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Banking Single Phase Transformers

Banking Single Phase Transformers

Banking Single Phase Transformers

(OP)
Can you bank three single phase transformers in a delta to grounded wye with one transformer having a different impedance?

RE: Banking Single Phase Transformers

I would think so, especially if they are not fully loaded. How much different impedance? I am not sure what would exactly happen though. Haven't ran into one yet. As long as voltage and current remains within the rating, it should be OK.
 

Rafiq Bulsara
http://www.srengineersct.com

RE: Banking Single Phase Transformers

It is possible, but you will get circulating currents.  From my slightly sketchy memory, I believe that we would limit the difference between transformers to no more than 10% of the impedance value.  You wouldn't want to try this on a heavily loaded transformer bank that is already running hot.

RE: Banking Single Phase Transformers

A four wire wye:delta will often have serious circulating currents.
A delta:wye is basically three single phase circuits. No circulating currents.
For unity power factor loads compare the regulation numbers rather than impedance. As the load changes you may have slight phase angle errors. Any motor loads will sacrifice some of their thermal capacity in an attempt to balance any unbalanced voltages and to correct phase angle errors.
It depends, a small impedance difference may not cause issues but an impedance difference large enough to cause a few percent voltage differences will cause hot running motors as the motors try to correct the errors.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Banking Single Phase Transformers

"A delta:wye is basically three single phase circuits. No circulating currents".

Did you mean to say wye:wye? The OP's delta:wye will certainly have any secondary zero sequence current transformed to the primary where at will be confined to circulate around the delta. Not a problem, as long as current is within the individual transformer ratings. The lower impedance unit(s) will hog the load, so this bank will not be able to run to a capacity based on the sum of the individual units.

RE: Banking Single Phase Transformers

You certainly can bank 3 single phase transformers in a delta grounded-wye connection.  We did so with a GSU some years back when we had a failure of one unit and the nearest available unit had a different impedance.

You can model this situation using symmetrical components.  Check out open conductor faults with impedance.

You'll probably want to model this so you can see how much negative sequence current you get and to make sure the settings of the negative sequence relays permit this.

RE: Banking Single Phase Transformers

A single phase load or fault will appear on the primary as a line to line load or fault.
I may have misspoke as part of the harmonic currents may circulate in the delta but that is the nature of harmonics. It doesn't matter if the harmonics are present on the line, or generated by your load. If harmonics are present, the mismatched impedance will not change that or in itself cause harmonics. The impedance mismatch may have a slight effect on the amplitude of the harmonic current.  

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Banking Single Phase Transformers

Waross,

Harmonics are not needed to have circulating zero sequence  primary delta current, just unbalanced secondary fundamental loading will work just fine. Yes they do not reach the primary conductors outside the delta where it looks like line to line current. Like Charlie they doomed to ride forever 'neath the streets of Boston within the delta.

'later

RE: Banking Single Phase Transformers

I am fairly familiar with the circulating currents that occur in four wire wye:delta banks. I am aware of the phantom delta effect found with three legged cores, but this is three single phase transformers.
If I hook a transformer line to line on a delta system I can draw single phase current from the secondary. I am at a loss as to how doing this three times with only one common point on the secondary windings will cause circulating current.
I hope that I am not about to learn something new. (Yes it happens and it's painful).
 

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Banking Single Phase Transformers

Assuming three transformers connected delta-wye, with the wye grounded.

If there is a zero-sequence component to the current on the wye side, there will be a corresponding zero-sequence current circulating in the delta of the delta side.  It can't leave the delta, so it never appears at the terminals, but it can certainly be measured by CTs in the delta.  This is the same for three singles as it is for a single 3-phase transformer.

Put the three singles in a black box, put a comparably rated 3-phase in another black box.  Three wires into the delta side, four wires out of the wye side in both cases, with the wye-point wire grounded in both cases.  You'd be hard pressed to determine which black box contains which type of transformer.  If the 3-phase unit were of shell construction it could be impossible.  If the 3-phase unit were of core construction there will be a difference between Z1 and Z0 for the 3-phase and not of the three singles.

RE: Banking Single Phase Transformers

It is really interesting to think over. What is the effect on no-load and under loaded condition with delta/grounded whye with different impedance in phases.In india we had an experience with a 600 MVA auto bank with stabilising tertiary.When one of the single phase units was replaced with a unit with different tertiary impedance,but with  the same HV-LV impedance,heavy circulating current was noticed in tertiary.(HV-try in one unit 60% and two units 45 %)

RE: Banking Single Phase Transformers

Hi fellows, help me here, please. No sarcasm intended, i respect both Steven and David.
I am visualizing a delta distribution system. The system carries the secondary neutral throughout the system.
On Maple street, there is a transformer connected from A phase to B phase. The secondary has the neutral point grounded. There is a line to neutral fault on the secondary. There is a corresponding current in the primary A and B lines.
If this is fed from a delta winding back at the sub, the load current will divide about 50/50 between A-B and A-C-B. This is not a circulating current, it may be measured on the primaries of the three windings.
But no circulating current in the single phase primary.
Now if we connect a similar transformer over on Bay Street from line B to line C and on Green Avenue we connect another similar transformer from C to A.
Now, if we have a line to neutral fault on Maple Street will we get a circulating current between Maple Street and Green Avenue by way of Bay Street?
This is the same circuit in that three transformers with a common connection on their secondaries are connected to the three phases of a delta supply system.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Banking Single Phase Transformers

Probably.  Would also depend on how those separated transformers are connected.  If the A-B is always H1 on A and H0 on B, while the B-C is H1 on B and H0 on C and likewise H1 on C and H0 on A you will get that circulating current.

The source, delta or wye, doesn't matter, wants to supply the fault current equally from all three phases.  The Maple St transformer has current flowing from A-B.  Some of the ground return will arrive at the X0 of Bay St, and current will flow from B to C.  Some of the ground return will also arrive at Green Ave and primary current will flow C to A.

Some circulating current.  One limiting case is that there is no impedance between the three X0 terminals.  The other limiting case would be infinite impedance between the X0 terminals.

With no impedance we have the case described in my previous post.  In the infinite impedance case there would be no circulating current as only one X0 would be participating in the fault.  Any real case will be somewhere in between and the amount of circulating current will vary.

RE: Banking Single Phase Transformers

Waross,

The situation you describe is somewhat different from the OP's, since your single phase secondaries are likely grounded mid-winding. Offhand I don't know the answer. Never analyzed a six phase system. To make it simple, let's ground each winding end so that the three secondaries form a three phase wye. Yes, circulating current will flow in the primary wires that connect the three streets.

For a graphical approach, draw your delta wye, show 1pu current leaving each wye leg in phase to represent the zero sequence currents. On the delta side you will have 1pu in each winding, all in the same direction. Summing the currents to zero at each delta node shows that no contribution from the primary lines are needed. If the nodes are remote from any of the windings, there is little difference.
 

RE: Banking Single Phase Transformers

Let's simplify things a bit.  Three transformers, one connected from A-B, one from B-C, and one from C-A.  Single phase secondaries with one side connected to a grounded neutral, common to all three transformers.  If these transformers were all in one place, they would be a delta-grd Y bank.  If they were in different places with negligible impedance lines, the situation would be essentially the same.

Say there is only load on one of the transformers.  There is clearly only current in one of the primary windings, right?  What is with this circulating current???  It is a mathematical construct.  In two of the transformers, the zero-sequence current is completely balanced by positive and negative sequence current.  The sum of the three sequence currents equals zero.
 

RE: Banking Single Phase Transformers

I think stevenal may have hit the nail on the head.  If Bill's three transformers are all center-tapped on the low side with the center tap grounded, then most of the discussion wouldn't apply.

RE: Banking Single Phase Transformers

David, ignore one single phase winding on each secondary. Then we have a three phase wye. Now consider a line to ground short on one transformer.
The primary of this transformer is a single phase load on the delta supply. On the one hand it will work just fine. Other single phase loads will work fine on the other phases. If we add transformers on the other phases, we expect to see any secondary current reflected accurately in the primary. I have never heard of or seen primary currents that were not in reasonable proportion to the secondary current that were attributed to circulating currents. That would be the case if there were any transformers anywhere in the system on the other phases.

Quote:

For a graphical approach, draw your delta wye, show 1pu current leaving each wye leg in phase to represent the zero sequence currents. On the delta side you will have 1pu in each winding, all in the same direction. Summing the currents to zero at each delta node shows that no contribution from the primary lines are needed. If the nodes are remote from any of the windings, there is little difference.
Steven, I like and respect you. But, respectfully I have to throw the flag on this one.
A transformer bank with full load on the secondaries and no contribution from the primary lines?? Is this patentable?
I was starting to doubt myself, but now, considering the high caliber of the other parties to this discussion, I suspect a massive misunderstanding of some sort.
 

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Banking Single Phase Transformers

Bill, I think its a matter of Apples, Oranges, and Kumquats.  All of my discussion dealt with two winding transformers where the there is only a connection at each end of each winding.  I've really no idea what happens when you ground a center tap and then fault from one end of a winding to the middle of the same winding.  Without doing a whole lot more work than I feel like at the moment, I have no idea even where to start to analyze the situation you presented 21 Jan 11 0:19 if you were in fact talking about transformers with center-tapped low sides.

RE: Banking Single Phase Transformers

Quote:

Steven, I like and respect you. But, respectfully I have to throw the flag on this one.A transformer bank with full load on the secondaries and no contribution from the primary lines?? Is this patentable?I was starting to doubt myself, but now, considering the high caliber of the other parties to this discussion, I suspect a massive misunderstanding of some sort.
You're being too harsh on Steven.  He was discussing only the zero-sequence currents.  The primary lines do not contribute zero-sequence currents.  As I noted in my previous post, you can have zero-sequence currents in a winding while the total current in the winding is zero.  Similarly, you can have current in the line without zero-sequence current in the line.

The problem with analyzing three separate transformers connected phase-to-phase in separate places with symmetrical components is that you are using a mathematical device applicable to three-phase systems, not three separate single-phase systems.

 

RE: Banking Single Phase Transformers

Well either ignore one winding on each transformer or connect it in parallel with the other winding as a typical 120:208 Volt bank of single transformers.

Let's look at it another way.
Connect a transformer A phase to B phase.
Connect a transformer B phase to C phase.
Connect a transformer C phase to A phase.
Leave the secondaries disconnected and ungrounded. What would cause a circulating current in the primaries?
Although the magnetizing currents may form a delta vector diagram and appear to sum to zero, the magnetizing currents are still fed from the primary lines.
Any load on any transformer will have a corresponding primary current. I don't see any reason for any circulating current at the fundamental frequency. I can't see how a load on one transformer secondary will have any effect on the other transformer primaries.
Now, any connection of one end of any winding will not cause any current to flow in that connection. The connection may be to another winding, to ground, or to both, it will not by itself cause any change in winding currents or voltages as it is open circuited at the other end of each winding.

I can see the phantom delta in a three legged core of a three phase transformer causing circulating currents and I have many times seen a four wire wye:delta bank develop circulating currents to the point of self destruction.
In a delta wye with three independant transformers there is no need for the voltage delta to close on itself. The delta may have phase angle errors and dissimilar phase voltages and it will have no effect on a delta primary winding unless there is also a delta secondary winding.
The OP asked about Delta:Wye, not Wye:Delta which does have serious circulating current issues.
As a comment regarding four wire Wye:Delta transformer banks that were common in the earlier part of the last century when utilities were upgrading 2400V systems to 4160V systems;
It was common practice to reconnect the 2400V transformers from line to line to line to ground/neutral. Most industrial plants were using delta services.
This caused system wide circulating currents to the point that a hard ground fault on the secondary of a large transformer bank would cause widespread fuse blowing throughout the system. Wye:Delta, not Delta:Wye.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Banking Single Phase Transformers

Ignore magnetizing currents in the following.

In a delta-wye transformer, or in a delta-wye bank of single-phase transformers, with unbalanced loading of the secondary, there will be zero-sequence currents in the secondary windings.  The zero-sequence currents in the secondary also result in zero-sequence currents in the primary windings.  Because the zero-sequence currents are equal in each phase (both magnitude and phase angle, by definition of the symmetrical components), they will sum to zero at the corners of the delta primary and there will be no zero-sequence current in the lines.

Just because there is zero-sequence current in a primary winding, this does not mean that there is any total current in the primary winding.  If there is no current in a secondary phase-neutral winding, there will be no current in the corresponding primary phase-phase winding.  There will still be zero-sequence current flowing in the primary winding.  The zero-sequence current will be offset by positive- and negative-sequence current.

Say you have a delta-wye transformer with no connection at all to one end of ØB and ØC of the secondary windings.  The other ends are connected to ground.  On the third phase there is a load connected to ground with 300A.  There will be 100A of zero-sequence current flowing in the ØB and ØC secondary windings, even though there is no connection to them.

Remember, the sequence currents are only a mathematical construct to enable using three sets of equal three-phase currents in situations where the phase currents are not balanced.  Each of the sequence currents has an equal current magnitude in each phase, by definition.

   

RE: Banking Single Phase Transformers

Thank you for your patience with me, friends.
Can you take a look at your signs in the last post please?
With the ends of the windings open, there will be no current flowing in the windings. If a proven mathematical construct indicates a current where there is none, possibly the sign is reversed so the current in phase B and phase C equal and cancel in the delta. Possibly your currents cancel and sum to zero so that there is no circulating current.
I have been responsible for systems with a combination of wye:delta and delta:wye transformers banks. We had many issues and failures with wye:delta banks but we never had any indication of circulating currents on our delta:wye banks. Due to a mixed load of commercial, industrial and residential, and a large part of our system running on two phases due to a failed undersea cable, we were never able to keep our system balanced over the course of the day.
We had transformer burnouts but mostly due to failed ventilation and/or overloads. Never an indication of circulating currents.
I regret that I am not able to make direct measurements at this time. The next time that I get back to the island I will do so.
PS; If the secondary of the wye:delta bank has any regenerative loads such as three phase induction motors, then I would expect to see some circulating currents in the delta.
Thanks again. I will be doing some further research on my own. I am becoming uncomfortable with this discussion. It's not good for the forum and I may find myself to be embarrassingly in the wrong.
Thanks again.
Yours Bill

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Banking Single Phase Transformers

100A of zero sequence current in one direction.  A total of 100A of positive and negative sequence current in the other direction.  Zero total current.  Mathematically very useful.

RE: Banking Single Phase Transformers

"A transformer bank with full load on the secondaries and no contribution from the primary lines?? Is this patentable?"

At last the solution to unlimited energy. Unfortunately zero sequence secondary energy comes from positive sequence primary energy, and conservation of energy still rules.

Transformer heating and possible burnout would be the result of total phase current exceeding the transformer capacity, and not the zero sequence component alone.

In Jghrist's example, I agree the sequence current in the unloaded phases sums to zero.

 

RE: Banking Single Phase Transformers

Jghrist,

I've been thinking about your "mathematical construct" comment, and I agree. But isn't some sort of mathematical construct needed to define delta circulating current? Assuming the three winding currents are different and non-zero, we would need some sort of manipulation to find the common component that makes up the circulating current. I had thought the zero sequence component would be the correct mathematical construct to use. How else should we define circulating current?  

RE: Banking Single Phase Transformers

No answer from Jghrist. Anyone else? How shall we define circulating delta current if we reject the use of the zero sequence component?

RE: Banking Single Phase Transformers

May I make a suggestion.
In the transformer feeding the bank in question the following condition may prevail.
First, consider that one transformer is missing and the three phase line is fed open delta. B-C is open and there is a single phase load on B-C. This load is fed from the open delta. The current is in phase with B-C but is leading in one of the open delta transformers and lagging in the other.
Now if the missing transformer is installed the cirrent will split, 50% will be supplied by what was the open delta and 50% will be supplied by the B-C phase transformer. This is a current split and not a circulating current. (Check the signs), but the zero sequence component will show up in three phases.
I suggest that when a phase to neutral load is applied to one phase of a delta wye transformer bank, there is no resulting current in two phases, but the zero sequence current will show up in three phases of a delta transformer feeding the system. This zero sequence current will be a current split rather than  a circulating current.
This makes me happy and follows field experience. Does it satisfy the zero sequence constructions?
Yours
Bill

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Banking Single Phase Transformers

Consider this scenario: The grounded wye side is networked with another grounded wye transformer winding located at the other end of a line. For simplicity, there is no load on or through this network. The joining line is mutually coupled with another unrelated line that experiences a line to ground fault. As in Jghrists example, I0 will flow in each phase while 3I0 will return through neutral/ground. On the delta side, I0 will circulate around the delta connected windings. Unlike Jghrist's example, there is no positive or negative sequence current to sum to the zero sequence cuurent to find phase currents. Ia = Ib = Ic = I0. Can we agree that in this case we have a delta circulating current equal to I0?

Open the delta, and this circulating current is blocked. I'm not finding a difference between the circulating current here and the "split current" above.  

RE: Banking Single Phase Transformers

We're getting pretty far from the OP situation.  In the OP, there are not two banks in parallel.  The concern is one three-phase bank with different impedances.  You don't have circulating currents in the sense of parallel transformers with different ratios where you can have current circulating between the two transformers.

If you have unbalanced loads on a delta-wye transformer, then you have current in each primary winding proportional to the current in the secondary winding.  Considering the currents to be "circulating" is unnecessarily confusing.  Whatever winding current that doesn't add up to zero at the delta corner goes out the line as line current.

If you want to analyze the circuit with equal three-phase quantities, you can do it by breaking the current into three separate components (symmetrical components) that each have equal magnitudes in each phase but add together to get the unbalanced total current.  One of these components, the zero-sequence component has equal phase angles as well as equal magnitudes for the three phases.  Each winding has the same zero-sequence current, so they add to zero at the corners of the delta.  This is how they get to be considered to be "circulating".  But this is only one of the three symmetrical components of the current.

You don't need to use symmetrical components to analyze the winding currents.  It is a method that can make analysis easier.  You don't need to consider any current "circulating" around the windings.  Just because the winding currents are not equal does not mean that you have to consider part of the current circulating around the windings.
 

RE: Banking Single Phase Transformers

Thank you jqrist. I think that that is what I have been trying to say.
One possible proof may be to "break" the delta. If there is a small voltage across the broken delta, (open delta in IEC land) then there will be circulating currents when the delta is closed.
If there is a large voltage across the break you have probably interrupted a legitimate load current with the break and there may not be cause for circulating currents.
A circulating current may be a zero sequence current but I don't accept the corollary. I have seen and cured a lot of circulating issues when the delta winding is the secondary or tertiary or when there is a phantom delta formed by a three legged core.
A wye:wye bank with a single phase load on the secondary will have a corresponding load on one phase of the primary. A delta wye with a load on one phase of the secondary will supply half the load from the corresponding primary winding and half the load from the resultant of the vectors of the two other phases.
The secondary currents will all be reflected in the primary line currents and primary winding currents and there will be no current left to circulate. Every amp will be accounted for by the primary line currents. The currents in two of the phases will be out of phase with the phase voltages but this will be accounted for by the current in the third phase line.
Zero sequence yes, circulating, no.
That is not to say that an unbalance will not cause a circulating current somewhere upstream in the system, but not in this bank.  

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Banking Single Phase Transformers

Waross,
If you break the delta in my example, full line to line primary voltage will appear across the break and all secondary current goes to zero. No circulating current then?

Jghrist,
The OP never said what what was downstream of his bank. We can only speculate on this point.

My example was not intended to throw in the concept of circulating currents between unmatched three phase transformers, but to provide an example of circulating delta current that I thought we could agree on. I guess we don't. Perhaps another example would work better?

All,
I remain troubled by the use of a phrase that apparently has no definition. On this point Jghrist has provided the only reasonable alternative; reject the use of the phrase. Waross has provided a method to determine if it is present or not, but no means to quantify it. Mgtrp introduced the phrase here. Mgtrp, please tell us what you meant.

RE: Banking Single Phase Transformers

In the case of a delta-wye transformer where there is a source on the grounded wye side, you have a grounding bank.  If there is a Ø-grd fault (or unbalanced load) on the wye side, and no load connected to the delta side, you will have current truly circulating in the delta.  There is no positive- or negative-sequence current.  Only zero-sequence current.  There will be equal current (magnitude and phase angle) in all three phases both on the wye and delta side.

This is an extreme example of currents observed by waross in wye-delta banks.
 

RE: Banking Single Phase Transformers

I must learn to think before typing. Breaking the delta in my example will not result in full line to line voltage because of the second source.

Jghrist,
Good example, much simpler than mine. So circulating current is equal to I0 in this case. So what is the general rule for quantifying circulating current when sources and loads may exist on either side?     

RE: Banking Single Phase Transformers

Sorry about the comment on the "Break the delta". The effect is very evident on the secondary of a four wire wye:delta. Breaking a primary delta will defeat the purpose.
I have always considered "Circulating currents" to be unwanted and possibly destructive currents, whether in a delta winding, core laminations, bus bars or whatever.
A circulating current, just like a load current, has a corresponding current in the companion winding.
Circulating currents that I have encountered and corrected are I believe, zero sequence currents, in that the problem current has the same phase angle in all three phase windings of the delta, both primary and secondary.
This circulating current will combine vectorily to the load current in the windings. The resultant current may be much greater than the expected load current in at least one phase and often less than the load current in the other phases.

A delta wye transformer bank is basically three independent single phase transformers. Connect a single phase transformer line to line on a three phase supply and the load current will still be accurately reflected in the primary winding, just as if the transformer were connected to a single phase supply. Other loads on the three phase system will not affect the currents in the transformer windings and the ratio between primary and secondary will still be accurate.

Anyone who has worked with the larger single phase generators that have been converted from three phase to single phase will be familiar with the current split when a single phase load is applied to a delta transformer winding or a delta connected generator stator. The connection does not create circulating currents at the fundamental frequency.

We may have a misunderstanding concerning terms.

Three phase transformers with three legged cores may have issues with circulating currents caused by the phantom delta but three single phase transformers with a delta primary and a wye secondary have no mechanism to create circulating currents in the delta primary, even if they are different impedances.

And the real, destructive, transformer destroying circulating currents that I have encountered do not cancel. Often line conditions on one phase will cause the current and determine the phase angle of the circulating current. The total current in each phase will be the vector sum of the circulating current and the load and exciting currents. I have seen the circulating current exceed rated full load current of a transformer.

But you don't get that type of current with a delta primary and a wye secondary on a three transformer bank.

 

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Banking Single Phase Transformers

I agree we have a misunderstanding of terms; exactly why I'm asking for a quantifiable definition. It's beginning to look like there is none that is accepted here, just a non-quantifiable "unwanted" zero sequence current.

I hear you on the magnetic circuit, and agree that three single phase transformers will have a different zero sequence impedance than a similar size three legged core three phase. This different impedance should be evident whether the zero sequence current is wanted or not.
 

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