Delta connections on transformers

[woftam]

I am setting up from practicals for my electrical apprentice students at the trade level and wanted help with two issues.
1. when doing a open delta zero volt check on the delta secondary of a star/delta transformer, I don't see zero volts even though the delta connection is correct. I observe about 170 volts (the phase voltage of the delta sec = 138 volts). If i connect the same transformer (training aid, lab-volt) in delta/delta, i observe zero volts across the opened delta connection on the secondary. So, why don't we see zero volts, and is there a way of proving correct delta connection of a star/delta transformer?
2. Why don't we observe a circulating current in a delta connected primary, if one of the phases are reversed? I observe incorrect voltages in the star connected secondary if the primary delta has a reversed phase.
Regards, Mike

 

[waross]

I. Open delta:
There should be full voltage across the open delta.
The advantages of an open delta:
Two primary conductors and a neutral and two transformers may generate three phase power.
-That is often a large saving on long rural primary lines.
For small three phase loads in an area of mostly single phase transformers and services, a small three phase load may be accommodated by adding a small transformer in open delta with a larger, existing single phase transformer. The existing ground on the center tap of the 120/240 Volt transformer will safely ground the delta.
You may see a 5 KVA or a 10 KVA transformer alongside a 25 KVA, 50 KVA or 100 KVA 120/240 Volt transformer serving single phase loads.
Motors often perform better on 240 Volts than on 208 Volts.
The open delta has no circulating current nor back feed issues that plague grounded-wye/delta connections.
.
Consider three transformers connected in grounded-wye delta.
Turn the power off and remove one of the transformers.
-You will see full voltage across the open side.
The single phase capacity of an open delta connection is equal to the capacity of either of the remaining transformers, or the smallest remaining transformer if the transformers are unequal.
.
By the way, a reversed connection on an open delta just reverses the phase rotation. You should still see full voltage across the open delta.
.
Quandry;
Observing about 170 Volts across an 138 Volt supply with an open delta.
Harmonics, metering error or both.
Put a reasonable load on the circuit and the voltages should match quite well.
This was common with analogue meters.
The magnetizing currents of the unloaded transformers distort the magnetizing waveform.
The analogue meters actually measured the average voltage and scaled the reading to display RMS voltage.
With the distorted wave-form cause by the magnetizing current the scale factor was no longer correct.
.
Concerning circulating currents in a delta wye primary;
Reversing either the one primary winding connection or one secondary winding connection shifts one secondary phase 180 degrees.
There is nothing to cause a circulating current.
.
.
By the way, the open delta connection is fairly often used.
The grounded-wye/delta is a connection to avoid using in the field.
It has serious problems anywhere but a perfect world.
The field is not a perfect world.
I have seen this imperfection proven too many times with grounded-wye/delta connections.
The shortcomings and issues with this connection were understood and the connection fell out of favour in North America 60 or 70 years ago.
I spent about 15 years in a land where the Y/D connection was common.
I made a lot of money helping customers deal with some of the problems.
Good luck.
.
Extra work for extra credit:
Connect three transformers in grounded-wye/delta. Primary voltage 120/208 Volts. The secondary voltage is unimportant.
From the three primary transformer terminals, take three leads to a switch.
Connect the primary wye point to neutral/ground, and connect only two primary phases.
Now with only two primary phases energized, you will be able to measure full three phase voltage at the switch connected to the transformer terminals.
Connect a small three phase motor to the switch and prove that it starts and runs.

The next time ask me about the Winnipeg connection.

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

 

[stevenal]

Open delta (one single phase transformer removed)or broken delta (delta is open between two of the three single phase transformers)?

 

[waross]

Hi Steven.
Good point.
I ASSumed the open delta was the two transformer open delta.
I may have suffered the consequence of ASSumption.
But adding to the confusion are the voltages.
With a bad connection on a broken delta and a 138 Volt supply, you would expect to see 278 Volts, not 170 Volts.
138 Volts and 170 Volts may be explained by metering errors. I have seen this a couple of times in the field with unloaded transformer banks.
The text suggests to me that he has a three transformer broken delta.
The voltages suggest a two transformer open delta or V-V connection.

The first time I encountered this was with a wye/wye connection.
I had sent students to the lab to do a wye:wye connection and verify the voltages.
The phase to phase voltages did not show the root three relationship with the phase to neutral voltages.
We put a scope on the transformers and readily saw the distorted waveform caused by the magnetizing current that caused the meter error.
I occasionally gave the students lab projects with unexpected results as a teaching technique to get their attantion.
Fortunately they assumed that I had done it again and that this was a planned lesson.
I was able to make lemon-aid out of that lemon.
The second time was as the contractor's rep. at a pumping station start-up.
The voltage was checked before closing the main switch and the same error was shown.
I had to teach the same lesson to three engineers and a couple of linemen before they would let me close the main switch.
As soon as there was a load on the transformer bank, it swamped out the error and the voltages became what they should be.

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

 

[stevenal]

I believe you will see 3V0 across a broken delta secondary with a grounded wye primary. It's an indication the primary voltage is unbalanced.

 

[waross]

In a properly connected wye/delta bank, breaking the delta should show zero volts across the break.
This is a good connection check before closing the delta.
If one winding is reversed, you will read twice secondary voltage across the break.

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

 

[stevenal]

Properly connected with unbalanced primary will result in non-zero voltage across the broken delta. In the extreme example, the primary voltages are all of equal magnitude and phase (sort of like V0). The secondary winding is then connected in series so the three voltages add linearly, giving you 3*Vnominal. Prior to microprocessor relays, VTs connected in this manner were used to provide polarizing quantities. 170 on a 138V system sounds like a lot of V0, though. What's the primary source?

 

[woftam]

Just to clarify the original post, the setup is 3 single phase transformers. It is not a "open delta" setup of only 2 windings. I am trying to verify that the delta connected secondary is connected correctly by leaving the delta connection open ("broken" connection). And as stated, i correctly observe zero volts across this broken delta ('open delta zero volt test' we call it down under), but only when the transformers are connected in delta/delta. If i connect them in star/delta (wye/delta), i do not observe zero volts across the broken delta connection on the secondary winding (even though it is correctly connected in delta). The voltage i observe across the broken delta is higher than the phase/line voltage i observe when the broken connection is reconnected, as described in op. So that is the main issue, why don't i observe zero volts across the broken delta on the secondary wye/delta setup, but do observe zero volts across the broken delta secondary on a delta/delta setup?

The second question relates to the first. We do the "broken" delta zero volt check to prove correct delta connection before closing the broken connection on the secondary to ensure a circulating current doesn't cook the windings if we had a reversed phase in the delta secondary. But why don't we need to do it on the primary? I am happy (unless otherwise convinced) that we don't need to do such a check, and as described i observe incorrect voltages in the secondary of a delta/wye setup, if the delta has a reversed phase, and also observed there was no circulating current in the incorrectly connected delta primary. My instinct tells me that the reason why there is no danger of a circulating current in a incorrectly connected delta primary is because the primary is not the source, and they are just 3 inductors connected to the supply. But the secondary is the supply, and therefore if not balanced (from a reversed phase in the delta connections) then a circulating current is setup (phasors explain this situation well). I just want to know if my thinking here is right? regards from down under

 

[waross]

Wye:delta connection.
Each secondary winding reflects the voltage across the corresponding primary winding.
If one primary phase is 10% low, the corresponding secondary phase will be 10% low.
With the neutral connected on the primary, the phase angles are locked in.
With balanced secondary voltages, the secondary head-to-tail vector sketch will close the triangle.
If either the phase voltages or the phase angles are not equal, the vectors will not close.
If the delta is then closed, the voltage across the broken delta will drive a circulating current around the delta limkited by three times the impedance of the transformer bank.
This circulating current will try to increase the voltage of the low phase, and will back-feed substantially full voltage into an open primary phase.

An important question;
Is the primary wye neutral connected to the transformer bank?
Measure the single phase voltage across each primary phase, line to neutral and measure the voltage across each single phase secondary with the delta broken, if possible.

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

 

[woftam]

Thanks Bill, connecting the neutral to the wye fixed it!!! And now i think of it, i have told my students on occasions that the reason we have the multiple earth neutral system here in Australia is because the supply system needs to be grounded (star point connected to earth) to provide voltage stability.

 

[waross]

The wye:delta is a terrible connection for distribution.
With the wye point connected to the system neutral you have bad circulating currents.
With the wye point floating you have switching transient over-voltages.
I saw one wye:delta bank with four fused cut-outs.
When the bank was de-energized locally for maintenance, a fuse was installed in the neutral cutout before re-energizing the bank.
That avoided the switching transients.
When all three phases were energized, the neutral cut-out would be opened and the fuse taken away with the service truck.
That avoided circulating currents and back-feeds.

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