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prc (Electrical)
29 Jul 10 6:41
During the commissioning of a 150 MW Generator-Transformer (delta -star 15.75/220 kV)set the following was observed.

A neutral current of 58 A is seen in the transformer as soon as the transformer is synchronised to the grid. Neutral Current remains the  same for the load range of 5 - 85 MW.At that time, REF relay shows 4 kV neutral voltage. Just before synchronisation,with TG under full RPM,before AVR made ON,there is no neutral voltage  indication in relay.But just after AVR made on with full voltage build up on the TG terminals,neutral voltage is found in REF relay and continues to be there after synchronisation too.There is current unbalance in 220 kV side R-Y & Y-B phases 10A and between B-R phases 20 A,throughout the load range of 5-85 MW.
Ths unbalance is seen on the generator side also R-Y & Y-B phases 100 A,B-R phases  200A.

Transformer was tested and ratio error was  same on all phases.
What  can be the reason for this abnormal behaviour?
rcwilson (Electrical)
29 Jul 10 11:42
The wye connected 220 kV winding may have neutral current flowing if the 220 kV system voltages are not balanced to ground.

How is the 4kV neutral voltage measured by the REF relay?  If the 220 kV winding neutral is solidly connected to earth, there should be no neutral to ground voltage. How is the wye point connected to earth?
Helpful Member!(2)  waross (Electrical)
29 Jul 10 12:50
If there is a voltage unbalance in the wye side of a wye:delta transformer there will be a circulating current in the delta side. This circulating is independent of the load. The current is dependent on the voltage unbalance, the impedance of each single phase winding of the transformer and the neutral path impedance.
I would expect these currents and neutral voltage to be there even if the transformer was put on-line without the generator connected. I have seen distribution wye:delta transformer banks with the wye point connected to the system neutral overheated to the point of destruction by circulating currents caused by unbalanced voltages on the wye side. (Not a recommended connection for distribution service, by the way.)
Possible causes?
Insufficient transitions in the 220 kV line.
Either your transformer or a grid transformer has a tap setting on one phase out of step with the tap settings on the other phases.
Note: if you are reporting 4 kV on the neutral I will assume that you have an impedance grounded wye point. That would be a reasonable voltage if a tap setting on one phase somewhere was one step off of the correct position.
But, it depends.

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

rockman7892 (Electrical)
29 Jul 10 13:22
waross

You are saying that circulating currents in Delta are a result of unbalanced voltages on wye side.  I know its not the preferred method but wouldn't grounding the neutral of the wye cause all the wye side voltages to be balanced as opposed to leaving the neutral ungrounded and letting the neutral float and therefore cuause unbalanced voltages acooss wye windings?
davidbeach (Electrical)
29 Jul 10 13:27
The current flows attempting to balance the voltages, but the transformer won't be a strong enough source and it will never succeed in actually balancing the voltages.  If the neutral is ungrounded, it floats to what ever voltage to ground provides a stable point given the unbalanced voltages on the terminal and no current flows as a result of the unbalance.
jghrist (Electrical)
29 Jul 10 13:29

Quote:

You are saying that circulating currents in Delta are a result of unbalanced voltages on wye side.  I know its not the preferred method but wouldn't grounding the neutral of the wye cause all the wye side voltages to be balanced as opposed to leaving the neutral ungrounded and letting the neutral float and therefore cuause unbalanced voltages acooss wye windings?
Grounding the neutral won't change the primary voltage.  Another way to look at this connection is that it is a grounding bank for the 220 kV system (with or without generators connected) and will provide zero-sequence currents for any unbalanced load on the system.  It will also provide ground fault current for any ground fault on the system.  This will probably cause more problems that a small amount of neutral current during normal operation.
 
waross (Electrical)
29 Jul 10 14:08
In this context I agree with you completely, David. However I have encountered distribution circuits that suffered unbalanced voltages due to unbalanced single phase loads. A wye:delta transformer bank did a very good job of balancing the circuit. (This wasn't planned. It was only discovered when the bank was switched off.)
In this case, although the transformer was small compared to the system capacity, its capacity was appropriate compared to the load unbalance causing the problem. But, in support of your post, when comparing a few tens of KVA load unbalance with a voltage unbalance on over 150,000 KVA of capacity the results may not be the same.

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

rockman7892 (Electrical)
29 Jul 10 14:36
jghrist

Are you saying that with a wye primary, grounding the neutral will provide zero-sequence current path for any unbalanced current/loads in the secondary delta of the transformer?

If the primary wye was left ungrounded wouldn't the neutral point float to a voltage to ground that satisfied the unbalanced condition similar to an 3 phase unbalanced wye connected load without a neutral?

Couldn't this neutral floating to ground then create unbalanced voltages across each of the wye windings depending on where the neutral floated to and how much unbalanced current was flowing in each winding similar to an unbalanced three phase wye connected load?

davidbeach

can you explain your last statement as to why no current flows as a result of the unbalance?
davidbeach (Electrical)
29 Jul 10 16:08
Nothing to drive the current.  For any three voltages, there is a "neutral" point for any three impedances connected in wye, where there is no current to/from the neutral (can't be if it isn't connected).  It's only when you then tie that neutral to some reference voltage (generally ground, could be anything) with a connection that can carry  current that there will be the currents flowing necessary to move that neutral point to the reference voltage.

For instance, say you have a 208V wye connection with a delta on the other side, voltage irrelevant.  If you have three balanced phase voltages, each 120V to ground and 120 degrees apart in phase, and the three winding impedances are exactly equal, there will be no current into/out of the neutral (connected or not) and no circulating current in the delta.

Now, lets say you reduce one of the voltages by 30 volts, but still at its phase angle.  And to begin with leave the neutral (wye-point) unconnected.  Unconnected, no current flow.  No current into/out of the neutral, no circulating current in the delta.  So, now that wye-point voltage won't be at ground.  It will be 30 volts away from ground and 120 volts from the reduced phase.  The voltages to the wye-point will be 120V, 108V, and 108V.  This can be found geometrically by drawing the voltage triangle and taking the perpendicular bisector of each side and seeing where they meet.  The point may or may not be inside the triangle, in this case it is.  Again, assuming equal impedances (Z) per winding, the currents in each leg have to sum to zero (you don't want Herr Kirchhoff spinning in his grave), this point is the only one where there would be no current.  No current in the wye, no current in the delta.

Now, connect that wye-point to ground.  Your current in the connection between the wye-point (neutral) and ground will be (Vag+Vbg+Vcg)/Z=3*I0 (30V/Z in this example), with 1/3 of that total (I0) in each leg and I0 (different magnitude by the turns ratio) circulating in the delta.  Small transformer and strong system, the transformer will do nothing to move the voltages, just allow a lot of current to flow.

Drop one phase all the way to zero, aka ground fault, and the wye-point voltage is 120V from ground if not connected.  If connected the current will be 120V/Z in the connection, probably a large number while the current in the delta will be 40V/Z times the turns ratio.  For zero volts to ground the geometric analysis fails as the perpendicular bisectors of all three sides wind up parallel to each other.

If on the other hand, we leave all three voltages 120V to ground, but vary the phase angle, to say 0, -110, and 110, we find (in the unconnected case) that the wye-point floats to 82V, 138V, and 138V.  When connected to ground the current in the connection will be 38V/Z, with 13.7V/Z on each leg.
jghrist (Electrical)
29 Jul 10 17:09

Quote:

Are you saying that with a wye primary, grounding the neutral will provide zero-sequence current path for any unbalanced current/loads in the secondary delta of the transformer?
No, I'm saying that it will provide zero-sequence current for unbalanced load in the primary 220 kV system.

Quote:

If the primary wye was left ungrounded wouldn't the neutral point float to a voltage to ground that satisfied the unbalanced condition similar to an 3 phase unbalanced wye connected load without a neutral?

Couldn't this neutral floating to ground then create unbalanced voltages across each of the wye windings depending on where the neutral floated to and how much unbalanced current was flowing in each winding similar to an unbalanced three phase wye connected load?
Not if the 220 kV system was effectively grounded through other ground sources.  If the 220 kV line tripped, leaving no other ground source, then the neutral would float.  You could put a zero-sequence voltage relay in to detect this situation and trip the generator.
 
waross (Electrical)
29 Jul 10 17:09
Think of a why:delta configuration with the wye neutral connected.
Draw the vectors for the secondary delta with a broken delta.
The vectors will close on themselves.
Now change one of the primary voltages slightly. One secondary vector in the delta will have a corresponding change. The vectors will no longer close, but there will be a voltage difference. Now close the delta. There is not much voltage but there is not much impedance in the transformer secondaries and a relatively large current will circulate in the delta.
Now if the primary wye point is allowed to float the vector position of the wye point will shift or offset to the position that does not cause current flow.
If the primary wye point is connected to the neutral/ground through a high impedance, the circulating current may be kept at a relatively  low level.
With 150 MVA and 220 kV, 4000 Volts is relatively low and the ground current and circulating current are also relatively low.

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

Rodmcm (Electrical)
30 Jul 10 4:50
I assume the generator is synched at 220kV. Is the 220kV supply balanced before conntection? Is it earthed? Are we looking at circulating earth current on the 220kV? Two or more earth points?
prc (Electrical)
30 Jul 10 5:49
Thank you waross and  all other experts who contributed.
1) Transformer  is 220 MVA 15.75/220 kV Delta/star,with three pole OLTC on HV.So chances of tap difference between the phases are high during installation work at site.But the ratio measured from 1 to 17 and then 17 to 1 taps shows no abnormality( This check on both directions is important.In another smilar problem that I confronted 2 years back,ratio was ok in one direction and in the other direction it was showing same ratio at certain taps and when transformer was on those taps neutral current was observed.)So such a possibilty in this case is ruled out from ratio measurements.

2) Question raised by rcwilson is my query too.Can experts throw some light?How REF relay is showing neutral voltage?

3) As waross wanted, I have already asked site to do a back charging of transformer.Before that, Iam planning a low voltage excitation from HV side with neutral earthed.Normally a 415 V application with tap changing on both directions with neutral earthed should show neutral current,in case of  phase discrepancies.

4) waross, 4 KV neutral voltage is reported even before synchronisation.So possibilty of unbalance from system seems not the reason.Can you explain how the inadequacy of transposition of transmission conductors can result in neutral current? Can you explain a bit more about the distribution transformer failure? My understanding was that  delta on HV and earthed star on LV is the normal connection for distribution transformers.

5) Transformer is synchronised to grid at 220 kV. Supply is balanced as another transformer is working on the same bus without neutral current.
pwrtran (Electrical)
30 Jul 10 11:57
prc
You may hit on the nail!  Un-symmetric transposition will cause unbalanced 3ph shut capacitance and will shift the neutral point normally less than 3% of the phase voltage.  So 4kV on neutral point falls in ballpark.
 
waross (Electrical)
31 Jul 10 3:20

Quote:

Before that, Iam planning a low voltage excitation from HV side with neutral earthed
I am always concerned when earth and neutral are used interchangeably. Earthing should make no difference.
Connecting the wye point to the supply neutral will make a difference. If connecting the wye point to the system neutral also earths it, no matter, but if someone feeds the 415V from a delta source, grounding the wye point may be meaningless for testing purposes.
If it is possible to energize the transformer from the 220kV with the generator disconnected please do so. If you have a voltage balance issue or a phase angle error on the 220 kV line it will show up as a neutral current.

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

prc (Electrical)
1 Aug 10 23:49
Thank you waross for cautioning me on the use of neutral and earth.In this part of the world we always use 3 phase 4 wire.
I agree with your views.I will come back after the backcharging.Meanwhile can you spare sometime to clarify  on the other queries  raised by me.
waross (Electrical)
2 Aug 10 18:15
Just to be sure that we are on the same page.
What do you mean be back-charging? The term may be misconstrued.
The circulating delta current issues show up when the wye side is powered with a non symmetrical source. The issue may be voltage, phase angle or both. Checking with a low voltage symmetrical source may show transformer ratio problems. It will not identify voltage or phase angle issues with the 220 kV line. If you direct connect the transformer neutral point to the source neutral point, you may be able to see a circulating current even when energizing with low voltage.
I have again reread your original post.  

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

prc (Electrical)
2 Aug 10 23:54
I am sorry.When we say back charging( a common usage in this part of the world) a GT (GSU) it means, charging from HV side.Normally a step up GT is always energised from LV side.

It is true that during low voltage excitation with neutral connected to source,there will be a neutral  current always.But in  case one voltgae step difference is there in one phase of winding,neutral current will shoot up.So in this case ratio is ok, I thought we may clear the transformer by this test.We always hesitate to back charge due to surge tarnsfer prolems to LV.

As I explained in 30th July mail, neutral voltage is appearing,even when the transformer is not connected to grid.So cant we rule out the effect of system to this problem? I was looking forward to your learned response to the queries raised vide by my  30th July mail.
waross (Electrical)
3 Aug 10 2:00
OK I. Thank you for the clarification. I may have misled you. The circulating currents that I was referring to occur due to line issues on the wye side.
If you have these issues before connecting to the 220 kV line I would check for a harmonic content in the generator output. Can you look at the neutral current with a scope? If the generator is developing third harmonics that may account for both the circulating current and the neutral current. If you see any 180 Hz currents or voltages, I will step aside and let a couple of our other posters who are more conversant with harmonic distortion take over.

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

waross (Electrical)
3 Aug 10 2:24
Can you explain a bit more about the distribution transformer failure?
First let's clarify the system;
Many distribution systems use phase to neutral voltage for all their single phase transformers.
Although a three phase transformer may have a line to line, delta primary, any three phase banks built with single phase transformers will be rated for phase to neutral voltage. That may be all that is available from stock.
Second; A wye:delta transformer or transformer bank has issues. If the wye neutral is floated, there may be damaging transient switching voltages when the bank is energized. This is the RUS (originally REA) recommended connection.
Some utilities use four fused cutouts on these banks. The neutral cutout is closed first. Then the phases may be energized without over voltage transients. Once the bank is online, the neutral fuse is removed and carried away in the service truck to avoid possible circulating currents and backfeeds.
Some utilities in the third world connect the wye point to the system neutral.
This is the cause of the circulating currents and burn outs.
Example: A bank of transformers has 2% impedance voltage. A 6% primary voltage drop in one primary phase will cause full load current to circulate in the delta. This is a simplification that may not be completely accurate but illustrates that a relatively small primary voltage error will cause a large secondary circulating current.
I am a little short of time. I will leave you to consider the result of a distribution circuit that loses one primary feed. Consider many single phase transformers distributed across the three phases and look at the wye delta bank as two transformers energized in open delta and one transformer across the open delta and back feeding the loads on the missing phase.
Draw a couple of diagrams. Open delta on phases "A" and "B".
Back fed transformer on "C" phase.

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

pwrtran (Electrical)
3 Aug 10 22:59
Can you check the frequency and PF for the neutral?

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