Ungrounded, corner grounded delta systems

[peter d]

Looking for a history lesson here.

Why were ungrounded deltas used? I think I may have read somewhere that they were preferred in old industrial settings because a ground fault would not cause a catastrophic shutdown of an industrial process.

Also, what was the benefit of a corner grounded system?

 

[iwire]

Looking for a history lesson here.

Why were ungrounded deltas used? I think I may have read somewhere that they were preferred in old industrial settings because a ground fault would not cause a catastrophic shutdown of an industrial process.

It is still used and for the same reasons and not just catastrophic shutdown. :smile:

 

[Buck Parrish]

While ago I was out side catching lightning bugs. Let's not get in to how they light up. Unless you know.
I was just looking at my transformer before it comes in to the house. Their is only one high voltage wire. It looks like they are useing the ground from the secondary side that is twisted with the two 125 volt conductors. As it goes the same way down the street. It is also used as correct me if I am wrong a messenger wire. Or the one that holds the wieight of the secondary wires.
The one lone high voltage is at the top .

 

[wirenut1980]

Looking for a history lesson here.

Why were ungrounded deltas used? I think I may have read somewhere that they were preferred in old industrial settings because a ground fault would not cause a catastrophic shutdown of an industrial process.

Also, what was the benefit of a corner grounded system?

Corner grounded systems were a cheap way of grounding an ungrounded system if that is what the owner wants to do. Corner grounded systems are sometimes is brought about unintentionally. :grin:

 

[wirenut1980]

While ago I was out side catching lightning bugs. Let's not get in to how they light up. Unless you know.
I was just looking at my transformer before it comes in to the house. Their is only one high voltage wire. It looks like they are useing the ground from the secondary side that is twisted with the two 125 volt conductors. As it goes the same way down the street. It is also used as correct me if I am wrong a messenger wire. Or the one that holds the wieight of the secondary wires.
The one lone high voltage is at the top .

The utility is probably using the secondary neutral also as the primary neutral. The transformer likely has a single primary bushing. Instead of an H2 bushing, the high side neutral connection is internally connected to the case, which is strapped to the low voltage neutral bushing.

 

[charlie]

The utility is probably using the secondary neutral also as the primary neutral. The transformer likely has a single primary bushing. Instead of an H2 bushing, the high side neutral connection is internally connected to the case, which is strapped to the low voltage neutral bushing.

Stated like a utility engineer, do you or did you work for Public Service Indiana . . . UH . . . PSI . . . OOPS . . . PSI Energy . . . UH . . . Duke (finally got it right)? :smile:

 

[quogueelectric]

While ago I was out side catching lightning bugs. Let's not get in to how they light up. Unless you know.
I was just looking at my transformer before it comes in to the house. Their is only one high voltage wire. It looks like they are useing the ground from the secondary side that is twisted with the two 125 volt conductors. As it goes the same way down the street. It is also used as correct me if I am wrong a messenger wire. Or the one that holds the wieight of the secondary wires.
The one lone high voltage is at the top .

I have been beating that drum for quite some time now yet I get mugged when I say current travels to ground. Judge for yourself One wire coming off the primary and a bare #6 down the pole to a rod.

 

[winnie]

I have been beating that drum for quite some time now yet I get mugged when I say current travels to ground. Judge for yourself One wire coming off the primary and a bare #6 down the pole to a rod.

No mugging from me. Just a gentle attitude adjustment

Current travels back to the source, via _all_ paths available to it.

The ground is a conductor. If the source is grounded, then current can flow back to the source via the ground.

If the source were not somewhere connected to the ground, then other than very small capacitive current flows, then the current wouldn't flow through the ground.

With most utility systems, the primary neutral and the secondary neutral share the same wires, and that common neutral wire is also grounded all over the place.

Since the ground is a conductor, this means that some fraction of the neutral current will flow through the parallel path represented by the ground. In these systems, current is clearly flowing into the ground...but it flows out of the ground someplace else, to balance this.

Finally, there are systems known as 'single wire earth return', where a single high voltage line is run out to a transformer, with no neutral return wire. The return is via a good grounding electrode (at both the source and the load transformer) and the earth acts as a conductor between the two. Again a case of significant current flow into (and out of) the earth, but the current is simply flowing through the earth as a conductor, not 'seeking' earth in any fashion.

-Jon

 

[massfd]

I do not like that the HV side has only 1 bushing and the other side of the primary is connected to the case. Just try to get away with that under the NEC. If the shared Primary/Secondary neutral goes open all the return current attempts to flow via the pole ground rods.

That was exactly what was found when we had 12 volts between a fence and the EGC on the site lighting poles at 1 of our sites. The utility found a missing neutral over 1000 feet away from our site was causing the primary current to return via the pole ground rod. We had, if I remember correctly about 5 amps flowing on our ECG with the main to our site open.

 

[ayerforce]

Regarding the original question, corner grounded delta systems use 2-pole OCP devices, thus sometimes saving cost in distribution systems. When using circuit breakers, though, they need to be rated straight 240volt, not 120/240 volt.

 

[iwire]

I have been beating that drum for quite some time now yet I get mugged when I say current travels to ground.

Only because your mistaken. :smile:

If I touch a hot to a neutral do we say current 'travels' to neutral?

No, current 'travels' back to the source.

If I touch phase A to phase B would we say current travels from A to B or B to A?

No, current 'travels' back to the source.

Judge for yourself One wire coming off the primary and a bare #6 down the pole to a rod.

All that proved is the earth works as a conductor back to the source.

The earth is not an electron sponge, it does not suck up any spare electrons, it does however conduct the current back to the source.

Try you same experiment from an ungrounded electrical system and you will find very little if any current 'to earth'.

 

[wirenut1980]

Stated like a utility engineer, do you or did you work for Public Service Indiana . . . UH . . . PSI . . . OOPS . . . PSI Energy . . . UH . . . Duke (finally got it right)? :smile:

Howdy neighbor!:smile:

Just started with them last September. The utility I used to work for in Missouri went through the same thing (10 different names). Now they are a part of KCP&L. Do you still work for IPL?

 

[charlie]

Do you still work for IPL?

Yes, this is part of my signature (I removed my street address and phone numbers): :smile:
Charlie Eldridge, Consultant
Indianapolis Power & Light Company
Standards & Code Compliance
Indianapolis, IN 46217

 

[dnem]

I have been beating that drum for quite some time now yet I get mugged when I say current travels to ground. Judge for yourself One wire coming off the primary and a bare #6 down the pole to a rod.

You won't get "mugged", and you'll sound like a professional that knows what he's talking about, if you just change one word: "to" changed to "thru"

"..... when I say current travels thru ground"
If you're an English major you can spend the extra half hour writing it out "correctly": through. . Who's the bozo that decided that I need to put a "gh" on the end of my "thru" ?

The earth is not an electron sponge, it does not suck up any spare electrons, it does however conduct the current back to the source.

I'm going to steal that phrase. . I kinda like it. :grin:

 

[charlie]

. . . we had 12 volts between a fence and the EGC on the site lighting poles at 1 of our sites. The utility found a missing neutral over 1000 feet away from our site was causing the primary current to return via the pole ground rod. We had, if I remember correctly about 5 amps flowing on our ECG with the main to our site open.

OK, now tell me how that would change if we used a two primary bushing transformer with one bushing connected to the neutral and grounding system on the pole? :-?

 

[dnem]

Regarding the original question, corner grounded delta systems use 2-pole OCP devices, thus sometimes saving cost in distribution systems. When using circuit breakers, though, they need to be rated straight 240volt, not 120/240 volt.

Yep, for corner grounded delta you get to use a 2pole main and 2pole breakers on your 3phase loads. . You get single and 3phase available but build the system just like 2hot single phase. . The drawback is only a single voltage available unless you add another transformer.

When you're building the corner grounded delta, make sure you phase tape the grounded phase leg white at every point. . That's a phase conductor tagged white. . So you'll tag 2phases 2 different colors other than white, gray, or green and the 3rd phase tagged white.

I'll step outside of my comfort zone and comment on the transformer. . If I'm wrong, somebody correct me. . It's my understanding that the switch from wye to delta also brings an advantage of a physically smaller, less copper inside, and less expensive transformer that provides the same amperage output as a wye.

For any one phase, the wye generates its amperage across a single coil and that coil "sees" the full amps that the load is drawing. . For any one phase, the delta generates its amperage across 2 coils and splits the full amps that it "sees" so that any one coil, at any pin point in time, has to provide only part of the full amps that the load is drawing.

 

[jim dungar]

It's my understanding that the switch from wye to delta also brings an advantage of a physically smaller, less copper inside, and less expensive transformer that provides the same amperage output as a wye.

For any one phase, the wye generates its amperage across a single coil and that coil "sees" the full amps that the load is drawing. . For any one phase, the delta generates its amperage across 2 coils and splits the full amps that it "sees" so that any one coil, at any pin point in time, has to provide only part of the full amps that the load is drawing.

In your example you have a three phase delta transformer (and assuming a 1-1 ratio transformer) feeding a single L-N load. Yes in this extreme case the delta windings are carrying 1/2 the current the wye winding is carrying. Add a second L-N load; now you have one delta winding carrying 1/2 the current for load 1 + 1/2 the current for load 2 , and you have the other (2) windings still carrying only 1/2 the current of the wye windings. Finally, add a third L-N load and now all of the delta windings are carrying the same current as the wye windings, so there is no savings.

 

[LarryFine]

. . That's a phase conductor tagged white . .

It's not that big a deal, really. It's no different than the neutral in a single-phase system in the bonding regard. In fact, most of the neutral-conductor rules apply to the grounded conductor of a corner-grounded Delta.

 

[dnem]

In your example you have a three phase delta transformer (and assuming a 1-1 ratio transformer) feeding a single L-N load. Yes in this extreme case the delta windings are carrying 1/2 the current the wye winding is carrying. Add a second L-N load; now you have one delta winding carrying 1/2 the current for load 1 + 1/2 the current for load 2 , and you have the other (2) windings still carrying only 1/2 the current of the wye windings. Finally, add a third L-N load and now all of the delta windings are carrying the same current as the wye windings, so there is no savings.

For the sake of an attempt at simplifying for clarity, let's just look at a single phase load powered from 2 phases.

With the wye, you have only one path thru the transformer. . An electron travels down one phase conductor to the transformer, into the transformer across a phase lug, thru one of the transformer coils, thru the center tie of all 3 coils, thru a second transformer coil, into a second transformer phase lug, back down a second phase conductor. . Every part of the single path will "see" the same amperage.

With the delta, you have two paths thru the transformer. . One path takes the route across a single coil. . The other path is across the other 2 coils in series. . The amperage is split between the 2 paths and no one coil "sees" the full amperage being drawn by the load.

I had heard that a switch from wye to delta brings an advantage of a physically smaller, less copper inside, and less expensive transformer that provides the same amperage output as a wye. . My description above seems to explain why that would be true.

Is there something wrong with my logic ?

 

[winnie]

For a balanced three phase single frequency load, wye or delta makes no difference. There are 'second order' differences with respect to how the different transformers will respond to unbalanced loading or harmonics or component failure.

Given the same three phase core and the same primary winding, you can build a secondary that is either wye or delta. Since we've specified the same core, we've fixed the 'volts per turn'. That means that the delta connection, which puts higher voltage across a coil, will need more turns of wire. For the same output power level, the delta connection has lower current flowing in each coil. So the 'Delta' connection will have more turns of thinner wire to get the same current density in the copper. The net result is that for the same output KVA rating, you have the same amount of copper as with the 'wye' connection.

The big place where delta would permit savings is that you can use an 'open delta' bank to get three phase power with only two transformers. This doesn't save you anyting in terms of pounds of copper and steel per KVA; but it is useful to get a little bit of three phase in an otherwise single phase application.

-Jon