Open Wye Open Delta Connection

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hpec

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Hopefully someone can help me with this. I first noticed this connection in South Carolina in the 80s. I have seen it in Ohio also. The utility will bring two phases of primary and a neutral conductor into two transformers and connect the two transformers in an open delta configuration and come out with three phase. If you put the system on a scope you will have a pure three phase sine wave. Excuse me, the way I look at it you have a single phase system turned into three phase. In other words if you have a 12,470 three phase network like we have in Ohio, you bring two phases of primary and a neutral 7200 volts if you will, a single phase system, and come out of two transformers connected in open wye on the primary and open delta on the secondary and produce three phase. Am I looking at this wrong? A friend of mine told me, "Herb if you do not look at this vectorially you will never understand it". Hopefully some of you can help me understand this.

Hpec
 
Re: Open Wye Open Delta Connection

Hpec, you're making me think here, and my head hurts. :)

Ed

[ December 07, 2004, 08:50 AM: Message edited by: Ed MacLaren ]
 
Re: Open Wye Open Delta Connection

Hpec
This is a common connection in many rural areas. While you are correct that you have only one phase, you actually have three shifts in phase, AB between phases; A-n, A to neutral; and B-n, B to neutral.
Your friend is right, to picture it you should draw it out with vectors, or at least little lines representing transformers.
First draw out a wye, ground the center, and then erase one leg, you have the open wye. Notice the grounded neutral keeps it in a wye shape.
Secondly, draw out a delta and erase one leg. You have an open delta.
The secret in making it work is grounding the wye.

Jim T
 
Re: Open Wye Open Delta Connection

Basically, you have two transformer primaries in series with each other in a single phase configuration. Then you attach the neutral to the mid-point and pull that point out like a bowstring to 120?. The result is a wye that is missing its third leg.

The two secondary coils are then connected in a delta configuration. This gives all three phases but the transformer from the high ? to one of the legs is missing. The other two transformers are required to make up the capacity to serve the 3? loads. On the secondary side, the center tap of the 1? transformer is normally grounded, taken out as the 1? neutral, and picks up the 120 volt loads.

Without pulling the bowstring, you lose the 3? and it becomes a 1? system with weird voltages. :D
 
Re: Open Wye Open Delta Connection

First off, I contend that an open wye is the same as an open delta. Now consider 3 transformers with their secondaries wired in a delta configuration with the primaries fed by 3-phases. You have 3-phases out right? Now disconnect one of the secondaries; you still have 3-phases right? Now disconnect the primary because it is not doing anything, and you still have 3-phases.

This is not as someone said a single phase problem, you must have the 120 deg. separation for this to work. That means 3 wires in, and 3 wires out.
 
Re: Open Wye Open Delta Connection

First off, I contend that an open wye is the same as an open delta.
Click to expand...
Not exactly. The open-wye is only used for primaries.
You couldn't connect transformer secondary windings in open-wye and get three phase out of it.

The reason that you can get three phase power out of two transformer windings with an open-delta connection is due to phase relationships that are unique to the delta connection.

Because the individual phase voltages are only 60 degrees out-of-phase with each other, the line-to-line voltages are correct with or without the third secondary winding connected.

Of course, load capacity is reduced to 58% of the closed delta capacity.

Ed

[ December 07, 2004, 01:49 PM: Message edited by: Ed MacLaren ]
 
Re: Open Wye Open Delta Connection

rattus

In addition to Ed's explanation, other differences are the phase shift between primary and secondaries. In a delta-delta the phase shifts are 0 or 180 degrees, in a wye-delta the primary-secondary phase shift is 30 or 210 degrees.

In an open wye delta, the transformer primaries are rated for line to neutral voltage, say 7200 volts in an open delta-delta they are rated line to line, an equivalent 12,470 volts.

Jim T
 
Re: Open Wye Open Delta Connection

Thanks everyone for your responses and so quickly.I understand the connection better now. It is the relationship with the neutral conductor that makes it work. It seems that the system sees the neutral conductor as the missing phase conductor and reacts accordingly. It seems that the system does not know that the neutral is missing.

Hpec
 
Re: Open Wye Open Delta Connection

Gents, I understand what you are saying. My point is that the schematic of an open wye looks like the schematic of an open delta, and you should be able to use a delta-delta example to demonstrate the point that the third phase can be derived from two phases 120 deg. apart. Some things are easier for me to think about.

Now, in my ignorance, I do not know why various wye-delta configurations are employed. Enlighten me please. We micro types have forgotten most of what little we learned in AC Machines.

[ December 07, 2004, 06:17 PM: Message edited by: rattus ]
 
Re: Open Wye Open Delta Connection

We use them all the time for smaller 3? services. It only takes 2? primary instead of dragging a third phase to the service and we can use just two transformers to serve the customer. Since most small 3? services have only one or two 3? motors, the kicker (the transformer that raises the high phase or B?) is usually very small (15 to 37.5 kVA). The majority of the loads are usually lights and the lighter (the center tapped transformer) is larger to serve the lights and motors.

This makes the whole installation very inexpensive for us. :D
 
Re: Open Wye Open Delta Connection

Charlie, I see the economics of the 2-xfrmr setup.

Now what are the considerations in deciding on a wye-delta, delta-wye, wye-wye, or delta-delta connection?
 
Re: Open Wye Open Delta Connection

rattus
reply to
My point is the schematic of an open delta looks like that of an open wye...
An open delta requires three phase wires 120 degrees apart, whereas an open wye requires two phase wires 120 degrees apart and a neutral. They seem the same, but they aren't.

My suggestion is "If it makes you feel good, think that way" You are so close, and the difference is so little.

Secondly, why do you choose wye delta, delta wye, etc.?
The secondary voltage is usually decided by the custoner, 120/208 three phase or 240 three phase, etc. Sometimes the utility will say also. The primary connection is decided on at least 2 factors, what effect will the high side have on the utility system, and what is the cost of line to line rated transformers vs line to neutral.
For example a 12,470 delta to 277/480 volt transformer bank will tend to balance any imbalance in the 480 volt side. This is due to the delta on the primary, and a benefit to the utility. Another effect is ferroresonance in utility systems with most connections except grounded wye-grounded wye in underground distribution.
However as cost becomes more of a factor, you might not want to purchase transformers with a primary rating of 24,900 volts if you wanted to connect a delta at that higher voltage. You might tend towards wye connections as the system voltage increases.
Jim T
 
Re: Open Wye Open Delta Connection

Jim, we do not have any delta connected 15 kV class configurations. If we have a delta secondary, we will have a floating wye primary (we don't connect the center point of the wye to anything).

The voltage available to the customer is dictated by the serving electric utility most of the time. It is dependant upon the existing facilities, what the customer wants, what the customer's loads are, etc. We try to accommodate our customers but we have to be practical also.

Rattus, we will change our standards when the industry changes the standard 15 kV class transformers (I don't think it will happen). Now, the standard is for a wye connection and we will not buck the system. Trust me, the conversion would cost everyone more than you can imagine. The big thing is that we can take a phase wire and a neutral down a street and feed 1? transformers with just a single wire. The grounding on the secondary side also connects the other side of the primary. To disconnect a transformer, it takes only one cutout to be opened. To cap it all, the secondary neutral is the same wire as the primary neutral. :D
 
Re: Open Wye Open Delta Connection

Charlie, the distribution system you describe sounds exactly like the system in our neighborhood. I would not even think of changing a standard. Just the cost of the paperwork would be overwhelming.

I would presume also that less wire would be required for these remote 2-xfmr sites.

Thanks to all for the edification.
 
Re: Open Wye Open Delta Connection

It is less wire if you would have to pull in another primary phase wire. It is less one transformer in any case. They do not have to be in remote areas. :D
 
Re: Open Wye Open Delta Connection

A good part of our overhead primary distribution is open delta. our wastewater folks want 480/277 at a liftstation, it will cost them $8,000 for the thrird line, it all has to be rebuilt.

The wild leg services have a large and small transformer. The large is for the single phase loads, the small picks up the three phase. If the load picks up the poco can install a third transformer. You get single and three phase from one bank. It was very common years ago, out poco will no longer use this system

We had a outage at at liftstation, due to a storm, the POCO rolled the high leg and caused the motor coils not to pull in. I would never think to check for the high leg moving...
 
Re: Open Wye Open Delta Connection

Charlie

I worked for a company that had a 15 kv class system and a 25 kv class system. The 15 kv was 7970/13,800 and the 25 kv was 13,800/23,900. We could take the transformers from the 25 kv system and connect them delta in the 15 kv system because the primary voltage was exactly the same and they had two bushings.
In your case, an ungrounded wye-delta will act like a delta-delta in your system without the problems that a high side ground brings.
If you provide a service to a 120/208 volt customer, you sound like the high side is wye also. It must be grounded to avoid problems, and when it is , the zero sequence network primary to secondary is connected. You drop line to ground current magnitudes on the secondary and allow for harmonics and other disturbances to travel between the 208 side and the primary.
There is nothing wrong with that, however some utilities do choose to make some delta connections at 15 kv.
 
Re: Open Wye Open Delta Connection

Hi Rattus: hope the following helps clarify some of the Wye-Delta issues: In the WYE configuration, all of the start or finish of the coil windings are connected to form the neutral point for connection of the neutral conductor. In a Delta configuration, the start and finish of the coil windings are connected together. So the two configurations are different as far as polarity. Therefore, the resultant vector values are different.
 
Re: Open Wye Open Delta Connection

If you provide a service to a 120/208 volt customer, you sound like the high side is wye also.
Click to expand...
That is correct, we use a grounded wye primary and it is 13.2 kV. We pump it out of the substations at 13.8 kV and use 7.62 kV transformers so we can go further before needing voltage regulators. We have never even experimented with 23 kV or 34.5 kV for distribution. :D
 
Re: Open Wye Open Delta Connection

I was just going to ask if you still used wye primary with wye secondary instead of delta. At some point I heard of some problem with wye-wye transformers. Can you say what that is and why that isn't a problem for this case?

Also, what makes the delta-wye the most commonly used transformer when you like wye primaries?
 
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