Phase angles on DYN11 transformers 4

[Area]

Dear all,

I have an interesting problem, that I cannot explain:
We have three transformers vector group DYN11, 10/0.4kV. All three transformers are supplied from the same medium voltage substation and each of them feeds one low voltage distribution. The star points of the transformers are connected to ground on the low voltage side.

A measurement on the low voltage site of the three transformers (between the transformers) results as follows:
Rotating field in all three low voltage distrubutions is equal!
U Tr1 - U Tr2 : 0V
V Tr1 - V Tr2 : 0V
W Tr1 - W Tr2 : 0V
(That seems OK so far)
U Tr1 - U Tr3 : 230V
V Tr1 - V Tr3 : 230V
W Tr1 - W Tr3 : 460V
U Tr2 - U Tr3 : 230V
V Tr2 - V Tr3 : 230V
W Tr2 - W Tr3 : 460V

It seems to me, as Tr3 is not connected in a proper way. But what might be the cause? A false vector group (DYN5 instead DYN11) would cause a 180° angle and therefore the difference in all phases should be 460V (correct?). A confusion on two phases (e.g. on the primary side) would cause a change of sense in the rotation field, which is not the case.

Any ideas?

 

[PwrCntEE]

A client had one of the tranformer fault and needed rebuilt. They got the rebuilt tranformer back and put it back in service.

We were installing a tie switch between this and another transformers. As part of commissioning, we noticed a similar problem. As a last resort, since it was just rebuilt, we consulted the rebuilder of the transformer. They picked it up right away. They had crossed the phasing inside the transformer. I do not remember our readings exactly, but yours sound familiar (there are several connections that will result in numerous combinations). The fact that the differnece is common to #1 and #2, I say #3 is improperly phased inside.

 

[busbar]

Sketch six phasors 60° apart.

0° - U1 - 3 o’clock
60° - U3 – 1 o’clock
120° - W1 – 11 o’clock
180° - V3 – 9 o’clock
240° - V1 – 7 o’clock
300° - W3 - 5 o’clock

 

[busbar]

Reversing two primary connections of one ∆-Y transformer with respect to the other may produce the described secondary-voltage relationship with 'normal' secondary-winding internal connections.

Are R-S-T the corresponding primary designations?

 

[rbulsara]

Not only the two connection have to be reversed, but also there has to be a 30 degree phase shift between T3 and other two tranfromers which will have the phasor diagram described by 'busbar' and the reading you are getting.

T3 internal connections are not the same as T1 and T2, regardless of what the namepate reads.

 

[Kiribanda]

Hi Area,

From your voltage measurements it can be proved that ONE transformer- ONLY ONE- is fed by the reverse phase sequence on the primary. As a result you have got a 3-vector set similar to an inverted star, which gives a hexagon with 230 V on each side. Therefore the remedial action should be to provide the “faulty” transformer with the standard sequence on the primary.

How to correct this in the field? My suggestion is this.

The most convenient way is to use a phase sequence meter connected to each secondary of the transformer out put terminal and identify which one is giving the reverse sequence. ( If the transformers are labeled properly then it is again easy to identify the u,v,w on the secondary bushings. Otherwise you have to stand facing the HV SIDE and then identify the corresponding u,v,w terminals on the secondary ) After identifying the reversed one, correct it ONLY from the primary and then confirm that all three are fed with the standard sequence of supply. Finally your voltage measurements should be all zero!!

Good Luck!

Kiribanda

 

[rbulsara]

One more piece of info needed:

Are all three transfromers fed from the same primary bus/feeder or different feeders?

If on different feeders, it is possible that one the T3 feeder has 30 degree phase shift, because of different trasnformation.

 

[jghrist]

A 30° phase shift wouldn't produce the reported voltages. Reverse phase sequence on one of the transformers as noted by Kiribanda would. This results in a 60° phase shift on the secondary. This is probably the cause - two phases are reversed on the primary of one of the transformers.

 

[rbulsara]

jgrisht:

There has to be a 30 degree shift AND reversal of two phases to get his 'steady' voltage results,as posted in previous posts.

Just reversing two phases will produce 0 volts in the between the phase that is not reversed and 460 in the other two.
(assuming fed from the same bus, so frequecny is locked)

 

[jbartos]

Suggestion: Three identical transformers connected the same way on the primary should have 0V readings among exactly corresponding secondary terminals. This is necessary requirement for transformers direct paralleling. The transformer winding termination at the transformer terminals should follow the specific industry standard.
Which industry standard are those transformers built to?

 

[stevenal]

I'll go along with busbar and jghrist on this. Been there and done it. Winding is okay, high side connection isn't. If we had a phasor diagram of this thing, we could tell you which two to swap.

 

[stevenal]

Maybe I can tell you anyway. Locate phasor W on wye side of your transformer phasor diagram. Find the phasor that parrallels it on the delta side. Reverse the two leads that connect to this winding on transformer three.

 

[mersin33]

U1: 3 o'clock (0 deg)
W3: 1 o'clock (60 deg)
V1: 11 o'clock (120 deg)
V3: 9 o'clock (180 deg)
W1: 7 o'clock (240 deg)
U3: 5 o'clock (300 deg)

As seen here, the voltages read between two trfs are 230, 230 and 460.

 

[jghrist]

If the primary is the normal (UVW) counterclockwise phase rotation, the vectors given by saltan indicate that transformers 1 and 2 have their primary phases swapped. This makes the secondary (UVW) clockwise phase rotation. Only transformer 3 has UVW counterclockwise rotation on the secondary.

 

[rbulsara]

Please review the sketch I uploaded at

http://www.geocities.com/neural3/vectors.doc

and please comment!

 

[rbulsara]

Try

http://www.geocities.com/neural3/mypage.html

 

[rbulsara]

Try

http://www.geocities.com/neural3/mypage.html

and click on vector.doc link

(sorry, I am trying this first time..linking uploaded docs...)

 

[rbulsara]

My apologies. Firstly, the vector diagram is drawn correctly and even the voltages are correct but anlges mentioned were wrong.

The phase difference has to be 60 degrees and which will be created by phase reversal on the primary side AND in order to correct the rotation the secondaries are reversed therefore creating a 60 degree phase shift between two of the phases and 180 on the third. This will give you the 230, 230, 460V results.

I will update my diagram.

 

[rbulsara]

Updated vector diagram:

http://www.geocities.com/neural3/mypage.html

and click on vector.doc link

 

[jbartos]

Suggestion: Visit

http://www.emsyst.com/products/netvision.htm

for an opposite rotation on the meter, namely, U1, U2, U3 in clockwise direction instead of A, C, B in clockwise direction.

http://www.themeterguy.com/Theory/therules.htm

Reference:
1. William D. Stevenson, J.R., Elements of Power System Analysis, 3rd Edition, McGraw-Hill Book Company, 1975,
page 27 Figure 2.14 and Figure 2.15
indicate the phase rotation in clockwise direction as A, B, C.