California utilities preference for ungrounded distribution 1

[mvsubstations]

Just curious if anyone knows why California prefers ungrounded delta for most of their 15kV class distribution, while much of the rest of the country is grounded wye system. Probably simply because "they always did it that way", or did California specifically change to ungrounded delta at some point in the past for a specific reason?

 

[davidbeach]

I don't know that you can make a general statement about "California". It's a huge place with lots of different utilities and they all have there own ways of doing things. There were at least some areas that were built out 80-100 years ago with a 12kV delta system. Some of that has slowly morphed into a 21kV grounded wye system. None of it gets changed simply for the sake of changing things. Load that is all connected phase-phase does allow for much more sensitive ground fault detection.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[cranky108]

I believe some of the older small hydro systems, like other parts of the country, were delta systems because it is very hard to drive a ground rod into rock.

I can tell you as someone who has worked on small hydro units in other parts of the country, putting a ground on the neutral of a small hydro can cause some unusual things to happen. Like sparks when opening the gate at the caretakers house.

At least that is why we still have some delta circuits.

Grounded systems also don't ensure there won't be any fires started, or that distribution circuits will clear when they fall on some surfaces.

 

[itsmoked]

It's simply because it's a big place. It would've required another 239,557 miles of wire to frivolously add a neutral.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

The added neutral is a pretty cheap way to gain a 73% increase in circuit capacity.
The neutral was added to many delta circuits along with a delta to wye conversion to quickly and economically increase circuit capacity.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[davidbeach]

Took a new transformer at the substation, that's not cheap. The house I grew up in went through one of those conversions; the two phase conductors on the cross arms became a phase and a neutral. The transformer gained a larger bushing on the side that was connected to the phase conductor. Where things had been three phase they had to string a neutral before the conversion; but must of the miles are single phase taps, and those already had two wires.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[cranky108]

That dosen't seem to make since, and seems out of place.

Usually there is a marker for a deleted thread.

Some utilities start with new two bushing transformers, where at least one is rated for the full phase to ground voltage. Likely both.
At least some companies keep these in stock, as we have both systems, and to keep fewer types of transformers in stock.

But david is right, the substation transformer is not cheap.

 

[Mbrooke]

This is part of it:

https://www.ecs.csun.edu/~bruno/MultiGroundedNeutralFinal_4-17-7.pdf

California's PUC-95 basically asks (requires) that current carrying conductors be insulated from earth. Second many places in Cali have groups or rules that deal EMFs and their mitigation.


Both of these combined tend to result in two bushing transformers either connected phase-phase or phase to a neutral up on an insulator with little or no grounding past the substation.

Personally, I think that all distribution system on earth should be wired this way. Earthing a normally current carrying conductor more than once is wrong on many levels. In the future an MGNs won't carry load current...

 

[waross]

Some thoughts.
Conversions:
Many early systems were 2400 Volts delta with two bushing transformers.
After WW2, the use of electrical energy increased rapidly and many existing circuits were reaching their capacity.
With the addition of a substation transformer and a multiple grounded neutral, many of these systems were successfully upgraded to 4160 Volt wye circuits.
The town that I lived in did such a conversion.
If for some reason the existing transformers were unsuitable for phase to neutral use, then the additional cost of replacing the distribution transformers was incurred.
For new construction there are savings in insulators and in transformers which may be fitted with only one insulator and may be insulated for line to neutral voltage rather than line to line voltage.
It depends.

Code and safety:

The Hazardous Multigrounded Neutral Distribution System
And Dangerous Stray Currents
Copyright Material IEEE
Paper No. PCIC-03-03
Donald W. Zipse, P.E.
Life Fellow, IEEE

Many of Mr. Zipse's ponts are valid, but he is pushing an agenda.
He cites anecdotes with no supporting facts.
What current was Edison pushing through the ground?
While I cannot recall the details, I have read of a number of investigations of livestock being injured by electricity but as I recall, the cause was generally wiring issues on the low voltage side and not influenced by primary wiring methods.
In specific regards to livestock, a common fault is not electrocution but a very low stray voltage (Not current) on the surface of stack waterers.
The tingle that the animals feel discourages them from drinking. The animals tend to suffer dehydration, not electrocution.
The cause is invariably local wiring issues.
Inappropriate calculations.
He tends to deal with resistance only and fails to consider reactance and impedance.
He gives a table of resistances but with no indication of the range of resistances typical of ground conditions.
At times he compares apples to oranges with a horse of a different colour.
Lightning is a real issue, but it may be argued that when lightning strikes a secondary conductor the effect on homes may be lessened by the effect of a multiple grounded neutral.

I will counter some of Mr. Zipse's inappropriate calculations with an inappropriate calculations of my own.
Consider a 10,000 Volt circuit.
In the event of contact between the hot line and ground at the end of the circuit, the hot line and the ground return will form a voltage divider.
With equal impedances between the hot line and the ground return, the voltage drop on the ground return will be 5000 Volts.
In the real world, the voltage drop on the ground return will probably be less, so this is a conservative estimate.
Mr. Zipse focuses on current and suggests that voltage is unimportant, it is the current that matters.
But, it is the voltage that drives the current and the voltage must be considered.
Let us now consider a fault about one mile from the substation, round it down to 5000 feet for convenience.
We have 5000 Volts dropping over 5000 feet.
That's 1 Volt per foot.
That is probably why we are not overwhelmed by cases of electrocutions of persons and animals due to multiple grounded neutrals.
Mr.Zipse uses single phase circuits for examples. This is somewhat misleading.
In a three phase circuit the diversity and cancellation of neutral currents from different phases results in the neutral current at the transformer being much less than the numeric sum of the individual neutral currents.

I suggest that both multiple grounded neutrals and insulated neutrals have strengths and weaknesses.
The choice is a compromise technically.

And by the way, I live in a ranching and farming area of the prairies.
Some of the farmers and ranchers are intelligent and well educated.
They also have sons and daughters who have become electrical engineers or electricians.
The farmers and ranchers also have a very strong political influence.
If multiple grounded neutrals were a problem they would have reacted long ago.
As for unity of action:
Who are the farmers (and by extension, the government) going to believe?
The wicked power companies or their sons and daughters who they paid to educate and can still quote the cost to the cent.
No dead or dehydrated stock to be concerned with.

But, actually we don't have the typical multiple grounded neutrals here.
What would Mr Zipse's reaction to know that we use the ground return system that failed for Mr. Edison?
Yes. No neutral carried back.
A typical installation is a pole mounted transformer with a grounding electrode at the pole.
An overhead neutral line is carried back one span and connected to a second grounding electrode.
That's it.
100% ground return.
No electrocutions.
No dead animals.

Like I said, it is a compromise.



Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Mbrooke]

Honestly, I've noticed multiple errors in his paper re-reading it over the years. I'll give you that.


But the basics are correct- current tends to flow 50/50 (or 40/60) more or less over the MGN and all else in parallel with it. A current carrying conductor will always have voltage relative to remote earth. An MGN is no exception. Further during a fault the system becomes a voltage divider, the short circuit voltage drop along the MGN is brought into each building.

Stray voltage is a very real issue in dairy farms and sometimes swimming pools. The elevated magnetic fields this system produces will also be forced into question one day. (California has already called it into question and put forth a solution)

 

[waross]

By far the worst stray voltages I have ever encountered were working under and near a 500 kV transmission line.
One of many examples:
There was a wooden warehouse a few hundred feet away from the line.
The electricians wiring the building tried to run a line of 14/2 NMD90 Romex from apanel at one end of the building to a light fixture above the main doorway at the other end of the building.
They were running the Romex through the rafters when the electrician got a shock from the Romex, through the insulation and jacket of the Romex.
The transmission line did not have a neutral conductor.
I understand that some wire fences below that line became unintended electric fences.

Stray voltage is a very real issue in dairy farms and sometimes swimming pools.

I agree, but I suggest that the cause is seldom from a multiple grounded neutral system.
But, I will think long and hard about the difference.
I may come around to your way of thinking, but it won't be quick.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Mbrooke]

Well, consider that transmission lines almost never have a neutral. Sure there is an electric field- but load wise- what passes zero sequence current during steady state operation? At most, perhaps, a few of these:

https://search.abb.com/library/Down...LanguageCode=en&DocumentPartId=&Action=Launch

Which in most utilities are actually not common. That leaves VTs and CVTs, which are negligible.

Now compare that to a distribution line, where 4,000,000 va worth of power going to neutral is not unreasonable if not approaching common.

 

[davidbeach]

"Transmission" lines almost never have anything connected line-neutral.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[waross]

Understood David.
My reference to transmission lines was to make the point that not all stray voltages and currents are the result of multiple grounded neutrals.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[FacEngrPE]

Transmission lines are usually star connected at the transformers. Likely for controlling voltage offset with respect to earth when faults occur. Thus has a side effect of making the systems sensitive to low frequency geomagnetic current saturation of transformers. A generating plant operator told me that during one of these geomagnetic events the generating plant substation transformers connecting to the bulk power system (500 kV here) growl.

The only practical solution that can be applied is to either temporarily lift the neutral, inserting a DC blocking capacitor on the neutral, or reduce the transformer load to the point where the degree of saturation does not overheat the transformers.

https://www.epri.com/research/products/1015938

Transmission lines usually have overhead wires intended to carry lightning strikes to earth. As the earthing wires are connected to the substation ground grids on both ends they may carry a portion of any current that might otherwise flow through earth.

Earthing of neutrals at substations is also likely needed for correct operation of some protection relays.

So just like at the distribution level transmission has a bunch of trade-offs to be made, and to some degree we need to live with history.

Fred

 

[Palletjack]

New Jersey, and other places in the northeast have some ungrounded delta also.
As stated before, the OP is a very broad generalization based on minimum observations.
There are many different utilities in the country that use different configurations.