Why does anyone use grounded delta transformers? A friend of mine was pulling his hair out on a service call that looked like a ground-fault on a B-phase of the service lateral and turned out to be a corner-grounded delta.

What is the utility of a corner-ground or center-tap ground delta? It seems like a waste of a perfectly good phase when a wye would have worked just fine.

I've heard that high-legs can be useful if you want to run a 208 single-phase lighting load. But even this seems questionable: I've only seen a couple of high-legs and they have never had any single phase loads on the "B" phase. Also, if you heavily load "B" phase, doesn't it ruin the ability of that phase to contribute to three-phase loads?

With a corner grounded delta, it seems like there is an advantage over a high leg in that there isn't the risk of inadvertantly burning up single phase loads put on "B" phase, but that phase is still useless otherwise.

I've heard that corner grounded delta banks were popular because it gave you the voltage stability of a wye, but it could be left running as an open-delta while one transformer was removed for service. Any truth to this? Does it also apply to high-leg deltas?

What's the deal?

-John

Center ground Delta allows you to serve single phase and three phase loads from the same service. It was used in areas of mixed residential and industrial. Use a larger transformer for the single phase loads and add a single small transformer to create the open delta three phase.

Corner grounded delta securely references the other two phases to earth potential. I believe it also allows you to save wire, but I am not sure. I think it also allows you to use single phase panel boards also. I believe that you are not supposed to connect any single phase loads between the high leg and the neutral.

I am not an electrician, so please take my information with a grain of salt.

Larry C

Larry,
Your answer applies to a high leg delta, not a corner grounded delta. There are only 3 conductors in a corner grounded delta system. There is only one voltage available and that is the phase to phase voltage. Most of the ones that I have seen were 240 volt. One advantage is that in many cases you can use single phase equipment for a corner grounded system. You only need overcurrent protection for the two ungrounded conductors.
Don

Corner delta is usually used where you only have 3 phase loads. It does save you one conductor and you only need O/C protection for 2 so the equipment does look like single phase. It still has to be listed for the purpose. I am not sure what that does to the cost. I saw 3p corner ground on sewer lift stations where the main load was a big pump or two. It will confuse you the first time you see it.
Center tapped delta is a lot more common. Usually you will see that done with 2 transformers, a big one for the single phasde loads and a smaller one on the 208 "red" leg. If this is "open vee" (2 transformers) you do not want any L/L loads on the high leg. It will unbalance the phases. I suppose if they had 3 it would be OK but if they had 3 they would probably just wire it wye.

BigJohn, did they answer your questions?

Here is the discussion thread in this site. Corner Grounded 3Ø 3 Wire Delta https://www.electrical-contractor.net/ubb/Forum15/HTML/000078.html

BigJohn,

The posts aleady made to this thread should be adequate for covering the topic, but I will add a few more points.

1: The 3 Phase 3 Wire Grounded Delta System is an optional type system, which offers the "Stability" of a Grounded AC System, and the use of a Delta Connected Secondary.

2: The "Stability" of a Grounded AC System refers to the issues of Stresses on the Conductor's Insulation, which are common on Ungrounded AC Power Systems.
With one Grounded System Conductor in use, the Voltage to Ground will be somewhat "Stable", as viewed by the Capacitive Reactance of the System.

3: Use of a Delta Connected Secondary: This would allow for the temporary use of an Open Delta Vee connection, incase of an emergency.
Also, the Permanent System may be setup as an Open Delta Vee, Open Delta Tee, or a full "Closed" Delta.

There are a few ways to achieve a Delta arrangement - as listed below:

A: Open Delta Vee, Corner Grounded: Using Two Single Phase Transformers, connected in a traditional "Vee" arrangement, with one of the phase lines physically Grounded,

B: Open Delta Tee, Corner Grounded: Using Two Single Phase Transformers, connected in a non-traditional "Tee" arrangement, with one of the phase lines physically Grounded,

C: Open Delta Vee, Center Tap Grounded: Using Two Single Phase Transformers, connected in a traditional "Vee" arrangement, with the Center Tap of one Transformer physically Grounded,


D: Open Delta Tee, Center Tap Grounded: Using Two Single Phase Transformers, connected in a non-traditional "Tee" arrangement, with the Center Tap of one Transformer physically Grounded,

E: Closed Delta, Corner Grounded: Using Three Single Phase Transformers, connected in a traditional "Parallel" Delta fashion, with one phase line physically Grounded,

F: Closed Delta, Center Tap Grounded: Using Three Single Phase Transformers, connected in a traditional "Parallel" Delta fashion, with the Center Tap of one Transformer physically Grounded,

G: 3 Phase 3 Wire Delta, Ungrounded: Any version of the Open Deltas (Vee or Tee), or a Closed Delta, where there is no Main Bonding Jumper between the Local Grounding Electrode System, and the AC Power System (i.e. no Grounded Conductor).

Please review the Technical Reference section's schematics, and check out the various types of Delta Systems.
The link provided by Larry will take you to a slightly detailed view of a basic "Corner Grounded 3 Wire Delta", with 2 views of Panelboard applications, 1 view of use with HID Lighting, and 1 view of use with a basic Motor application.

Feel free to make additional comments if needed.

Scott35

Thanks for the replies, they definitely answered some questions. I've never worked on the actual transformers for a center-tapped delta, but I've seen the diagrams, and they were always shown as a closed-delta made of three transformers. Is it more common for the actual installation to be an open-delta?

So you would install a center-tapped delta bank when you have small three-phase loads? You use a small second transformer to derive the other two phases, and then they take their large single phase loads from the one transformer with the center tap? Is that right?

It definitely makes sense that if they had three transformers they would simply make a "wye" has anyone worked on a three-transformer center-tapped delta? What were the advantages over "wye"?

-John

I've worked on exactly one corner-grounded delta in my career, when I was a very green apprentice. The journeyman I was with wondered why one phase was taped white-- what did I know? Well, it was the grounded conductor, of course!

This was a plant that made all those telescoping dentist mirrors and magnets that auto mechanics use. I'm sure they've moved all the production to China by now.

So, 240V phase-to-phase, 240V phase-to-ground. If you want 120V, you need a transformer.

Sometimes a building with an existing 3-wire single-phase service will need one 3-phase motor, for whatever reason. It's too much expense to rewire the whole building. Add one transformer and one wire, and you can supply that special load. This is common in auto shops. You need 3 phase to run the lift pump, and maybe an air compressor. Everything else is single phase.

It might also be common in rural areas served by single-phase. If you need 3 phase, and the primary run is long, you only need run one extra wire.

The whole scenario of being able to do with two transformers in an emergency where one burns up might just be a rarity. Probably half of the deltas I see around here are "open deltas" to begin with.

Here in Austin, there are a few large houses built in the early '50s with center-ground deltas. It seems that the earliest central air conditioning systems required 3 phase. Usually, there will be 2 weatherheads, one for the 3 phase load, and one for the single phase load. I changed out one where the 3 phase had long since been disconnected. It had a huge metercan for 2 A-base meters, but only the single-phase meter was connected. Of course, the new service I built was single phase only.

There are many areas where utilities do not supply delta connections, and I'm not sure they exist at all outside the US.

Delta isn't used here in the U.K. for any sort of public domestic/commercial supply. All of our HV distribution is without neutral, so if a large industrial consumer has its own transformers and takes its supply at 11kV, then the xfmr primary will be delta-connected.

Once you get to the normal LV connections from the utility though, we really have only three distribution arrangements in use:

#1. Single-phase xfmr, 240V secondary with one side grounded, found almost exclusively in rural areas where the xfmr feeds just one or two isolated homes.

#2. Single-phase xfmr, 240/480V secondary, center-tap grounded as the neutral, 3-wire distribution. Fairly unusual, and again only found in rural areas. Individual homes are tapped for normal 2-wire 240V service, some from one side of the supply, some from the other. Farms etc. might get a 3-wire connection with load distributed between the two legs. Pretty rare, and really just a holdover from the past.

#3. 3-phase 4-wire wye secondary, 240/415V, grounded neutral. The standard distribution used just about everywhere except for isolated homes as described above. Even relatively small villages will have a sub-station which feeds a 3-ph 4-w network like this. Homes and light commercial are tapped for a 2-wire 240V supply while 3-phase users get a 4-wire wye 240/415V connection.

We ran into an issue recently regarding grounding of a delta, and had quite the heated argument regarding center-grounding of a delta connection. The switchboard was 3W delta, which is unusual for us. Motors and distribution transformers clearly don't care if the delta drifts, but the issue was that TVSS measures voltage from the lines to ground, as does the static switch of the delta-fed UPS on the switchboard, and both could malfunction if the delta became unbalanced and any of the phases drifted high or low with relation to ground. The generator was wired in WYE, so it was easy to ground the neutral tap to center the delta (no neutral connected, just the grounding jumper), but doing that causes some other issues...

Grounding one leg of a delta secondary eliminates the circulating current inside the transformer. This helps to reduce heat and losses internally. There can be a small amount of current that moves inside the transformer due to imbalanced load that will continually circulate thru the transformer if it is not grounded.

Wow, that's a great point, I'd have never thought of that! It's completely counterintuitive, but makes perfect sense now that I think about it- grounding a corner causes a wider rotation, which places the triplens into destructive interference. Nice tool to have in the toolbox the next time we're having harmonic problems at a transformer

...Although now that I'm thinking about this more, I'm getting confused again. Gonna have to chew on it a while longer.

To the original question, the advantage of the corner grounded delta as opposed to a totally ungrounded system is voltage stability to ground and better lightning resistance. In an ungrounded system transient voltages between any phase and ground are essentially indeterminate and limited only by the breakdown voltage of the insulation. Consider motors or transformers where the phases are enclosed in and wound on grounded metal equipment. More of a problem on older systems (and insulation materials) it can still present issues today. Corner grounding doesn't buy you anything operationally.

Did some electrical/safety training in a small food-processing plant a couple of decades ago. It had an original {LA Dept of Water&Power} 800A 240V 3ø 3w corner-grounded ∆ for motors and forklift-battery chargers, with odds’n’ends 120V dry-type transformers for lighting and convenience outlets.

Later, a 400A 120/240V 1ø 3w service was added. This gave the plant three separate sets of grounded-circuit conductors [all correctly white] to keep track of. At the end of training, I had the chief maintenance mechanic sort of clear on what was what, {none there were originally hired as ‘electricians’} with the agreement that he would call me if he needed further explanation.

Big John, on the west coast, [CA/OR/WA] there are thousands of utility-owned 4-wire open-delta-secondary [two-transformer] banks in service—primarily for smaller farms (house, barn and water pump), isolated pump motors, dairy barns and small, individual retail firms. The ANSI-standard voltage designation for both delta (three transformer) and open delta (two transformer) is 240/120-volt 4-wire, [3ø is implied in the designation] where 120/240 3-wire is the common residential 1ø designation.

The tradeoff between the two configurations for electric utilities is purely economic—open-delta banks are usually limited to loads of ±20 horsepower.

For open-delta services, the primary-winding configuration may be either open-delta or open wye. (A slang term for open-wye primary is “v-phase,” and must also include a primary-side neutral conductor connected to both transformers.) An easy way to differentiate 2-transformer, crossarm-mounted-bank primaries is by the number of fused cutouts—two means open-wye—three indicates open-delta.