Hi,

Can anyone explain what are the differences between a Delta/Delta, Delta/Y, Y/Delta or Y/Y transformer? What are the pro's and con's? Which systems are used and which ones are prohibited in your country? And why? Here in Belgium the only system used and allowed is Y/Y. The other systems are prohibited. Don't ask me why, because I don't understand the difference. That's why I'm asking you.

HV lines in Britain are 3-wire only, no neutral, with the step-down transformers to provide 240/415V power wired delta/wye.

The HV-HV xfmrs are a little out my line, buy I think that most are either delta/wye or delta/delta, the latter with the appropriate balancing added to provide a neutral reference.

Scott35 has some configurations cataloged at https://www.electrical-contractor.net/ubb/Forum15/HTML/000064.html E-C.net Technical Reference Area.

It’s half a megabyte, but there is also http://www.cooperpower.com/Library/pdf/R201902.pdf and is a good collection of common and mutant transformer arrangements for electrical-distribution applications, using combinations of various 1ø transformers. The figures generally apply to a set of transformers with external jumpers or internally connected like in 3ø unit-substation or padmount installations.

Figure 9 is a ∆-∆ connection and common for older industrial systems or in rural areas where one transformer set is used to feed a medium-sized motor. Figure 19 is a Y-Y connection, and one use is to interconnect different transmission voltages…a variation is a wye autotransformer arrangement like figure 24.

Figure 22 is a ∆-Y connection, and probably most common over the range of transformer configurations used to directly serve utility customers—at high-to-medium- or medium-to-low-voltage. Another ∆-Y application is generator-stepup service to feed transmission circuits where the delta side is connected to medium-voltage generator(s) and the wye side faces the 69-345kV transmission system.

Figure 17 is a Y-∆ connection, and in distribution systems is a little less popular than ∆-Y because of its tendency to act as a sometimes-undesirable ‘ground source’ on the wye side.




[This message has been edited by Bjarney (edited 08-04-2003).]

I sure ain't no expert but a Y (or wye) system is really only need when you want to use single phase loads with a lower voltage than the phase to phase voltage. This is rarely the case in MV and HV distribution. As far as I know, only delta is used for the primary in Sweden. At least, there are always just three wires on the poles.

In some countries a wye MV system is used in order to make it possible to run single phase lines off it. The difference between doing with a delta and a wye system is that in a wye system, the voltage on one wire will be low enough not to require a real insulator. This means that you save an insulator per pole. And, the voltage across the single phase transformers is lower than it would have been with a delta system. This mean you can use a simpler transformer. (Which presumably is cheaper)

If most or all transformers are three-phase I can't see any reason to use a wye system on the primary. It justs add a wire. With a delta you get a certain balancing between the phases: If someone hooks up a big load on one of the phases on the secondary side, this load is divided between two phases of the primary. Better load balancing saves money.

One, albeit far-fetched, reason to require Y-Y I can imagine is that you can have a common earth between the MV and LV sides. This would mean you have a lot of earth rods connected to each other via the MV system. This is probably good in a TT system. In a TN system, you have plenty of earth electrodes in the LV system connected to each other.

Single-phase branches from a wye system also allow the use of a xfmr with only a single hi-pot bushing, which also helps to save costs.

Single-phase spur lines in countries like England rarely extend more than a very short distance to serve a couple of xfmrs. But in places such as the U.S., Canada, Australia, etc. where the population density is much lower and distances much greater, I understand that the savings add up considerably.

You might find this document interesting: www.ieconsult.fr/etc155.pdf

P.S. The site seems rather slow to respond at the moment. I guess with the heatwave sweeping across France (over 100 in places yesterday!) that the electrons can't be bothered to travel at their usual speed.


[This message has been edited by pauluk (edited 08-06-2003).]

Sorry, I wasn't clear. What I wrote was applying to step down transformers from MV to LV.

Eli,
Do you mean that all MV-LV xfmrs in Belgium are Y/Y? So you have a distributed neutral on all MV lines?

I don't know much about xfmrs but that's what I heard, yes , that they all are Y/Y. I'll try and back it up with written documentation.

What do you mean by a distributed Neutral? And why?
The center point of the Primary Y and secondary Y are connected to ground. We are with a TT system.

Here is an image of two types of 1ø overhead distribution transformers used on the North American continent.


Fig 1 is a ‘two-bushing’ 1ø transformer that can be used in a delta or wye 3ø primary configuration, or for 1ø as a spur, connected phase-to-phase on a wye or delta distribution circuit, or phase-to-neutral on a wye-multigrounded circuit—also referred to as primary multiple earth.

Fig 3 is a ‘single-bushing’ 1ø transformer {the side porcelain device jumpered to the bushing is a surge arrester} where the other primary connection is routed through the metal can, that must always be used in a grounded-wye distribution circuit.

This is the Y-Y configuration that Belgian describes—used in many regions. In much of Europe, the secondaries are wye connected to provide ~415Y/240V 3ø 4-wire service, generally to serve 240V 1ø 2-wire and 3ø 4-wire in buildings.

When describing 3ø and 1ø systems, understand that 3ø circuits may be 3 or 4 wire. Also, remember that a 1ø 2-wire circuit can be tapped from 3ø system by two means—phase-to-neutral {from a 4-wire wye system} OR phase-to-phase {from a wye or delta system.}

A 3-wire circuit is most often comprised of three ungrounded phases. There is an uncommon practice using two phases and the neutral connection, but by convention describing the circuit as ‘2-phase’ is incorrect. Another type of 1ø 3-wire circuit is common in the US as 120/240V, using one 1ø transformer with a mid-winding tap. A slang term for this arrangement is ‘spilt phase.’ In Europe, it is possible to find rural 3-wire split-phase 240/480V service, but the application appears very limited.

These few points are just about universal in discussing 1ø and 3ø power distribution around the planet. Regional practices are built from these basic relationships.

[Paul, C-H and Belgian, if any of these points need correction for European practice, please don’t hesitate to respond.]




[This message has been edited by Bjarney (edited 08-06-2003).]

Just one comment: European practice is to use three-phase transformers, not three single phase transformers. They also tend to be large and pad mounted, unless serving only a small number of houses.

The advantage of a three-phase transformer is the lower total cost.

The advantage of single phase transformers is that you can have the same transformers for both single phase and three-phase lines. The more single phase lines per three phase lines (on the LV side) you have, the more likely you are to go for this solution.

That goes for the U.K. too. We have single units for 3-phase delta-wye MV-LV xfmrs, like this pad-mount type:


or this pole-mount unit:

(Both of these are 11kV delta primary, 240/415V wye secondary.)

Single-phase transformers are normally used only where a single-phase 240V service is required for one or two isolated houses.

Re "distributed neutral" -- I'm refering to the situation where MV distribution is 3-phase 4-wire, so that a neutral is run along with the phases, thus enabling a 3-ph xfmr to be connected either delta or Y on the primary (and single-phase xfmrs to be connected either between two hot phases or from phase to neutral).

In my travels, it is only in the U.S.A. where I've seen a distributed neutral on MV lines. Admittedly my travels on this side of the Big Pond are restricted to the U.K., France, Ireland, and Spain.

Could it be that your rules are just specifying a wye system on the LV secondary side? That seems much more likely.

Delta LV supplies seem to have been a particularly American system.


[This message has been edited by pauluk (edited 08-06-2003).]

I have informed myself further and it seems that I got it wrong. All MV/LV xfmrs are here Delta/Wye. Sorry for the misinformation.

Belgian,
Over here, Y/D(or D/Y, depending on which sort of supply you are after) transformers form the bulk of our Distribution system.
D/D transformers are used on really long stretches of line, as a voltage stabiliser and these are normally run through some sort of an automatic tap-changer.
Just a note about transformer earthing, regardless of where a tranny is installed, it has to be earthed by a driven earth stake, this earthing is regularly tested.
According to ECP 35 over here, the use of the Earth Potential Neutral as a common between the LV and MV sides of a transformer, is to be avoided, because of the very real risk of back-feeds.