can 2 Bucket Transformers share a neutral? House to house backup generator circuit.

[mivey]

On single phase circuits your neutral simply becomes phase two, on 3 phase circuits you end up with 3 wires instead of 4. At least how I imagine it. But if you say so I'll take your word for it.

You can run an ungrounded system but it is a different setup and protection scheme. There are delta distribution systems but there are pros and cons for either setup.

I doubt you would just let a multi-grounded system become an ungrounded system because it is not just a simple thing. You sure wouldn't mix the two.

The neutral doesn't just "simply" become a phase conductor. You have to change the insulation and the location on the pole for the neutral. Your system protection scheme will have to be completely changed. All of your arrestors will have to change. Any transformer connected wye will have to be changed. Any other equipment rated line-ground will have to be changed. I'm sure there is more but I'm not awake yet.

 

[mbrooke]

You can run an ungrounded system but it is a different setup and protection scheme. There are delta distribution systems but there are pros and cons for either setup.

I doubt you would just let a multi-grounded system become an ungrounded system because it is not just a simple thing. You sure wouldn't mix the two.

Why not uni-grounded? You have a ground reference for easy relaying, but dispense the neutral.

The neutral doesn't just "simply" become a phase conductor. You have to change the insulation and the location on the pole for the neutral. Your system protection scheme will have to be completely changed. All of your arrestors will have to change. Any transformer connected wye will have to be changed. Any other equipment rated line-ground will have to be changed. I'm sure there is more but I'm not awake yet.

I know for an ungrounded delta...

However I was thinking that 336 is raised up to a crossarm and insulated for a single phase line. Still two conductors.

 

[mivey]

Why not uni-grounded? You have a ground reference for easy relaying, but dispense the neutral.

Not a direct replacement. Near the station it acts like a MGN and far from the station it acts like an ungrounded delta.

Some "delta" systems actually start out wye at the station and are actually uni-grounded systems.

The protection scheme is different and so are the insulating, arrestors, etc.

I know for an ungrounded delta...

However I was thinking that 336 is raised up to a crossarm and insulated for a single phase line. Still two conductors.

Sounds simple till you get in a bucket and actually have to do it. Also, the neutral tends to be sized smaller than the phase conductors.

 

[mbrooke]

Not a direct replacement. Near the station it acts like a MGN and far from the station it acts like an ungrounded delta.

Some "delta" systems actually start out wye at the station and are actually uni-grounded systems.

The protection scheme is different and so are the insulating, arrestors, etc.

You have a point here. As you go further from the station earth resistance adds up gradually- go some miles and its like an HRG. 1.73 x rise. So the noddle is needed.

Sounds simple till you get in a bucket and actually have to do it.

Well, way back when, it was all ungrounded delta. I know 4.8kv CU laterals (probably #6) from 40s that go for a few miles. I don't think they used buckets but could be wrong. Understand where you are coming from though.

Also, the neutral tends to be sized smaller than the phase conductors.

For a single phase line with L-N connected loads this is a no-no in my book.

 

[Besoeker]

What's a bucket transformer?

 

[mivey]

For a single phase line with L-N connected loads this is a no-no in my book.

Yeah, not usually for single-phase.

 

[JFletcher]

What's a bucket transformer?

Pole mounted transformer I presume. Also sometimes called pole pigs, pots, or trash cans.

I did like your joke about transforming them pails.

 

[Besoeker]

Pole mounted transformer I presume. Also sometimes called pole pigs, pots, or trash cans.

I did like your joke about transforming them pails.

Thank you, sir.....
I sometimes try to lighten the mood here and elsewhere. Doesn't always work....

 

[ActionDave]

I'm still missing something.

Yes. The POCO system (a MGN system anyway) uses a multi-point grounded system. Even without a MGN, the system gets capacitively coupled to earth. On the NEC system, we use a single-point grounded system.

With only one bonding point, the SPG keeps load current off of safety grounds and other things like telecom shields, building steel, pipe, etc., at least except for the short hop from main to transformer. However, if the service neutral breaks we can see load current running all over the place as it finds a combined path back to the transformer secondary (the premise source) as it tries to replace the neutral.

This is the case if you use a generator or grid power, no?

Not lifting the neutral at house #2 puts load current everywhere as it tries to get back to the source at house #1. However, a ground fault follows that long parallel path(s) between houses even with the neutral lifted at #2.

How is this different than multiple houses fed from the grid? And with a dedicated equipment ground between house #2 and the generator aren't we putting most of the fault current on that low impedance path?

A separate gen for #2 is better. Or even better yet just directly plugging in #2's appliances is safer (equivalent to lifting the line, neutral, and ground at house #2).

When the power comes back on we're back to sharing the ground with the neighborhood.

On the POCO primary side, load current runs on the multi-grounded neutral conductor as well as on other ground paths. Some things come to mind that help mitigate the ground currents.

For one, we try to limit the neutral/ground current (balance feeders, use shorter single-phase feeders if loads are high, use two-bushing transformers as needed, etc.).

As for currents on everything attached, a high NEV is caused by high neutral current. Limiting the NEV helps limit excess ground current away from the main line (where the NEV is carried from the main line by conductor bonds). Some NEV is expected and is not classed a stray current source because it is considered normal.

Maintenance to check for broken/poor main line neutrals helps mitigate localized abnormal high ground currents.

Also, any ground currents tend to stay near the power line. It is a physical phenomenon and can be modeled during system analysis.

How effective these efforts are gets called into question with all the NEC rules surrounding commercial farms and in ground pools. Meanwhile, if the power goes out and the neighbor powers up his house and Old Mrs. Franklin at the same time with his generator we've got too much neutral current in the dirt?

 

[GoldDigger]

You have a point here. As you go further from the station earth resistance adds up gradually- go some miles and its like an HRG. 1.73 x rise. So the noddle is needed.
...
...

That does not fit with the generally accepted approximation that the bulk resistance through the earth of the current path will be close to zero, and that all of the resistance associated with that current path can be approximated (at great distance) by fixed resistances associated with each individual rod.

The neutral is generally needed for a metallic fault current path to supplement the ground electrodes and incidentally to bond the ground electrodes onto a unified system, reducing the overall impedance to earth by paralleling many individual electrode resistances. It also allows for efficient carrying of normal current, which would not be practical using only the earth as a return path because of the electrode resistance and resulting step and touch potentials.

 

[mbrooke]

That does not fit with the generally accepted approximation that the bulk resistance through the earth of the current path will be close to zero, and that all of the resistance associated with that current path can be approximated (at great distance) by fixed resistances associated with each individual rod.

Correct- in that I'm sure the resistance of the earth a whole is on the low side. However, picture a single ground rod just bonded to a pole pig. The resistance of the rod may be very high considering its only 8 feet of top soil and the surface area of the rod in contact with the dirt is minuscule compared to the area electrons can flow in the earth itself. A fault to the pig can raise the voltage on the bushing and arrestser by 1.73x, effectively acting like an ungrounded system.

The neutral is generally needed for a metallic fault current path to supplement the ground electrodes and incidentally to bond the ground electrodes onto a unified system, reducing the overall impedance to earth by paralleling many individual electrode resistances. It also allows for efficient carrying of normal current, which would not be practical using only the earth as a return path because of the electrode resistance and resulting step and touch potentials.

Correct, I am in agreement on this one.

 

[GoldDigger]

Correct- in that I'm sure the resistance of the earth a whole is on the low side. However, picture a single ground rod just bonded to a pole pig. The resistance of the rod may be very high considering its only 8 feet of top soil and the surface area of the rod in contact with the dirt is minuscule compared to the area electrons can flow in the earth itself. A fault to the pig can raise the voltage on the bushing and arrestser by 1.73x, effectively acting like an ungrounded system.

What I was specifically taking issue with was your assertion that "As you go further from the station earth resistance adds up gradually- go some miles and its like an HRG."
In fact the association of the bulk of the resistance with the immediate vicinity of the electrode means that for identical electrode conditions at each endpoint the earth resistance of a 500 foot stretch may be not detectably smaller than the resistance of a 100 mile stretch. The distance from the station does not really affect the earth return portion of the circuit at all on this scale.

 

[mbrooke]

What I was specifically taking issue with was your assertion that "As you go further from the station earth resistance adds up gradually- go some miles and its like an HRG."
In fact the association of the bulk of the resistance with the immediate vicinity of the electrode means that for identical electrode conditions at each endpoint the earth resistance of a 500 foot stretch may be not detectably smaller than the resistance of a 100 mile stretch. The distance from the station does not really affect the earth return portion of the circuit at all on this scale.

Mivey claims that it does, and I am inclined to believe him considering his experience with MV/HV. But I hate having to pick sides as this is not my area of strong understanding. He would have to clarify further.


Actually now that I think about it, your theory makes more sense. I genuinely don't know :?

 

[electrofelon]

You can run an ungrounded system but it is a different setup and protection scheme. There are delta distribution systems but there are pros and cons for either setup.

I doubt you would just let a multi-grounded system become an ungrounded system because it is not just a simple thing. You sure wouldn't mix the two.

The neutral doesn't just "simply" become a phase conductor. You have to change the insulation and the location on the pole for the neutral. Your system protection scheme will have to be completely changed. All of your arrestors will have to change. Any transformer connected wye will have to be changed. Any other equipment rated line-ground will have to be changed. I'm sure there is more but I'm not awake yet.

Here in upstate ny it's the land of 4800 delta distribution. Often you will see transformer plarforms that are fed from a newer 13.2/7.62 line and feeding the older 4800 lines. I used to wonder if they were auto or isolation banks, but after careful looking, all the ones I have been able to trace out are isolation. What would be the implications of using autos in this situation? Clearly, the originally ungrounded 4800 would have the ground reference from the 13.2 MGN system.

 

[mivey]

I'm still missing something.

This is the case if you use a generator or grid power, no?

The point was that there is added current on the parallel paths when the neutral breaks ahead of the main. The generator should not have a parallel path past the N-G bond but I wasn't really addressing the generator.

The utility service has a N-G bond at the pole and at the customer main. The ground and neutral are shared conductors and are both used to carry load current. The generator will either have a bond at the generator (SDS) or use the service bond. It does not use the ground for load current.

In other words, if the generator neutral breaks, the load should stop because there is no path unless you have a fault. If the POCO neutral breaks, the load can keep running even without a fault (how well it runs will depend).

How is this different than multiple houses fed from the grid? And with a dedicated equipment ground between house #2 and the generator aren't we putting most of the fault current on that low impedance path?

The generator is not supposed to put load current on the ground.

I could model it to see what happens during a fault.

When the power comes back on we're back to sharing the ground with the neighborhood.

Yes.

How effective these efforts are gets called into question with all the NEC rules surrounding commercial farms and in ground pools. Meanwhile, if the power goes out and the neighbor powers up his house and Old Mrs. Franklin at the same time with his generator we've got too much neutral current in the dirt?

You are not allowed to connect your generator to run load current across grounds and around the neighborhood. Just the way it is.

 

[mivey]

That does not fit with the generally accepted approximation that the bulk resistance through the earth of the current path will be close to zero, and that all of the resistance associated with that current path can be approximated (at great distance) by fixed resistances associated with each individual rod.

The neutral is generally needed for a metallic fault current path to supplement the ground electrodes and incidentally to bond the ground electrodes onto a unified system, reducing the overall impedance to earth by paralleling many individual electrode resistances. It also allows for efficient carrying of normal current, which would not be practical using only the earth as a return path because of the electrode resistance and resulting step and touch potentials.

Unigrounded...as in grounded once at the substation...not many paralleled electrodes.

Such a system where you drop the neutral as Mbrooke proposed (even if you bring out a static wire) will act like a wye system near the substation and like a delta system far from the station.

When specifying arresters you have to calculate the impact. Near the station you use an arrester rated for line-ground voltage. Far away you use arresters rated for line-line voltage. In between that you calculate how much the neutral point swings for a fault and use the appropriate rating.

 

[mivey]

Here in upstate ny it's the land of 4800 delta distribution. Often you will see transformer plarforms that are fed from a newer 13.2/7.62 line and feeding the older 4800 lines. I used to wonder if they were auto or isolation banks, but after careful looking, all the ones I have been able to trace out are isolation. What would be the implications of using autos in this situation? Clearly, the originally ungrounded 4800 would have the ground reference from the 13.2 MGN system.

An auto needs to keep the same configuration. delta-delta or wye-wye but not a mix.

You could certainly use an autotransformer but I'm sure it would be odd for distribution levels. Autotransformers are used in transmission stations

 

[electrofelon]

An auto needs to keep the same configuration. delta-delta or wye-wye but not a mix.

You could certainly use an autotransformer but I'm sure it would be odd for distribution levels. Autotransformers are used in transmission stations

I thought most auto tap changers were autos? We have lots of those too because of our relatively low distribution voltage.

 

[mivey]

I thought most auto tap changers were autos? We have lots of those too because of our relatively low distribution voltage.

Yes, but they make relatively small changes in voltage, not a large change like a step-down bank.

Load Tap Changers (LTC) are usually a few percent, like +/- 2.5 and 5 percent. Voltage regulators are usually +/- 10 percent with something like 32 steps.

add: the no-load TCs are usually +/- 2.5 & 5 percent. The on-load TCs are +/- 10% in 32 steps

 

[electrofelon]

Yes, but they make relatively small changes in voltage, not a large change like a step-down bank.

Load Tap Changers (LTC) are usually a few percent, like +/- 2.5 and 5 percent. Voltage regulators are usually +/- 10 percent with something like 32 steps.

add: the no-load TCs are usually +/- 2.5 & 5 percent. The on-load TCs are +/- 10% in 32 steps

what is the difference between the LTC and voltage regulator, besides the percentage difference mentioned?