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?

 

[stevenal]

Rbulsara,

Try drawing the delta side first, and reversing the connections there. Then carry it over to the wye side. Simply swapping two leads on the delta side will give you the 60 degree shift and phase reversal shown in your last diagram.

 

[rbulsara]

Stevenal:

Let us review it again.

Remember, you need to SUBTRACT the phasors not 'add' (we measuring the differnce in voltage at two points) as it is misleading to look at the phasors with 60 degree difference and just add them.

A simple phase revesal on the primary side, should and would give a simple phase revrsal on the secondary side!!
That will give you the voltage readings of 0, 400 and 400V. As seen in the second picture on the top (on the right)in my diagram.

You have to reverse phasing again on the secondary side to get my last diagram.

 

[stevenal]

Thanks for the review, rbulsara, but I wasn't adding anything. I use a simple graphical approach.

This phenemonon puzzeled me when I first encountered it. Phasing sticks read line to ground and double line to ground voltages across an open switch. While I was still scratching my head, operations rolled two high side connections and cleared the problem. Determined to avoid that particular embarassment again, I sketched out the phasors. The drawing is a tool I keep handy for when it reoccurs. Unfortunately, it has reoccured, but at least I had an answer. Not really the answer my boss wanted to hear, but correct anyway. You may persist in your attempts to prove that bumblebees can't possibly fly, but I prefer to understand how they do fly.

Get it fixed yet Area?

 

[jghrist]

rbulsara,

As Stevenal says, you have to go back to the delta primary. Your diagrams do not show the proper phase relationships between the delta and wye sides. Try this:

Turns ratio n = 10000/230

Primary side voltages:

V_A = 5774@0°
V_B = 5774@-120°
V_C = 5774@120°
V_AC = V_A-V_C = 10000@-30°
V_BA = V_B-V_A = 10000@-150°
V_CB = V_C-V_B = 10000@90°

Secondary side voltages:

V_a = V_AC/n = 230@-30°
V_b = V_BA/n = 230@-150°
V_c = V_CB/n = 230@90°

This is the normal Dyn11 (US Std) connection. Now if we reverse voltages B & C:

Primary side voltages:

V'_A = 5774@0°
V'_B = 5774@120°
V'_C = 5774@-120°
V'_AC = V'_A-V'_C = 10000@30°
V'_BA = V'_B-V'_A = 10000@150°
V'_CB = V'_C-V'_B = 10000@-90°

Secondary side voltages:

V'_a = V'_AC/n = 230@30°
V'_b = V'_BA/n = 230@150°
V'_c = V'_CB/n = 230@-90°

The secondary side voltages are now reverse rotation acb counterclockwise 60° shifted from before and:

V'_a-V_a = 230@90°
V'_b-V_b = 230@90°
V'_c-V_c = 460@-90°

 

[stevenal]

Nice work jghrist. Note that V'c to Vc has the largest error, like phases W3 and W1 in the question. Vc is at 90 degrees, corresponding to VCB on the primary, and C and B are the swapped phases. Likewise Area just needs to swap whichever two primary phases line up with W.

 

[rbulsara]

stevenal/jghrist:

Your explantion explains voltages, except the last line of the original question by 'Area' that "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."

So if he just reverses two phases on the primary only, what happens to any motor rotations (if any connected to T3) that are running in 'correct' direction now?

 

[jbartos]

Question to the original posting: Please, would nameplate data of each transformer be available?

 

[mvcjr]

To jghrist:
I think Dyn1 is the standard delta-wye transformer connection in the U.S.A. In Dyn1, VAN leads Van by 30 degrees.

 

[stevenal]

Rbulsara,

They would reverse of course. I would suggest Area recheck this part. I can see no combination of correctly rotating phasors that will give these magnitudes.

 

[jghrist]

mvcjr,

You're right. And, my calculations are for a Dyn1 connection. For a Dyn11 connection:

V_A = 5774@0°
V_B = 5774@-120°
V_C = 5774@120°
V_AB = V_A-V_B = 10000@30°
V_BC = V_B-V_C = 10000@-90°
V_CA = V_C-V_A = 10000@150°

Secondary side voltages:

V_a = V_AB/n = 230@30°
V_b = V_BC/n = 230@-90°
V_c = V_CA/n = 230@150°

This is the normal Dyn11 connection. Now if we reverse voltages B & C:

Primary side voltages:

V'_A = 5774@0°
V'_B = 5774@120°
V'_C = 5774@-120°
V'_AB = V'_A-V'_B = 10000@-30°
V'_BC = V'_B-V'_C = 10000@90°
V'_CA = V'_C-V'_A = 10000@-150°

Secondary side voltages:

V'_a = V'_AB/n = 230@-30°
V'_b = V'_BC/n = 230@90°
V'_c = V'_CA/n = 230@-150°

The secondary side voltages are now reverse rotation acb counterclockwise 60° shifted from before and:

V'_a-V_a = 230@-90°
V'_b-V_b = 460@90°
V'_c-V_c = 230@-90°

 

[stevenal]

You can find a UVW dy11 phasor diagram here:

http://www.riedel-trafobau.de/english/pdf_eng/in_general/explanation.pdf

Looks U and W are reversed on winding 1 (high side) in Area's case.

Don't know why I'm bothering. No responses from Area. Probably had it fixed long ago.

 

[Area]

Dear all,

sorry for the late reply, but I was out off the office for some days and did not have the possibility to check the internet.
Special thanks to stevenal for all the bothering, I really appreciate it. Sorry again for the late reply.

Hope I can answer all your questions so far:

To rbulsara: Yes all transformers are connectet to one (the same) medium voltage busbar. But each transformaer is connected to this busbar via his own short circuit breaker.
I tried to open your sketch, unfortunately our network adimistrator seems to have banned geocities from our proxy server; I only receive a proxy error.

To jbartos: Unfortunately, all three transformers are under tension, meaning that the nameplate is not accessible. I have to wait for the next plant shutdown (mid of October)

To jghrist: Thanks a lot for the phaser description. Excellent work. It is very helpfull. I did not know how to transfer the phasers from the primary to the secondary voltage side. I was also not aware, that a DYN1 vector group exists: As also mentioned in the file that stevenal is referring in his last post, I only knew DYN5 and DYN11 vector groups, where the phase angle between primary and secondary side is -150 / -300 degree. I did not know, that also a DYN1 exists.
It explains perfectly the voltages that me measured, unfortunately, as also mentioned by rbulsara, it does not explain the non-reversed sense of rotation.

So today, we checked again the sense of rotation with an external consultant.(I did not trust in our own measurement any longer). But he came to the same result. We also checked the busbar connection between the low voltage side of the transfomers and the distribution panels: There seems to be a reversed phase on the low voltage side of the faulty transfomer, as the busbar is connected in a different way. The consultant explained me, that we also have a reversed phase on the medium voltage side. This would explain the right sense of rotation.

To stevenal: I fully agree with you, that the phases on the secondary side must be reversed, if a phase on the primary side is reversed. I have now two good explanations:
The phasor diagram of jghrist and the explantion/observation of the consultant. Unfortunately, they do not match. If we have a double reversing of phases, and the phasor diagram of jghrist is correct (what I believe) I would expect to have 400V between U-U, 400V V-V and 460V between W-W.

It still remains I mistory to me. I can reassure you stevenal, problem is not solved yet. I will again contact our consultant and confront him with the diagram of jghrist.

Nevertheless, thanks so far to everybody for the interesting discussion and help.

 

[jghrist]

The only difference between a Dyn1 and a Dyn5 is what you call the secondary phases. The difference between a Dyn11 and a Dyn1 is that the primary phases are reversed internally, producing a 60° phase shift on the secondary. The same thing would occur if you externally reverse the phases on a Dyn11 connection.

 

[stevenal]

I think it takes more than two dimensions to explain these readings. The S plane is no longer a plane. Have you checked phase to phase across the tie point? U1 to V3, U1 to W3, V1 to W3 and so on? I assume that all phase to phase and phase to neutral readings are normal on T3?

Is transformer 3 presently powering three phase motors?

 

[isoA120]

Hi everyone

I am sorry to jump in after the discussion is probably over but after reading it I am a bit confused.

as recommended by stevenal (Electrical) I visited the following site and looked ath the phasor diagram of dy11 etc.

http://www.riedel-trafobau.de/english/pd...

In Protection Relay Application Guide - Alstom, a completely different figure and connection has been given for the same vector group. It also says " The Power transformer vector group references correspond to those specified in IEC 76:1967 and BS 171:1970."

I shall be very thankful if anyone could please claify this.

With kind regards
isoA120

 

[isoA120]

Hi everyone

Pls ignore my question. I just made a mistake reading the PRAG.

Regards
IsoA120