Open delta vs Y question

[Electric-Light]

Does an open delta have the same voltage between the legs where the transformer is missing?

On paper, it looks the same as 208Y/120 that is fed with two legs and a neutral.

It was explained that open delta allows continued service with one of the three transformer out of service.

Assuming
A
CB
order
In a closed delta, A-B, B-C and C-A are 240v.
If secondary on A-B burns out, what's the voltage between A-B?
Would they add up to 416v as if its two of the three windings on 416Y/240 system?

How is it any different from this with s3 not pulled in?( As in individual tenant suite in an office building)

 

[Hv&Lv]

Does an open delta have the same voltage between the legs where the transformer is missing?

On paper, it looks the same as 208Y/120 that is fed with two legs and a neutral.

It was explained that open delta allows continued service with one of the three transformer out of service.

Assuming
A
CB
order
In a closed delta, A-B, B-C and C-A are 240v.
If secondary on A-B burns out, what's the voltage between A-B?
Would they add up to 416v as if its two of the three windings on 416Y/240 system?

How is it any different from this with s3 not pulled in?( As in individual tenant suite in an office building)

Those pictures don't do the relationship between these banks much justice.

In the first bank, the transformers secondary inductors are connected in series on one pot, and the second pot is center tapped. We configure this bank from the outside. The length of the inductors should be 240. Draw a line straight down from the top of the triangle to the center of the horizontal inductor, and measure it. in a perfect world, it should be 207.84.

In the wye bank, the XF inductors are connected in parallel. If you look at a 208Y120 bank on a pole, you will see the leads shifted to to one side with one secondary bus not connected to anything. We actually go inside the tank to configure the pot for this bank to be built. The length of the inductors should be 120. Draw a triangle with 120 degrees and sides 120, the hyp should be 207.84

In the open delta bank, if the polarity is connected wrong, and the resulting angle is 120 instead of 60, the voltage will be 317.49 from top of triangle to center of horizontal inductor. It will be 415.69 across the missing side.


In your bottom bank, If the inductors were 240 long, rather than 120, the resulting hyp would be 415.69


Yes?

 

[fmtjfw]

Does an open delta have the same voltage between the legs where the transformer is missing?

On paper, it looks the same as 208Y/120 that is fed with two legs and a neutral.

It was explained that open delta allows continued service with one of the three transformer out of service.

Assuming
A
CB
order
In a closed delta, A-B, B-C and C-A are 240v.
If secondary on A-B burns out, what's the voltage between A-B?
Would they add up to 416v as if its two of the three windings on 416Y/240 system?

How is it any different from this with s3 not pulled in?( As in individual tenant suite in an office building)

Delta line-to-line voltages are typically 240V or 480V.

The voltage across the lines in delta with the missing transformer is typically poorly regulated and can vary a good bit.

A variation on an open delta is a center tap on one of the secondaries that is a grounded conductor and forms the neutral for a 120/240V single phase circuit. This configuration is typically used for a load that is mostly single phase, with just a small three phase load.

WYE voltages are typically 120, 277, and 347V line to ground (208, 480, and 600V) line-to-line.

This way most WYE and DELTA configurations can be supplied by standard transformers with 120/240/480 secondaries.

 

[Electric-Light]

Let's assuem top is 240 straight
bottom is 415Y/240

Just used those pictures as they were available.

So, in delta they're like

1----2--3-----4
1-4=208v

but in wye
they're like

1----2--4-----3---1


1-2, 4,3 = 240v
1-3 = 208v

do I have this right?

In an office space fed from 208/120 single ph... could one use
two 120:240 transformers to generate open delta 230 to run a 3 phase motor from?

 

[gar]

121117-2355 EST

Electric-Light:

Yes.

Our open delta to the shop is fed from a Y source. Just that it is step down and not up.

.

 

[Smart $]

Let's assuem top is 240 straight
bottom is 415Y/240

Just used those pictures as they were available.

So, in delta they're like

1----2--3-----4
1-4=208v

but in wye
they're like

1----2--4-----3---1


1-2, 4,3 = 240v
1-3 = 208v

do I have this right?

Can't make sense of your coding: What is 1, 2, 3, 4, and what do the number of dashes mean?

In an office space fed from 208/120 single ph... could one use
two 120:240 transformers to generate open delta 230 to run a 3 phase motor from?

Yes. Let's say the office is fed L1(A) at 0? (relative reference) and L2(B) at -120?. Your 120-240 stepup transformers have X1 and X2 primary connections. Connect L1 to 1X1 and N to 1X2, then N to 2X1 and L2 to 2X2. Note hot and neutral primary connections are reversed. This will give your secondaries a 60? phase displacement rather than 120? if you connect opposing terminals (e.g. 1H2 to 2H1). Thus an open-delta output.

 

[mivey]

The voltage across the lines in delta with the missing transformer is typically poorly regulated

Quantify poorly. How about comparing the percent regulation vs. a closed bank, and considering that the open bank will normally be oversized?

Just for fun, use a 45 kVA load and compare the voltage regulation of a 45 kVA closed bank (three 15 kVA pots) with an open delta with two 37.5 kVA pots. FWIW, I would probably install two 50 kVA pots if it was an improperly sized motor (i.e. sensitive to voltage unbalance) on a V-phase line so you might look at that case as well.

and can vary a good bit.

I guess that depends on what you mean by a good bit. What percentage to you consider to be a good bit?

To the best of my recollection, I can keep the voltage unbalance due to the transformer to less than 2% without too much effort. I have a chart around here somewhere but could not put my hands on it just now. Granted, there are loads that have the motors maxed out and are sensitive to voltage unbalance.

The key with the open delta is to upsize the pots.

 

[fmtjfw]

Quantify poorly. How about comparing the percent regulation vs. a closed bank, and considering that the open bank will normally be oversized?

Just for fun, use a 45 kVA load and compare the voltage regulation of a 45 kVA closed bank (three 15 kVA pots) with an open delta with two 37.5 kVA pots. FWIW, I would probably install two 50 kVA pots if it was an improperly sized motor (i.e. sensitive to voltage unbalance) on a V-phase line so you might look at that case as well.

I guess that depends on what you mean by a good bit. What percentage to you consider to be a good bit?

To the best of my recollection, I can keep the voltage unbalance due to the transformer to less than 2% without too much effort. I have a chart around here somewhere but could not put my hands on it just now. Granted, there are loads that have the motors maxed out and are sensitive to voltage unbalance.

The key with the open delta is to upsize the pots.

It's been a while, but as I recall the situation was pretty strange:

Open Delta with 50KVA center tapped secondary 30ft away and 10KVA 3 spans away, secondary probably only #4.
Voltage across open (missing transformer) was pretty off maybe 10--15%. And the voltage across the 10KVA secondary was low as well.

The 50KVA was on a street with only single phase primary. the 10KVA was up the street and down a block on the 3 phase primary. 10KVA served us and a church on the 3 phase street.

We talked to the POCO about putting the two transformers on the same pole, but they wanted too much money to run the second phase primary and add a 10KVA for us.

We just changed out pumps to 1 phase motors, it was cheaper. (The 3 phase motors ran hot and failed.)

 

[richxtlc]

Does an open delta have the same voltage between the legs where the transformer is missing?

On paper, it looks the same as 208Y/120 that is fed with two legs and a neutral.

It was explained that open delta allows continued service with one of the three transformer out of service.

Assuming
A
CB
order
In a closed delta, A-B, B-C and C-A are 240v.
If secondary on A-B burns out, what's the voltage between A-B?
Would they add up to 416v as if its two of the three windings on 416Y/240 system?

How is it any different from this with s3 not pulled in?( As in individual tenant suite in an office building)

In an open-delta circuit, nothing changes between the primary and secondary voltages. The only difference is the the kVA decreases to 57.7%. There is no phase-to-neutral voltages, all are phase-to-phase.

 

[mivey]

It's been a while, but as I recall the situation was pretty strange:

...and 10KVA 3 spans away, secondary probably only #4.
... the 10KVA was up the street and down a block on the 3 phase primary.

That is strange. No wonder there were problems.

 

[Electric-Light]

That is strange. No wonder there were problems.

How does the second one from bottom work here/ wouldn't this be single phase primary?

http://insayne_kokane.tripod.com/transformers.shtml

What's the differece between

with the center point connected to neutral vs the third pase?

 

[Hv&Lv]

How does the second one from bottom work here/ wouldn't this be single phase primary?

http://insayne_kokane.tripod.com/transformers.shtml

What's the differece between

with the center point connected to neutral vs the third pase?

I looked at the second one from the bottom, and I believe it may be drawn wrong. Still trying to wrap my head around why the green line is installed on the secondary side. I can't see how it would work the way it is drawn.

For the one in the dstar site, it is like this one:http://www.powertransformerdesign.net/2012/04/wye-delta-open-transformer-banking.html

 

[richxtlc]

I looked at that same one and if you look at the green wire, it shorts A-phase and C-phase together. In addition, the primarys are connected in series, not in delta. For it to be an open-delta, the common between T1 and T2 need to be connect to C-phase as drawn.
The proper phasing would be T1 to A-B phases, T1 to B-C phases for a correct open-delta. The same would apply to the secondary.

 

[Hv&Lv]

I looked at that same one and if you look at the green wire, it shorts A-phase and C-phase together. In addition, the primarys are connected in series, not in delta. For it to be an open-delta, the common between T1 and T2 need to be connect to C-phase as drawn.
The proper phasing would be T1 to A-B phases, T1 to B-C phases for a correct open-delta. The same would apply to the secondary.

The bank is little more than single phase connected line to line as the OP suggested. Without the center point grounded, one could only guess what the voltages would be.

An open wye-open delta doesn't need all three hot primary phase connections. Connect any phase to one side of the pot of XF1, ground the other bushing(if a double bushing pot). Pick any other phase for the XF2. again as in the first XF,connect it and ground the other bushing. Only two phases needed. Just make sure polarity matches to get correct voltages.

 

[richxtlc]

The bank is little more than single phase connected line to line as the OP suggested. Without the center point grounded, one could only guess what the voltages would be.

An open wye-open delta doesn't need all three hot primary phase connections. Connect any phase to one side of the pot of XF1, ground the other bushing(if a double bushing pot). Pick any other phase for the XF2. again as in the first XF,connect it and ground the other bushing. Only two phases needed. Just make sure polarity matches to get correct voltages.

The diagram that I was looking at was the Delta- Open-Delta, second from the bottom.

 

[mivey]

How does the second one from bottom work here/


http://insayne_kokane.tripod.com/transformers.shtml

The bottom two won't work. The drawings are incorrect.

wouldn't this be single phase primary?

yes

What's the differece between

with the center point connected to neutral vs the third pase?

Using the phase connection is when using a delta configuration on the primary . Using the neutral is for a wye configuration on the primary. Evidently, the drawings were supposed to show open-delta -> open-delta and open-wye -> open-delta but are completely screwed up.

 

[Electric-Light]

So, according to this, you can make the third phase from a two phase source?

By two phase, I mean that existence of phases shifted, as opposed to "two phase system" as defined in the industry.


So, an alternator with 0deg and 120deg voltages, or inverter which produces the same. If they're fed into the transformer like above, it will create a true 3 phase with 0,120 and 240degs of shifts?

 

[mivey]

So, according to this, you can make the third phase from a two phase source?

By two phase, I mean that existence of phases shifted, as opposed to "two phase system" as defined in the industry.


So, an alternator with 0deg and 120deg voltages, or inverter which produces the same. If they're fed into the transformer like above, it will create a true 3 phase with 0,120 and 240degs of shifts?

Correct

 

[Smart $]

...it will create a true 3 phase with 0,120 and 240degs of shifts?

Correct(#2).

This is not hard to realize. If you plot out the waveform of each secondary, using the common-connection terminal as the reference (e.g. the negative lead of your meter), the waveforms represent the across-the-winding voltages. Now take the difference between these two waveforms (instantaneous values) and plot it out. This represents putting your meter across the "open" terminals of the two secondary windings. You will get a third time-shifted waveform with a 120? and 240? shift to the original two.