds and wire theft we see many utilities practice grounding every primary pole in places. This will vary, even internally to an individual utility. Soil conditions, lightning frequency, population density, economics, reliability index, etc. all play a role.

Question- how does the reliability index play a role? As you know reliability is my primary concern.

 

[paulengr]

You need to move beyond basic text book stuff. It is more complex than you realize.

That is not how it works. You would do well to study Carson's equations and the associated calculations for overhead transmission line impedance and fault modeling. I have quite a few posts about that here and even went through the formulas and calcs (I think I completed the series anyway...don't remember).

You can't ignore either one.

I would be interested in hearing about these experiences as I have not worked in pit mining.

However, these experiences seem to have led to some peculiar conclusions. There may be something different you are talking about but in the context of the OP they appear at first glance to be way off. One would expect empirical data to generally align with accepted engineering theory and practice.

A straw-man argument. That is not practice by standards but practice by experience in certain areas. The standard is still 4/mile plus at equipment poles. In today's world of broken grounds and wire theft we see many utilities practice grounding every primary pole in places. This will vary, even internally to an individual utility. Soil conditions, lightning frequency, population density, economics, reliability index, etc. all play a role.

Well its a good thing no one is suggesting that then isn't it?

Your premise is not supported by engineering standards or analysis.

You are simply wrong. Near a ground fault, the current travels mostly in the earth but quickly climbs the pole grounds on the way to the station. The current tends to follow the line and if long enough, about 1/2 will travel in the earth and 1/2 on the neutral in the middle of the circuit. More will travel on the neutral at the endpoints

Another straw-man argument and not what I have said at all.

The model I use is the same model used by transmission and distribution engineers all over the world. It is not something I came up with.

The model(s) you have been supposing I'm using comes from who knows where.

You are making assumptions about a model that has not been proposed by me so you are arguing against some strange model that you came up with, not one I use.

It's a single phase system with a neutral. Agreed Carson's equations apply but the assumption is no intentional connection between the various conductors or Earth within the transmission line because it's a transmission line.

Sent from my SM-T350 using Tapatalk

 

[mbrooke]

Two threads on touch voltage:


https://www.eng-tips.com/viewthread.cfm?qid=305712


https://www.eng-tips.com/viewthread.cfm?qid=438149

 

[mivey]

I'm still a bit skeptical- the voltage to remote earth has to be higher- though the odds of faster clearing do go up.

But for what design parameters? 34.5 kV can push more power further but also has a different design.

For a constant fault impedance, the higher kV will yield a higher minimum ground fault true enough. That is for a relatively large fault impedance. But the GPR scenario discussed is for bolted faults with normal source impedance.

My thought was that the conductor size and distance for either voltage would be designed to supply adequate power to a load without overdoing it. Also, for the higher voltage you would increase the source impedance at the station to keep the available fault from being too high. As such, my first thought was that when all is said and done you would wind up with a similar power supply profile at the load for either design and would thus have a similar result.

This may not be true but I did not feel like running a bunch of scenarios. It would take more than just changing the source voltage because the two would be designed differently.

To a degree though- I've found that at 12.47kv you are generally limited to 40-80MVA loading tops, 100-120MVA at 23kv, and at 34.5kv I've seen 200MVA and over.

Yes you can push more power from a single transformer at a higher voltage. But you can also have multiple transformers in a sub. I like about a 20-40 MVA unit. When the transformer gets too big, you may be better off to locate a new sub closer to a load center and spend less on large distribution.

You have to run the economics of large distribution lines vs building a new sub elsewhere. A large concentrated load may very well need 200 MVA in a small sq mile area but you could also invest too much in one substation and then spend a lot of distribution system cost getting that power out to remote load centers. Every system is different.

But how do we know this will always result in safe values of neutral to remote earth voltage?

At what Thevenin impedance?

Nothing is always safe. People demand electricity. They demand it at a reasonable cost. They demand it with reasonable safety. They demand it with reasonable reliability. There are a lot of demands and not all can be met perfectly at the same time. And not everyone defines reasonable the same way.

An example of one of the most dangerous things people demand is cars. Not always safe but we still allow them because people demand them. It is not a perfect world.

 

[mivey]

Question- how does the reliability index play a role? As you know reliability is my primary concern.

One example would be equipment on a hill that is being taken out by lightning surges. Adding grounded arrestors on poles on either side can reduce the failure rate.

Poor soil (sandy soil) would need additional grounds to help dissipate lightning surges and reduce equipment failure/flashover.

Redundant grounds can help compensate for stolen/broken grounds until repaired/replaced on the next inspection cycle.

 

[mivey]

It's a single phase system with a neutral. Agreed Carson's equations apply but the assumption is no intentional connection between the various conductors or Earth within the transmission line because it's a transmission line.

Sent from my SM-T350 using Tapatalk

We were talking about MGN distribution, not ungrounded transmission. I used a 3-phase model because it was handy. The mutual impedances with the added B & C phases make a slight difference but not enough to worry about for this L-N fault discussion.

 

[mbrooke]

But for what design parameters? 34.5 kV can push more power further but also has a different design.

For a constant fault impedance, the higher kV will yield a higher minimum ground fault true enough. That is for a relatively large fault impedance. But the GPR scenario discussed is for bolted faults with normal source impedance.

GPR? :?


Thats my thinking- a higher voltage will always yield higher remote earth voltage.


I'm still stuck on the 50/50 phase neutral drop that I've always assumed.

My thought was that the conductor size and distance for either voltage would be designed to supply adequate power to a load without overdoing it. Also, for the higher voltage you would increase the source impedance at the station to keep the available fault from being too high. As such, my first thought was that when all is said and done you would wind up with a similar power supply profile at the load for either design and would thus have a similar result.

This may not be true but I did not feel like running a bunch of scenarios. It would take more than just changing the source voltage because the two would be designed differently.

30/40/50 MVA unit for 12.47kv 10% Z giving about 12.5ka

50/65/80 MVA unit for 23kv 10% Z about 12.5ka

80/100/120 MVA unit for 34.5kv 10% Z about 12.5ka


But- you are correct come real world. Despite the voltage; 30/40/50 and 40/50/60MVA is the most popular size by far regardless of 15, 25 or 35kv class.

So in theory 34.5kv has a higher source impedance for the same MVA transformer? More voltage drop at the source for a bolted fault?

Would closed bus ties (3 60MVA units in parallel) produce a higher remote earth voltage for any given voltage?

Yes you can push more power from a single transformer at a higher voltage. But you can also have multiple transformers in a sub. I like about a 20-40 MVA unit. When the transformer gets too big, you may be better off to locate a new sub closer to a load center and spend less on large distribution.

You have to run the economics of large distribution lines vs building a new sub elsewhere. A large concentrated load may very well need 200 MVA in a small sq mile area but you could also invest too much in one substation and then spend a lot of distribution system cost getting that power out to remote load centers. Every system is different.

True- and yes, every system is different based on many, many factors :happyyes:

At what Thevenin impedance?

Explain.

I'm imagining fairly code minimum. California soil, 4 grounds per mile and at every pole pig/arrestor/switch and plastic water mains.

Nothing is always safe. People demand electricity. They demand it at a reasonable cost. They demand it with reasonable safety. They demand it with reasonable reliability. There are a lot of demands and not all can be met perfectly at the same time. And not everyone defines reasonable the same way.

An example of one of the most dangerous things people demand is cars. Not always safe but we still allow them because people demand them. It is not a perfect world.

But if a line to neutral fault has the ability to kill or injure code should address that in a reasonable way... >>>

Courtesy of, Tony:

https://talk.electricianforum.co.uk...-connections/neutral-earth-connections-1-r25/

 

[mbrooke]

One example would be equipment on a hill that is being taken out by lightning surges. Adding grounded arrestors on poles on either side can reduce the failure rate.

Poor soil (sandy soil) would need additional grounds to help dissipate lightning surges and reduce equipment failure/flashover.

Redundant grounds can help compensate for stolen/broken grounds until repaired/replaced on the next inspection cycle.

Got it, and makes sense.


I believe you when you say 2.4kv will show up with the conditions assumed, but myself I am skeptical of the clearing times what would make it safe. I mean look at it like this: can you design a system that will open fast enough if I were to touch a 4.16kv busbar while referenced to ground?

 

[romex jockey]

I mean look at it like this: can you design a system that will open fast enough if I were to touch a 4.16kv busbar while referenced to ground?

this is interesting>>>

As there is no neutral earth fault detection is by a CT and relay on the star point tap

~RJ~

 

[electrofelon]

this is interesting>>>

~RJ~

What is that thing? Please elaborate....

 

[mivey]

GPR? :?

Ground Potential Rise

I'm still stuck on the 50/50 phase neutral drop that I've always assumed.

Run the numbers and you will find out differently for a MGN.

30/40/50 MVA unit for 12.47kv 10% Z giving about 12.5ka

50/65/80 MVA unit for 23kv 10% Z about 12.5ka

80/100/120 MVA unit for 34.5kv 10% Z about 12.5ka

and knowing that I_fault = E / Z, and keeping I_fault the same, and raising E, the Z must increase. Indeed, you get

0.5183 ohms, 1.0580 ohms, and 1.4878 ohms for those. Remember Z_base = V_LL_rated^2 / VA_rated so Z_ohms = Z_base * Z_pu

So in theory 34.5kv has a higher source impedance for the same MVA transformer?

(34.5/12.47)^2 => 7.65 times the impedance assuming the same %Z.

More voltage drop at the source for a bolted fault?

Not really voltage drop. For a bolted L-N fault, the A phase is faulted to N so the voltage is zero. So you go from 34.5 kV to zero or 12.47 kV to zero. Not sure what you are asking.

Would closed bus ties (3 60MVA units in parallel) produce a higher remote earth voltage for any given voltage?

I would think so...more fault current = more NEV

Explain.

I'm imagining fairly code minimum. California soil, 4 grounds per mile and at every pole pig/arrestor/switch and plastic water mains.

You have body impedance, power line impedance, and the impedance of the path from power line to the body. For a bolted L-N fault at the service pole we get a voltage V. The current is not just V / R_body but the divisor must also include contact resistance, path resistance, etc.

 

[mivey]

can you design a system that will open fast enough if I were to touch a 4.16kv busbar while referenced to ground?

No. But I can design one that will keep you safe in a substation if you touch a grounded metal frame that has 4.16 kV impressed on it.

 

[mbrooke]

No. But I can design one that will keep you safe in a substation if you touch a grounded metal frame that has 4.16 kV impressed on it.

So how is 2.4kv acceptable to remote earth? :?:blink:

 

[mbrooke]

this is interesting>>>

~RJ~

No. But I can design one that will keep you safe in a substation if you touch a grounded metal frame that has 4.16 kV impressed on it.

Hypothetical scenario- What if the substation neutral was earthed via a correctly tuned Peterson coil and high speed clearing? I'd think the current that would pass through the person would be rather small.

 

[mivey]

So how is 2.4kv acceptable to remote earth? :?:blink:

It must have been determined that the probability of that happening in conjunctio. with the failure of high-speed clearing is low. I have not researched it.

 

[mivey]

Hypothetical scenario- What if the substation neutral was earthed via a correctly tuned Peterson coil and high speed clearing? I'd think the current that would pass through the person would be rather small.

Those are for ungrounded systems. We were discussing MGN systems.

What person currents did you calculate and for how long?

 

[mbrooke]

It must have been determined that the probability of that happening in conjunctio. with the failure of high-speed clearing is low. I have not researched it.

But if you can't design a system to clear fast enough to save a person from touching a 4.16kv busbar, then how can a distribution system clear fast enough to protect a person?

 

[mbrooke]

Those are for ungrounded systems. We were discussing MGN systems.

What person currents did you calculate and for how long?

Yes, because the massive amount of current in an MGN produces tremendous neutral to remote earth voltage, where as in an ungrounded system the voltage to remote earth is small during a fault.

 

[mivey]

What is that thing? Please elaborate....

A Petersen coil tries to cancel the capacitive current on an ungrounded system by paralleling an inductive coil.

The idea is to keep the current low during transient line to ground faults because the coil current is opposite in phase to the capacitive current and thus, ideally, no net ground current due to line capacitance would flow.

 

[mivey]

But if you can't design a system to clear fast enough to save a person from touching a 4.16kv busbar, then how can a distribution system clear fast enough to protect a person?

There is a difference between touching a 4 kV line while standing on the ground vs. touching a grounded line in contact with a 4 kV line.