Open delta?

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peter d

peter d

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New England
On my trip to California last summer, I saw a transformer setup serving a single load (a well pump in the desert) that had only 2 transformers on the pole, yet it was a 3-phase load (the service drop had 3 insulated wires plus bare neutral.)

I'm guessing this is an open delta, (which we don't use in this area AFAIK) but I'm a little fuzzy on some of the details.

Is this connected across only two phases on the high side?

How is the grounded conductor derived in this setup? (grounded conductor still needs to be brought to the service per NEC requirements for fault clearing, right?)
 
This might help

deltad2.gif


A wealth of info can be found here
 
Sparky's linked page has a major problem: every drawing below the Delta and Y are the same image.


Peter, the best way to picture an open Delta is to ignore the high leg, which we'll call the B phase, along with the smaller transformer that supplies it, for a moment.

The A and C phase, along with their center tap, are exactly, and I mean exactly, identical to the 120/240v service in your home. The primary is supplied by the single high-voltage wire and the system neutral.

Now, by bringing in a second tranformer, and one more primary conductor, and connecting one end of the secondary to, let's say, the A phase, and ignoring that the center-tap of A-C is grounded, you have 3-phase power.

The primaries are basically two thirds of a Y system, each connected to a phase wire and the neutral. The two secondaries are connected as two thirds of a Delta system. The "missing" third primary is not needed.

The beauty of the open Delta is, or I should say was, it allowed converting a single-phase system to 3-phase without requiring all three primary phases. Today, nobody specs open Delta in a new service.
 
LarryFine said:
Sparky's linked page has a major problem: every drawing below the Delta and Y are the same image.


Peter, the best way to picture an open Delta is to ignore the high leg, which we'll call the B phase, along with the smaller transformer that supplies it, for a moment.

The A and C phase, along with their center tap, are exactly, and I mean exactly, identical to the 120/240v service in your home. The primary is supplied by the single high-voltage wire and the system neutral.

Now, by bringing in a second tranformer, and one more primary conductor, and connecting one end of the secondary to, let's say, the A phase, and ignoring that the center-tap of A-C is grounded, you have 3-phase power.

The primaries are basically two thirds of a Y system, each connected to a phase wire and the neutral. The two secondaries are connected as two thirds of a Delta system. The "missing" third primary is not needed.

The beauty of the open Delta is, or I should say was, it allowed converting a single-phase system to 3-phase without requiring all three primary phases. Today, nobody specs open Delta in a new service.
Click to expand...
Any open delta I came across had 3 primary lines into 2 transformers,with 3 phase secondary out. Usually the reason for open delta is a failed third xfmr or a need to cut costs.Removing a transformer out of a 3 xfmr bank reduces available K.V.A. by 33%.
 
Worse than that:

Worse than that:

boboelectric said:
Any open delta I came across had 3 primary lines into 2 transformers,with 3 phase secondary out. Usually the reason for open delta is a failed third xfmr or a need to cut costs.Removing a transformer out of a 3 xfmr bank reduces available K.V.A. by 33%.
Click to expand...

Dropping one transformer forces the line current through the remaining transformers. With a balanced load, this increases the phase currents by a factor of 1.73.

1/1.73 = 0.577-->57.7%

To compensate, we must drop the max line current by 42.3%
 
An open delta system will deliver true 3 phase power on the secondary side. The primary side is where the real advantage can be.

To connect the primary of an open delta, you need 3 wires from any 3 phase system. Most distribution around northern Nevada is one of the following; 2400/4160 4 wire wye; 7200/12470 4 wire wye; 14400/24940 4 wire wye; and 34500 3 wire.

If the secondary is either a wye or a closed delta, and it's fed from one of the 4 wire primary systems, the primary is typically wye connected and all 4 wires must be run. This can get expensive if the distance is long. An open delta needs only 3 wires, typically 2 phases and the neutral. Considerable cost savings.
 
Thanks for the info. I did check the diagrams at ECN, etc, but I was still unsure about the primaries. I guess it would depend on how the utility is set up? (delta primaries, wye primaries)

It looks like the open delta is good for the situation I saw - a single, relatively small 3 phase load. It saves the poco a transformer and the associated loss from it.
 
peter d said:
Thanks for the info. I did check the diagrams at ECN, etc, but I was still unsure about the primaries. I guess it would depend on how the utility is set up? (delta primaries, wye primaries)

It looks like the open delta is good for the situation I saw - a single, relatively small 3 phase load. It saves the poco a transformer and the associated loss from it.
Click to expand...


Peter, just for fun facts, CL&P no longer offers this type of service... (but after re-reading your first post you already new that... :)
 
iwire said:
This might help

deltad2.gif


A wealth of info can be found here
Click to expand...

With the open Delta example in this schematic will there be a voltage present between the B and C phases. Can anyone give an explanation to accompany this schematic?
 
I'll take this one... :)

I'll take this one... :)

mull,
VBC = VBA + VAC
VBC = 240V @ 0 degrees + 240V @ 120 degrees = 240V @ 240 degrees
JM
 
mayanees said:
mull,
VBC = VBA + VAC
VBC = 240V @ 0 degrees + 240V @ 120 degrees = 240V @ 240 degrees
JM
Click to expand...

Jim, I believe that,

Vbc = 240 @ -120,

then,

Vbc = 240 @ -60

or,

Vcb = 240 @ +120

gotta get those arrows pointed in the right direction.
 
A bit of clarification:

A bit of clarification:

In any 3-ph system, the sum of the line to line voltage phasors is zero. In this case,

Vba + Vac + Vcb = 0

then,

Vcb = -Vac -Vba

or,

Vbc = Vac + Vba

These phasors are examples of "fixed" or "static" phasors. The phase angles are constants; they are not functions of time.

Yeah, I know we have been through this ad nauseum, but this topic will never go away.
 
peter d said:
It looks like the open delta is good for the situation I saw - a single, relatively small 3 phase load. It saves the poco a transformer and the associated loss from it.
Click to expand...
I've posted before that I see the open delta commonly used on duplex pumpstations where the load is primarily a pair of 3-phase motors, with minimal 120V control.
In DE, the local CO-OP has a rule of thumb where they will only supply up to a 20 HP motor on open delta, stating that beyond that the system is unstable and the summation phase may drop out.
JM
 
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