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Star/Star TX - Zero Seq circuit
3

Star/Star TX - Zero Seq circuit

Star/Star TX - Zero Seq circuit

(OP)
Could someone please explain the equivalent zero sequence circuit for a 2-winding Star/Star TX (HV ungrounded and LV grounded). The confusion is that PRAG shows that no zero sequence current will flow for ph-E faults. But, the J&P TX Book shows a link to the zero bus on the LV side. (I found several other books showing these circuits both ways).

The only explanation for this is that J&P might assume the  tank acting as a delta and therefore the closed link to the zero bus. The problem is, if you assume the PRAG is correct, then for a ph-E fault on the LV, no current will flow. But the winding has basically a short across it - it does not make sense...

Could someone please explain this better.

Thanks in advance.

RE: Star/Star TX - Zero Seq circuit

No current will flow.  Think of the 3Ø circuit of a 1:1 ratio ungrd Wye-grd Wye transformer.  It will help to draw a diagram of the transformer windings.  Assume a short from secondary ØC to ground but no connection to the other secondary phases.  Since ground is connected to the secondary neutral, there is a completed circuit shorting the CØ winding.

The same current will flow in the primary CØ winding as in the secondary.  But with no connection to ground or a direct path to the source, the only place the CØ primary winding current can flow is through the neutral connection to the AØ winding or the BØ winding.  Since there is no current in the secondary AØ or BØ windings, there can be no current in the primary windings.  With no current in the primary AØ or BØ windings, there can be no current in the neutral connection to the CØ winding and no current in the CØ winding.

RE: Star/Star TX - Zero Seq circuit

Jghrist
Just to clarify, with a Wye-grd Wye (Yyn), if a phase -ground fault occurs you say no fault current will flow as it cannot be supported by the primary winding.
Im struggling a little with this theory, have I read your reply correctly or have I got hold of the wrong end of the stick.
thanks

RE: Star/Star TX - Zero Seq circuit

Basically, because there is nowhere for the current to flow in the primary winding, there can be no current in the secondary winding.

The zero-sequence equivalent circuit is

CODE

                  Z0
   o-------o  o--www---o

   o-------------------o
No connection to the neutral bus.

RE: Star/Star TX - Zero Seq circuit

Umm...
Assuming this is not some sort of April fools...

If you short one phase to ground on the secondary of a Y-Ygnd xmfr, you DO get significant fault current.

We often refer to these as single-line-to-ground faults.

They are very very common.

Imagine a single-phase xmfr with the neutral (X2) grounded on the secondary side.  If you were to then ground the 'hot' (X1) bushing, you are effectively shorting out the full secondary winding with a very small impedance.  I think it is obvious that there will be significant current flow if the xmfr is energized.

A Y-y transformer is effectively three separate 1-phase xmfrs sharing a common tank and core for three-phase applications.

RE: Star/Star TX - Zero Seq circuit

I'd say no current.

Look at jghrist's example a different way. If you short the C phase secondary winding then the primary winding becomes very low impedance compared to the other two windings. Therefore, the equivalent circuit is almost the same as directly connecting the neutral point to the C winding.

So, you will increase the primary and secondary voltages of the A and B phases. The primary current will likely increase due to saturating the transformer core but not to fault current levels.

The connection seems useless though. There's no way to keep the voltages on the LV side balanced.

RE: Star/Star TX - Zero Seq circuit

@LionelHutz

I must respectively disagree.  My answer was not a theoretical one, but based on years of direct empirical observation working at several power utilities.

Many POCOs use Y-Ygnd xmfrs for all of their distribution (120/208V or 277/480V secondary) transformers.  I can assure you that if one phase faults on the secondary cables or in the customer premise, you do get a spectacular display of fault current.

RE: Star/Star TX - Zero Seq circuit

This is what I find in IEC 60076-8 Table-1.

The current will flow and the zero sequence impedance for Yyn connected transformer is magnetising impedance of relatively high value ~ 60%.

RE: Star/Star TX - Zero Seq circuit

tinfoil, what you are referring to is a wye-wye transformer that has both H0 and X0 grounded.  This is as shown in figure 11-30 of your reference and performs as you describe.  The OP question refers to the case when H0 is not grounded, as shown in figure 11-31 of your reference - note the open in the zero-sequence diagram.  With an open in the zero-sequence network there can be no zero sequence current.  Current can not exist on the secondary of the transformer if it does not exist on the primary.

RE: Star/Star TX - Zero Seq circuit

I was of the impression that the 'star-points' (brought out as X0 and 'H0')  inside most distribution-class Wye-Wye transformers (as well as small station-class units) were internally connected (and case-bonded) at the factory, which is why
a) The 'H0' bushing is often just a nominal 'stud' welded to the side of the case
b) The utility only needs to connect to neutral / ground the X0 bushing to achieve both high and low side grounding.

I made the assumption that the OP of this thread was observing the single point of grounding made outside of their xmfr, and therefore referred to it as a Y - y(gnd) unit.

RE: Star/Star TX - Zero Seq circuit

@davidBeach:

Even if the primary side is ungrounded, then you are going to have the situation shown in fig 11.31, only with HV and LV 'reversed'.  There is still a sequence path.

Looking at it from my three 1-phase xmfrs on a common core point of view, you have the X2 (X0 on the real 3 ph xmfr) and X1 (X1,X2,X3 on the 3ph) bushings connected across one leg of the secondary winding, and then 'shorted' through the fault impedance.  There WILL be a current flow (or else how do you pull single-phase load through a 3ph xmfr?).  The secondary fault current can be thought of as a big unbalanced 'load' from the  primary point of view, but its magnitude will be largely limited by the xmfr impedance.

RE: Star/Star TX - Zero Seq circuit

If X0 and H0 are internally connected, grounding X0 will ground H0 and you have the normal grd wye - grd wye connection.  I have seen ungrd Wye - grd Wye transformers with a delta tertiary.  This is an entirely different case because the tertiary makes it a ground source.

The diagram in my previous post is from Fig. 13.17 of Elements of Power System Analysis, William D. Stevenson, Jr., 2nd Edition, 1962, p. 297.

I with the short on the HV (grounded) side of Fig. 11.31 of Tinfoil's source.  This indicates that the transformer is a ground source.  Fig. 11.31 is the same as Fig. 11.33 (grd Wye - Delta) except for the phase shift.

Maybe the University of Idaho has a text more up-to-date than mine, but I'll trust Stevenson.

RE: Star/Star TX - Zero Seq circuit

@jghrist:

I assume you accept that a three-phase ungrd Wye - grd Wye xmfr is capable of delivering unbalanced current, if one of the three secondary phases had more load than the other two.

Stated differently, there will be more current on one leg of the secondary if you connect a smaller net impedance (more load) to it w.r.t. the other two phases.

A SLG fault is a special case of 'more load/less impedance', where this impedance is at or near zero.  The only choke on this current level is the internal impedance of the transformer in series with the primary network impedance, plus the whatever we assume for fault impedance.

Otherwise, I believe that you are asserting that you can short one secondary leg of the xmfr and see no (significant) current flow.

If I am misunderstanding what you are saying, please correct me.

RE: Star/Star TX - Zero Seq circuit

It's possible to have unbalanced current without ground current.  

Current flowing into the ground requires zero sequence current.  For any transformer, for current to flow in the secondary it must also flow in the primary.  This applies to sequence currents the same as "real" current and is required  because the corresponding flux required in the core.  So in most cases, there will be very little ground fault current when the primary wye is not grounded.  

But... for a three-phase core-type transformer, the zero sequence flux in the ungrounded primary can complete its path outside of the core.  This does give rise to what can appear to be a high-impedance "phantom" delta tertiary that will allow some zero sequence current to flow in the secondary.  GE data I have indicates an equivalent zero sequence reactance of about 5 times the positive sequence.  This would not be true for a bank of three single-phase transformers or a shell-type transformer.   

RE: Star/Star TX - Zero Seq circuit

Quote:

There WILL be a current flow (or else how do you pull single-phase load through a 3ph xmfr?).

That was my point, the connection under discussion doesn't seem useful because it doesn't allow single phase or phase-neutral loads and it doesn't keep the voltages on the transformers balanced.

Now, according to dpc some current can flow if it's a common core transformer but I still say none will flow if it's 3 single phase transformers in use. Well, in reality magnetizing currents and load currents on the other phases can cause some current to flow but not the same fault levels as you'd see with the primary neutral connected.

RE: Star/Star TX - Zero Seq circuit

Let's briefly set aside the symmetrical component model, including the zero-sequence artifact that we use to ease our computations, and look at the real equipment that we are modelling:

We have any three phase wye-wye transformer.  Assume that Ho and Xo are not connected, and that neither Xo nor Ho is grounded.  Assume the transformer is energized by the primary, and that the X1, X2, and X3 points are not connected to anything. Therefore, there WILL be (nominal)voltage from X1 to Xo.

We then place a finite impedance across X1 to Xo(such as the very small impedance we assume during a fault).  Regardless of the state of the Xo and Ho grounds, current WILL flow from X1 to Xo due to the voltage difference, with the primary side H1 leg furnishing the power.  If Ho is ungrounded, then the other two primary legs will have to furnish this current.

Grounding the Xo bushing and then introducing an X1-to-earth fault is the same thing, except that it involves the earth path in determining the finite impedance.

RE: Star/Star TX - Zero Seq circuit

Yes, some current will flow but not fault current. The voltage from X1 to X0 will collapse due to the load you just connected which will limit the possible current flow.

The voltages measured from H1, H2 and H3 to H0 will not remain equal with an unbalanced secondary load.

RE: Star/Star TX - Zero Seq circuit

Try this exercise: When present, I0 is equal in magnitude and direction in the three phases. Kirchoff tells us the sum of the currents entering a node (such as the wye point)is zero. This is easily satisfied if the wye point is connected; to ground for example. The I0 leaving on each phase is balanced by 3I0 returning through the ground. If the wye point is floating, though, the path carrying the 3I0 is open circuited. If 3I0 is zero, so is the I0 in the phases. Of course the two windings do not exist in isolation. If I0 = 0 on one side, it must be zero on the other side as well even if the wye point is connected on that side.

You can also look at the primary neutral to ground open circuit as an impedance that is transformed by the square of the turns ratio to the secondary side where it is still an open circuit.

RE: Star/Star TX - Zero Seq circuit

I agree with Lionel.  The voltage across both primary and secondary windings will collapse (neglecting the effects of the phantom tertiary in core type transformers, magnetizing current, stray capacitance, and saturation).  That's why there is no current in the shorted winding.

Yes, you can serve unbalanced load and get a current, but the voltages will adjust so that there is no neutral current.  You can serve unbalanced load from a wye-delta transformer, but there won't be any zero-sequence current in the primary.

By the way, the next to last paragraph of my previous post should start "I disagree with the short on the HV (grounded) side of Fig. 11.31..."  Certainly, the zero-sequence circuit of a wye-wye transformer is not the same as that of a delta-wye transformer.  There is no short to neutral on the grd wye - grd wye diagram Fig 11.30.

RE: Star/Star TX - Zero Seq circuit

I am confused.

jghrist said 'The voltage across both primary and secondary windings will collapse'.  

Our typical model has an infinite bus connected through an impedance (call it Zgrid) to the area under study.  If the voltage on H1 has collapsed, then the infinite, unchangeable bus now has to drop the full pre-fault system voltage across Zgrid.  Wouldn't this have to compute out to an SLG on the primary side?


I never tried to imply that 'zero seq. current' was flowing through the xmfr; only that a lot of 'real' current will be passed through.

RE: Star/Star TX - Zero Seq circuit

BTW, when you say 'collapsed', do you mean 'has reached the point of the vector sum of the H2 and H3 legs'?

I do not have access to the quoted IEEE document, but I thought that this table was interesting:http://www.ieee-kc.org/library/pq/singlelinegroundfault.htm

RE: Star/Star TX - Zero Seq circuit

The primary winding neutral point (which isn't connected to the source neutral) shifts to the voltage of the faulted phase.  If ØA is faulted, primary A-n voltage becomes zero, B-n and C-n voltages equal the system phase-to-phase voltage.

Your referenced table is interesting.  Note that Van is non-zero in some grounded wye secondary connections, but zero in all grounded wye primary cases (except delta secondary).  This must mean that the single-line-to-ground fault that is referenced is a single-line-to-ground fault on the transformer primary.  A single-line-to-ground fault on the transformer secondary would make Van zero for all grounded wye secondary connections.

IEEE Explore doesn't find any paper by those authors and title, but it does find Voltage sag analysis case studies by Lamoree, J.; Mueller, D.; Vinett, P.; Jones, W.; Samotyj, M.; Industry Applications, IEEE Transactions on, Volume 30,  Issue 4,  July-Aug. 1994 Page(s):1083 - 1089.  This paper doesn't have the table but does discuss sags of customer systems caused by faults on the utility distribution and transmission systems.  A quick read doesn't indicate any mention of the effect of transformer connection.


RE: Star/Star TX - Zero Seq circuit

The "real" fault current you speak of, for the faulted phase of an SLG, is I1+I2+I0 where I1=I2=I0. The other phases see no additional current from the fault.

Your infinite bus model maintains the line to line voltages and possibly the line to ground voltages. The unconnected transformer neutral is free to move about based on load or fault conditions. As LionelHutz said, not a very useful connection.

RE: Star/Star TX - Zero Seq circuit

There are a couple of points here that should be clarified.
Do not confuse grounding with connection to the neutral.
A ground fault is often a line to neutral fault with the grounding system forming part of the circuit, particularly at utilization voltages.

If the primary wye point of the transformers is connected to the neutral (but not grounded) then fault level currents will flow.

If the transformer wye point is not connected to the system neutral then in the event of a line to neutral/ground fault on the secondary, the transformer windings will act as a three phase voltage divider.
An easy example to follow and one with which most of us are familiar is a 120/240 volt secondary with an open neutral. The voltages in each 120 volt circuit are inversly proportional to the impedance when the neutral goes open. If there is a high wattage load such as a toaster on one line and a low wattage load such as a stereo set on the other line the resulting voltage across the toaster will be very low and the voltage across the stereo will be around 200 volts or more. The toaster will "Toast" the stereo.

The voltages, both secondary and primary will behave similarly. Current may only flow in the primary of the faulted phase if it also flows through one or both of the other phase windings.
The current resulting from the line to neutral fault will not be at fault levels. The currents on the unfaulted phases will increase as a result of the increased voltages.
If the increased voltage across the primaries of the unfaulted phases drives the transformers into the knee of the saturation curve, there will be increased current. This increased current will result in a higher voltage across the faulted phase and the system may be expected to find a voltage balance point with the unfaulted transformers in the knee of the saturation curve. The currents will still be at load or overload levels.

In the real world:
A wye/wye transformer with a floating primary wye point is only suitable for a balanced secondary load.
A wye/wye transformer with a floating primary wye point has no primary return path for harmonics. Overheating may be expected. In a large bank, the harmonics generated by the magnetizing current may be an issue.
A wye/wye transformer designed to operate with a floating primary wye point will have a delta connected tertiary winding that will hold the voltages balanced and fault current will flow in the primary, the secondary and the tertiary windings.
respectfully

RE: Star/Star TX - Zero Seq circuit

The performance of a three-phase STAR - STAR transformer (YNyn) with primary star un-grounded and secondary star grounded during an earth fault (or phase to neutral load) on the secondary side will depend upon the transformer type construction; shell or core.

For shell type, the zero sequence impedance will be very high, equal to the open circuit reactance (i.e. magnetising reactance) i.e. near enough open circuit so you will get very little earth fault current (i.e. effectively zero).

For core type the zero sequence flux passes through to the tank wall of the transformer and this acts as a tertiary.  The effect is that the reactance will be somewhere in the range between 50% to 200% and you will most definately get earth fault current for an earth fault on the secondary side.  There will be no zero sequence current on the primary side.

The effects are described in the J&P transformer book and also on pages 73 - 75 in a brilliant text "Electrical Power Systems Engineering Problems and Solutions" by Alvin Knable (McGraw Hill 1967).  The GE paper referenced by dpc is a good read, well worth its suggested price of 50 cents (thanks for the link dpc), and it explains things as per above really well plus all sorts of other goodies.

The problem these days is that analysis software packages seem to model star - star transformers with primary star unearthed as having infinite zero sequence impedance hence no earth fault current.  This is OK for a shell type but very wrong for a core type (how did we get by before there was software....).  To get around this for a core type transformer you need to cheat a little.....either model it as a star - star with a fictitious tertiary winding or as a delta - star with the secondary neutral resistively earthed.

Core type star - star transformers without a tertiary do exist and are not all that uncommon.  They owe their existance to the "best" of reasons...apparently they cost a bit less to manufacture than a delta - star of the same capacity (don't know exactly why...maybe you can use graded insulation on the primary?).  Anyone ever contemplating buying such transformers had best get some additional testing done by the manufacturer to establish the zero sequence reactance so that the transformer may be adequately modelled.      

RE: Star/Star TX - Zero Seq circuit

Waross
I have just been to site to check a transformer rating plate on a 14.5MVA 33/11Kv transformer, Yyn0. It has a resistance grounded neutral with a value choosen (6.35-ohms) to allow a maximum of 1000amps earth fault current. There is no mention on the rating plate of a tertiary delta winding. I checked the value of earth fault current which flowed in to the last fault, 797A. According to the above a tertiary winding must be present for this current to flow.
Is it normal practice for the manufacturer to ommit imformation regarding tertiary windings on rating plates.

RE: Star/Star TX - Zero Seq circuit

isquaredr,

Is the primary neutral grounded?  Or connected internally to the secondary neutral?  If so, you don't need a tertiary to get ground fault current on the secondary, so long as the primary system has a ground source.

RE: Star/Star TX - Zero Seq circuit

jghrist
No according to the rating plate and schematic diagram of both HV & lv windings there is no connection to the HV neutral, and therefore not grounded.

RE: Star/Star TX - Zero Seq circuit

isquaredr,

You need to look beyond the nameplate and look at the installation. You have four primary air bushings with H0 left unconnected? Or is it connected with three elbow style bushings with insulated cable?

RE: Star/Star TX - Zero Seq circuit

The transformer in question is cabled both HV & lv, 3-connections HV, 4-connections lv (1-being the neutral which is grounded via a resistor)
I have also gone to many other sites within this group and all except one are identical, the odd one out being Dzn0, no mention at any site about a tertiary windings.

RE: Star/Star TX - Zero Seq circuit

isquaredr,

Perhaps the transformer is core-type, in which case the phantom tertiary as mentioned by dpc will give a zero-sequence impedance about 5 times the pos-sequence.  This would still be small compared to the grounding resistor and could result in your measured fault current.

RE: Star/Star TX - Zero Seq circuit

If by "cabled" you mean insulated cable with elbows, each connector provides a connection to the concentric neutral.

RE: Star/Star TX - Zero Seq circuit

jghrist
I accept that 5 times the pos-sequence value supplied via the phantom tertiary would be more than enough to source zero sequence current (ground fault) as our neutral resistor restricts ground faults to 19MVA max anyway.

Stevenal
All our connections at this voltage level are cabled, and I accept that the HV cable neutrals are bonded together but there is no connection what so ever to the star point of this winding, it is just not available.



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