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Delta wye transfomer during a grounded phase

Delta wye transfomer during a grounded phase

(OP)
What happens when one or two phases fault to the MGN (multi grounded neutral) of a distribution system with the effected phase fuse blowing (but still grounded) while feeding a delta wye transformer? Will any inductive tank heating take place? Will a 3 limb vs 5 limb core make a difference?


Basically I am entertaining the idea of using a delta wye padmount instead of a conventional 5 limb wye grounded wye grounded, however I am unsure if certain fault scenarios create special protection concerns. Both the over head feeder and underground riser are protected via fuses, so single phasing and shorted phasing is a concern.

RE: Delta wye transfomer during a grounded phase

I don't see where you would get any tank heating.

Assume you lose A phase on the primary due to the open fuse. The delta will actually collapse. A-B and A-C will add up to B-C voltage. I don't think the ground fault matters at this point other than causing the fuse to blow. Result on secondary is two phase to neutral voltages at half value and the other one at rated voltage.

RE: Delta wye transfomer during a grounded phase

He is talking about tank heating during core saturation due to zero sequence voltage. Zero sequence voltage can't saturate the delta side due to it not impacting the delta windings. The delta windings are acted on with line to line quantities and those are only composed of positive and negative sequence.

You'll have some trade offs going to delta wye ground wye ground. Your phase voltages will be impacted by unbalanced load. Delta wye ground I think is preferable in industrial systems due to the loads being mostly 3 phase (motors) and balanced and it segregates the grounding and providing a ground source.

RE: Delta wye transfomer during a grounded phase

(OP)
Correct, tank heating from zero sequence voltages. I know this is possible with a 3 limb wye-wye, hence why a 5 limb or tri-plex is specified.


Also my worry is not just an open fuse, but that phase back feeding (shorted) into the MGN before line crews show up. As long as the transformer itself does not become damaged I am content.


And- for those who know about ferrorsaonance. As long as the transformer remains under about 15-20% load, the resonant condition will be dampened out?

RE: Delta wye transfomer during a grounded phase

For ferroresonance, you don't need that much load. Less than 5% load is adequate. What primary voltage are you dealing with?

RE: Delta wye transfomer during a grounded phase

(OP)
22.5kv primary voltage.

RE: Delta wye transfomer during a grounded phase

2
Here are a couple of good papers we reference here when look into distribution transformer configuration and resonance.

http://s000.tinyupload.com/index.php?file_id=30823...

This link works. Just don't click on the bait. The zip file downloads when you click on the file name.

RE: Delta wye transfomer during a grounded phase

(OP)
Thank you, and that this is a life saver, would be an serious understatement! I owe you 2x now :)

RE: Delta wye transfomer during a grounded phase

You will have zero-sequence voltage with one phase connected to the MGN. See the attached calculations that assume ØB is connected to the MGN (VB=0V). I've assumed 100A secondary current in each phase, in phase with the secondary voltage, to calculate some currents.

There will be no zero-sequence current, even though there is current from ØB to the MGN (it is all pos- and neg-sequence).

RE: Delta wye transfomer during a grounded phase

(OP)
Perfect and spot on assumptions. This is gold :)

RE: Delta wye transfomer during a grounded phase

Jghrist,

Line to line quantities on the delta would never be affected by zero sequence because zero sequence doesn't produce a voltage differential across each winding.

The thing when I looked over your calculations that I had never thought of before is that the type of load I think could dictate what happens on the secondary of the transformer. The load that you used in your calcs was 100 A load in phase with the voltage on each of the phases, a constant current load, probably single phase. Zero sequence current on the secondary is generated by the change in angle of the secondary voltages and zero sequence current is produced due to the currents not adding up to zero. If you had just a passive load, I don't think you would have seen any zero sequence currents on the secondary because the secondary voltages with just positive and negative voltages would have only produced positive and negative sequence currents. I suppose you might see something similar with constant power loads. I suppose this isn't profound or anything but I guess the type of load you have could determine if you should care what is happening on the high side in respect to tank heating.

RE: Delta wye transfomer during a grounded phase

(OP)
@HambergerHelper: great point. I am assuming single phase resistors in that simulation, but what if you had other loads?

RE: Delta wye transfomer during a grounded phase

Hamburgerhelper- Thank you for that old Westinghouse paper. There is a similar GE paper of 1967
http://www.geindustrial.com/publibrary/checkout/Wh...–The Why of the Wyes: The behavior of transformer Y connections. All these incorporated in the IEEE Standard C57.105-1978(R2008) Transformer Connections in 3 phase distribution systems.

RE: Delta wye transfomer during a grounded phase

Mbrooke,

I am looking at this wrong. The zero sequence voltage at the transformer secondary due to the load would be zero because it is grounded. It can't be anything but zero. The load is generating the zero sequence voltage due to how it interacting with the voltage imbalance. I find it curious that the secondary voltage consisting of just positive and negative components could cause zero sequence currents to flow but I don't think that has anything to do with tank heating. I don't think the loads can impact your problem.

RE: Delta wye transfomer during a grounded phase

(OP)
So in a nut shell tank heating is not a concern- even with varying loads and various line faults. Thus a 3 limb core will be of no issue- unless of course someone sees a 5 limb being superior.

RE: Delta wye transfomer during a grounded phase

I don't think it is for a delta-wye-grounded transformer since it is a ground source. Zero sequence voltage can't affected the high side because they are connected line to line. The zero sequence voltage on the low side will be the zero sequence current times the transformer's impedance to ground and you have current circulating in the high side delta which will reduce zero sequence flux in the transformer core. So, zero sequence can't do anything on the delta side and on the secondary I think you would have to pull a lot of zero sequence current to get stray heating flux.

RE: Delta wye transfomer during a grounded phase

(OP)
Makes sense. Or we can view it like this: if a wye-wye had a delta tertiary, would we still get stray flux?

RE: Delta wye transfomer during a grounded phase

I have seen wind farms with wye-gnd-delta-wye-gnd transformers. I don't know why exactly they select those transformers but the impedance of the tertiary delta makes it act like a wye-gnd-wye-gnd if it has really high impedance or a back to back wye-gnd-delta to a delta-wye-gnd for really low impedance. Anything, though that circulates in a delta provides counter flux to your transformer core and reduces its flux density.

RE: Delta wye transfomer during a grounded phase

With a Dyn connection,or YN(d)yn and a Ynyn 5 limbed core, you will not have straying zero sequence flux from core and hence tank heating. But with a Ynyn connection with 3 limbed core, zero sequence flux in 3 limbs(say under unbalanced secondary loads)have no return path through core and it goes to tank to close the path ie tank acts as a virtual tertiary delta( one turn winding,carrying current and heating)
In case of wind farm transformers, please check the core construction. If it is 4 or 5 limbed, stabilizing delta will be required to bring down the zero sequence impedance. With a 3 limbed core it may not be necessary.

RE: Delta wye transfomer during a grounded phase

What is the effect of the back EMF of any motors until the thermal over current trips or thermal differential trips clear? Won't you be back feeding the fault with a current limited by the motor impedances and the transformer impedance?

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

RE: Delta wye transfomer during a grounded phase

(OP)
@waross: X2, this has also been on my mind the whole time.

In so far I'd like to thank everyone for the replies. :) I've learned a lot in this thread and still am.

RE: Delta wye transfomer during a grounded phase

Quote (HamburgerHelper)

Line to line quantities on the delta would never be affected by zero sequence because zero sequence doesn't produce a voltage differential across each winding.
The zero-sequence primary voltage results from the unbalance caused by VBn going to zero because of the fault. VAn = VAB and VCn = VCB.

Quote (HamburgerHelper)

The thing when I looked over your calculations that I had never thought of before is that the type of load I think could dictate what happens on the secondary of the transformer. The load that you used in your calcs was 100 A load in phase with the voltage on each of the phases, a constant current load, probably single phase. Zero sequence current on the secondary is generated by the change in angle of the secondary voltages and zero sequence current is produced due to the currents not adding up to zero. If you had just a passive load, I don't think you would have seen any zero sequence currents on the secondary because the secondary voltages with just positive and negative voltages would have only produced positive and negative sequence currents. I suppose you might see something similar with constant power loads. I suppose this isn't profound or anything but I guess the type of load you have could determine if you should care what is happening on the high side in respect to tank heating.
The secondary voltage depends only on the primary winding voltages. I have neglected any voltage drop in the transformer winding from load current. There is no zero-sequence secondary voltage because VAB + VBC + VCA= 0. There can be zero-sequence secondary current regardless of the secondary voltage. Consider if there is only load connected from one secondary phase to neutral. The zero-sequence current would be 1/3 of the load current.


RE: Delta wye transfomer during a grounded phase

Waross,

Wouldn't you just include some grounding impedance if you were worried about back feeding fault current?

RE: Delta wye transfomer during a grounded phase

We are discussing a multiple grounded neutral system.

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

RE: Delta wye transfomer during a grounded phase

I have a query with reference to the Westinghouse paper of 1978 posted earlier. It is said (slide 15) when there is a break on A phase primary, Vo will be applied on primary and this cause zero sequence flux resulting in tank heating. Since primary neutral is solidly grounded physically there is no voltage on the A winding and limb of the core. What ever flux generated in B &C limbs can return through the flux empty C limb of the core. Then how zero sequence flux can go to tank to create heating?

RE: Delta wye transfomer during a grounded phase

(OP)
The whole "no voltage" term is confusing me. While it may be correct; grounded or not, in either delta or wye it forms some type of circuit. In wye its just a shorted winding while the others remain energized and in delta the other phase is feeding that winding.

In fact in wye, I would be concerned about back feeding load heating the winding itself from being fully shorted out. I have never though about this, but it now just ran through my mind.


RE: Delta wye transfomer during a grounded phase

As per slide 15,in YN winding A phase is shorted to neutral and no connection to incoming supply as fuse opened out. Hence my view no voltage and flux in limb A.

RE: Delta wye transfomer during a grounded phase

(OP)
But what about load back feeding it? Think a motor or lots of delta loads. The shorted primary would produce a very low impedance on the secondary allowing back fed current from the load to flow easily.

RE: Delta wye transfomer during a grounded phase

PRC,

I think the confusion is due to what you think V0 is. V0 is the same magnitude and angle on all three phases. Whatever is on the third leg that is unconnected will be come mix of zero, negative, and positive sequence flux. The zero sequence flux by definition will be the same in each leg and it can't return through the open or faulted phased leg. So, there will be flux that passes in the third leg but it won't be just the zero sequence flux. The voltage on phase A can be zero, but that is due to all the sequence components adding up to zero for that phase. It is just like how a single line to ground fault has I1=I0=I2 but current is only present on one of those phases. The sequence components in the unfaulted phases add to zero so you have what you expected, only current in the faulted phase.

RE: Delta wye transfomer during a grounded phase

Realistically given the great number of delta/wye transformers in service I don't anticipate a problem.
There are problems with a wye/delta connection when the wye neutral is connected.
This may be mitigated by floating the neutral.
Back feeding will be primarily from motor contributions that do not have a neutral connection so the wye/delta issues will be mitigated.
As to secondary voltages during a fault.
With one phase open the two windings connected to the open phase will be in series across line voltage. The voltage division will be determined by the loads on the individual transformers.
With one phase faulted to ground on an ungrounded system the result will be the same.
With one phase faulted to the neutral, the voltages across the windings connected to the faulted phase will drop from line to line to line to neutral voltage.
The phase angles will shift. With B phase faulted the winding at A-B will become A-N and the winding at B-C will become N-C.

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

RE: Delta wye transfomer during a grounded phase

(OP)
@waross, thanks for the explanation. Even though two limbs are phase shifted there is no actual concern and the transformer can run like this for hours? Reason I keep thinking this is that normally on a 3 limb core everything is 120* apart and thus all net flux is equal to zero. But with two limbs now phase shifted by (30?) degrees I can't see the net flux equaling zero. Unless my understanding of 3 phase transformer theory is off?

RE: Delta wye transfomer during a grounded phase

(OP)
@Jghrist: what program did you run that simulation through? Or is just hand calculated? Just curious, I'd like to run it on that same program if possible.

RE: Delta wye transfomer during a grounded phase

Mathcad

RE: Delta wye transfomer during a grounded phase

(OP)
Thanks :)

RE: Delta wye transfomer during a grounded phase

That's the phantom delta on a three legged core.

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

RE: Delta wye transfomer during a grounded phase

(OP)
Thanks- and a 5 limb Y-Y will not back feed-right?

RE: Delta wye transfomer during a grounded phase

I don't believe so. The 5 limb core eliminates the phantom delta. I am not aware of any other type of back feed.
A delta fed from a wye with the neutral connected has the ability to transfer power from one or two phases to the third phase.
The phase windings in a grounded wye are independent of each other, unless the phantom delta is present.

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

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