Open Wye - Open Delta Phasors

[rockman7892]

I'm a bit rusty on drawing phasor diagrams based on 3-phase transformer winding configurations. I understand the phase shift and relationships with phase shift related to delta vs wye configuration but somewhat loose sight of representation of such on phasor diagrams for different winding configurations.

can someone help with giving me a high level description of how to come up with the phasor diagram based on attached open wye-open delta transformer configuration or point me towards a good reference?

I understand that the primaries are in a wye configuration and this have the wye phasor diagram shown on top. From there however the secondary of xfmr is connected to phases 2 & 3 so i'm not quite seeing how the 2-3 phasor on the secondary open delta is derived. It would appear that it is 90 degrees out of phase with 2-3 reference on primary but i may be missing something here?

 

[stevenal]

X1 to X3 is in phase with H1 to H2, so the phasors are parallel. Once you connect 1X3 to 2X1 without disturbing the angles, you have your secondary phasor diagram complete.

 

[davidbeach]

If it’s a true open wye-open delta then you can start out by adding the third single phase transformer to make it a simple why-delta and work out the vectors as per normal. Then remove the “helper” transformer and its associated vectors.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[waross]

When I hear "Open Wye" I first think of what I have heard called the "Winnipeg Connection".
This is a circuit to produce three phase wye power from two phases and a neutral.

Both circuits are criticized for voltage drop problems.
Actually, the source of issues is the impedance of the primary side neutral.
Open delta works well on city grids, but not so well at the end of long rural lines.
The irony is that possibly the major use of an open delta is to save a conductor and insulators when feeding a three phase load a long way out on a circuit.
More damaging then voltage drop, is the phase angle errors caused by the neutral conductor voltage drop.
This is mostly corrected by the customer's moor at the expense of circulating currents and added motor heating caused by the circulating currents.
Moral. If you are faced with a rural open delta project a long way out on a rural line, remember to over-size the motor.
If that is not feasible, a small wye-delta transformer bank floated across the open delta will do much of the phase error correction and allow the motor to run cooler.


--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[wcaseyharman]

I helped troubleshoot one of those open delta banks on a rural circuit. Turns out it’s really important to balance the load on the primary, especially if you have open delta banks - the phase shift is a killer.

 

[waross]

Unbalanced loads and voltage regulators are problematic.
If the three regulators are not on the same tap, you get phase shifts, line to line as well as line to neutral.
Then run a motor and the back EMF, acting as an induction generator, draws unbalanced currents trying to correct the phase shifts.
Even without voltage regulators, unbalanced loads cause neutral currents which cause a voltage drop on the neutral which causes phase shifts.
With voltage regulators, you may have equal line to neutral voltages but unequal line to line voltages and line to line phase shifts.
Motors really don't like that.
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[wcaseyharman]

That’s pretty much what happened - when the weather got cold and the load went up. We (the utility) had to move loads around to balance the circuit in order for the customer to be able to run their pumps. LG voltages were fine, LL voltages were awful.

 

[waross]

A wye delta transformer bank floated across the line will go a long way towards correcting the issues.
The wye delta bank will cause a neutral current to flow in such a magnitude and at such an angle as to balance the line to line voltages and angles.


--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[jghrist]

A wye delta transformer bank floated across the line will go a long way towards correcting the issues.

A grounded wye - delta bank will also put a ground source on the line and may cause transformer fuse blowing or transformer damage during ground faults on the primary line. Open wye - delta banks provide a way to serve 120/240 V single phase loads along with a small 240 delta three phase load without having a ground source. Utilities (in North America) will generally prefer to furnish 120/208 V three phase services and use grd wye - grd wye banks, but some customers have 240 V three phase equipment that isn't suitable for 208 V.

 

[wcaseyharman]

My utility went through and removed YgndD distribution banks probably 20 or 30 years ago. There are now very few, the only ones I know are associated with distributed generators that are capable of islanding in order to provide a ground path for the microgrid. I believe the concerns were:
- YgndD banks will backfeed the primary if a there’s an SLG fault. This can cause a couple of things:
- desensitization of the upstream ground overcurrent protection due to the remote infeed. This could prevent the substation breaker from seeing faults at the end of the feeder, for example.
- If the upstream protection is not 3 pole clearing (for example, a fused lateral) and one phase operates for a fault, the YgndD bank can backfeed the fault, and due to the much higher impedance may not clear. This could potentially damage the transformer but more importantly could create a potentially lethal situation, possibly in public right of way.
Thus the utility I work for is pretty reluctant to install YgndD banks on distribution circuits.
Balancing the circuit also has other benefits besides correcting phase imbalance - reducing current in the neutral potentially reduces the neutral to earth voltage (important in rural areas with dairy farms) and slightly reduces losses among other benefits.
However, that being said, it’s not clear whether the suggestion was to put the grounding bank in the primary or secondary. If I recall correctly, the customer was fed from an open delta, not a Y, so I am curious if the YD bank would be helpful in this case. I would think not since the delta provides no ground path but I’ve been wrong many times before.

 

[waross]

I agree with your comments regarding the shortcomings of the wye delta bank on distribution circuits, wcaseyharman.
I can add to that.
I spent about 15 years in wye/delta land, helping customers cope with the issues.
The national power utility loved star delta.
Much line maintenance was done on Sunday on dead lines.
Typically, a line would be de-energized locally by pulling the fused cut-outs.
This was a refrigerator killer.
When only one primary phase is energized the backfeed is at about 50% of nominal voltage.
On a typical outage lasting several hours, almost all of the domestic refrigerators and freezers would be trying to start.
Two phases of the circuit would be hit with 50% voltage and would stall instead of starting.
Now the linemen would be hanging another cut-out fuse on the hot stick and replacing it.
This is not a fast operation.
Finally the second phase would be energized, but the entire circuit would be backfed by any wye delta banks.
Typically the transformers backfeeding would be overloaded and the voltage would be low.
Now the refrigeration motors have been sitting stalled and the voltage steps up, but is several percent low.
The motors can't start.
The thermal protection operates, the head pressure dissipates and when the thermal Klixon cools, the motor starts and runs normally.
BUT
It was common for one or two refrigerators to fail.
Now for the local colour.
As I mentioned, the backfeeding transformer would be overloaded.
Sooner or later, but mostly sooner, the primary fuse would blow and the bank would revert to open delta.
Problem solved until the next election.
The next election? WTF?
Well if the government changed, all of the top people at the National Utility would be replaced with political favourites.
The new appointees would notice one fuse hanging down at each transformer bank all over the country and issue directions for them to be reinstalled.
For several months there would be a rash of refrigerator burn-outs until the field crews were able to train their new bosses.
Of course this begs the question:
"Why didn't the transformers fail on overload on open delta?"
More local colour.
It was a time honoured tradition in the country for under the counter kickbacks to any electrician or contractor buying a transformer.
Virtually all of the three phase banks were sized for at least 200% of the load.
Anecdote alert.
I left that country, but still visited from time to time.
Whenever I was back in the country, the small independent island utility that I had done much consulting for would fly me out to the island, all expenses covered and have me "walk" the system.
One one walk-around I spotted a new wye/delta bank.
I asked the manager about it.
"That belongs to the National Telephone Company.
They demanded a wye/delta and we are not big enough to buck the National Government."
"Do we want to start burning out refrigerators again?"
"Lord NO."
"Let me see what I can do."
For those not familiar with line hardware, A "Cut-out fuse holder" has a toggle that is held in position by the fuse link.
When the fuse blows, the toggle collapses and the fuse holder swings down and hangs from the cut-out.
I checked the customer's load and determined that an open delta bank would carry the load.
I had the crew take a fuse holder, without a fuse link, and tack weld then toggle in position.
When the dummy was installed, the bank looked like a full delta but was actually an open delta.
I mentioned oversized transformers;
The customer's load, single phase and three phase combined was 17 KVA.
The transformer bank capacity was 150 KVA.


--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[waross]

There are now very few, the only ones I know are associated with distributed generators that are capable of islanding in order to provide a ground path for the microgrid.

When a generator is looking into the delta of a wye/delta bank, single phase loads and the unbalanced portion of three phase loads are better distributed on the generator. The in-phase winding takes one half of the load while the other two phases between them supply the other half of the load.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[wcaseyharman]

I for one very much enjoy these anecdotes, thank you, very interesting and instructive.

 

[waross]

Open Wye or "Winnipeg Connection".

Imagine a horizontal phasor, labelled X1, X2, X3. A center tapped 120-240 Volt winding on "A" phase of the primary supply.
Now imagine two 120 Volt transformers on phase "B" of the primary supply.
The first 120 Volt transformer is connected to the X2 center tap of the 120-240 Volt transformer so that the phasor is pointing up and to the right..
The second 120 Volt transformer is connected to the X3 terminal of the 120-240 Volt transformer so that the phasor is pointing down and to the left.
You now have three phase, 120-208 Volts three phase from two primary phases.
The capacity is 3/4 of the sum of the transformers.
eg: Use a 50 KVA and two 25 KVA transformers for a capacity of 75 KVA from a sum of 100 KVA.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!