Introduction
Three phase transformer consists of three sets of primary windings, one for each phase, and three sets of secondary windings wound on the same iron core. Separate single-phase transformers can be used and externally interconnected to yield the same results as a 3-phase unit.
The primary windings are connected in one of several ways. The two most common configurations are the delta, in which the polarity end of one winding is connected to the non-polarity end of the next, and the star, in which all three non-polarities (or polarity) ends are connected together. The secondary windings are connected similarly. This means that a 3-phase transformer can have its primary and secondary windings connected the same (delta-delta or star-star), or differently (delta-star or star-delta).
But when the primary and secondary windings are connected differently, the secondary voltage waveforms will differ from the corresponding primary voltage waveforms by 30 electrical degrees. This is called a 30 degree phase shift. When two transformers are connected in parallel, their phase shifts must be identical; if not, a short circuit will occur when the transformers are energized.”
Basic Idea of Winding
An ac voltage applied to a coil will induce a voltage in a second coil where the two are linked by a magnetic path. The phase relationship of the two voltages depends upon which ways round the coils are connected. The voltages will either be in-phase or displaced by 180 degree.
When 3 coils are used in a 3 phase transformer winding a number of options exist. The coil voltages can be in phase or displaced as above with the coils connected in star or delta and, in the case of a star winding, have the star point (neutral) brought out to an external terminal or not.
Six Ways to wire Star Winding:
Six Ways to wire Delta Winding:
Polarity
An AC voltage applied to a coil will induce a voltage in a second coil where the two are linked by a magnetic path. The phase relationship of the two voltages depends upon which way round the coils are connected. The voltages will either be in-phase or displaced by 180 deg.
When 3 coils are used in a 3 phase transformer winding a number of options exist. The coil voltages can be in phase or displaced as above with the coils connected in star or delta and, in the case of a star winding, have the star point (neutral) brought out to an external terminal or not.
When Pair of Coil of Transformer have same direction than voltage induced in both coil are in same direction from one end to other end. When two coil have opposite winding direction than Voltage induced in both coil are in opposite direction.
Winding connection designations
- First Symbol: for High Voltage: Always capital letters.
- D=Delta, S=Star, Z=Interconnected star, N=Neutral
- Second Symbol: for Low voltage: Always Small letters.
- d=Delta, s=Star, z=Interconnected star, n=Neutral.
- Third Symbol: Phase displacement expressed as the clock hour number (1,6,11)
Example – Dyn11
Transformer has a delta connected primary winding (D) a star connected secondary (y) with the star point brought out (n) and a phase shift of 30 deg leading (11).
The point of confusion is occurring in notation in a step-up transformer. As the IEC60076-1 standard has stated, the notation is HV-LV in sequence. For example, a step-up transformer with a delta-connected primary, and star-connected secondary, is not written as ‘dY11’, but ‘Yd11’. The 11 indicates the LV winding leads the HV by 30 degrees.
Transformers built to ANSI standards usually do not have the vector group shown on their nameplate and instead a vector diagram is given to show the relationship between the primary and other windings.
Vector Group of Transformer
The three phase transformer windings can be connected several ways. Based on the windings’ connection, the vector group of the transformer is determined.
The Determination of vector group of transformers is very important before connecting two or more transformers in parallel. If two transformers of different vector groups are connected in parallel then phase difference exist between the secondary of the transformers and large circulating current flows between the two transformers which is very detrimental.
Phase Displacement between HV and LV Windings
The vector for the high voltage winding is taken as the reference vector. Displacement of the vectors of other windings from the reference vector, with anticlockwise rotation, is represented by the use of clock hour figure.
IS: 2026 (Part 1V)-1977 gives 26 sets of connections star-star, star-delta, and star zigzag, delta-delta, delta star, delta-zigzag, zigzag star, zigzag-delta. Displacement of the low voltage winding vector varies from zero to -330° in steps of -30°, depending on the method of connections.
Symbol for the high voltage winding comes first, followed by the symbols of windings in diminishing sequence of voltage. For example a 220/66/11 kV Transformer connected star, star and delta and vectors of 66 and 11 kV windings having phase displacement of 0° and -330° with the reference (220 kV) vector will be represented As Yy0 – Yd11.
The digits (0, 1, 11 etc) relate to the phase displacement between the HV and LV windings using a clock face notation. The phasor representing the HV winding is taken as reference and set at 12 o’clock. Phase rotation is always anti-clockwise. (International adopted).
Use the hour indicator as the indicating phase displacement angle. Because there are 12 hours on a clock, and a circle consists out of 360°, each hour represents 30°.Thus 1 = 30°, 2 = 60°, 3 = 90°, 6 = 180° and 12 = 0° or 360°.
The minute hand is set on 12 o’clock and replaces the line to neutral voltage (sometimes imaginary) of the HV winding. This position is always the reference point.
Example
- Digit 0 =0° that the LV phasor is in phase with the HV phasor
Digit 1 =30° lagging (LV lags HV with 30°) because rotation is anti-clockwise. - Digit 11 = 330° lagging or 30° leading (LV leads HV with 30°)
- Digit 5 = 150° lagging (LV lags HV with 150°)
- Digit 6 = 180° lagging (LV lags HV with 180°)
When transformers are operated in parallel it is important that any phase shift is the same through each. Paralleling typically occurs when transformers are located at one site and connected to a common bus bar (banked) or located at different sites with the secondary terminals connected via distribution or transmission circuits consisting of cables and overhead lines.
| Phase Shift (Deg) | Connection | ||
| 0 | Yy0 | Dd0 | Dz0 |
| 30 lag | Yd1 | Dy1 | Yz1 |
| 60 lag | Dd2 | Dz2 | |
| 120 lag | Dd4 | Dz4 | |
| 150 lag | Yd5 | Dy5 | Yz5 |
| 180 lag | Yy6 | Dd6 | Dz6 |
| 150 lead | Yd7 | Dy7 | Yz7 |
| 120 lead | Dd8 | Dz8 | |
| 60 lead | Dd10 | Dz10 | |
| 30 lead | Yd11 | Dy11 | Yz11 |
The phase-bushings on a three phase transformer are marked either ABC, UVW or 123 (HV-side capital, LV-side small letters). Two winding, three phase transformers can be divided into four main categories
| Group | O’clock | TC |
| Group I | 0 o’clock, 0° | delta/delta, star/star |
| Group II | 6 o’clock, 180° | delta/delta, star/star |
| Group III | 1 o’clock, -30° | star/delta, delta/star |
| Group IV | 11 o’clock, +30° | star/delta, delta/star |
| Minus indicates LV lagging HV, plus indicates LV leading HV | ||
Clock Notation 0 (Phase Shift 0)
Clock Notation 1 (Phase Shift -30)
Clock Notation 2 (Phase Shift -60)
Clock Notation 4 (Phase Displacement -120)
Clock Notation 5 (Phase Displacement -150)
Clock Notation 6 (Phase Shift +180)
Clock Notation 7 (Phase Shift +150)
Clock Notation 11 (Phase Shift +30)
To be continued…
Sugandh
How can I tell “Yd11″(which you have mentioned as a step-up) is not a step down transformer.Please explain
Dhimant Gondaliya
sir how can i connect the auto transformer at phase shifting of 40 degree for -20 and +20 degree for deferential delta connection
a Hamd
I am reading an AUTO transformer Name plate and it says;;;; YNa0d11
Can someone explain what does letter a and digit 0 stand for.
Thanks
Santhosh Ramayanam
“a” stands for auto-transformer.
M. Husnain
Why Vector group Yd3, Dy3 are omitted however they are possible connections?
S Srinivasan
Sir. Why all the distribution transformers in India is generally Dyn11? Any specific reason for that? Why can’t other vector groups not preferred?
Anand K V
Hello Sir,
How shall we mention Vector Group for a 3-Phase LT Transformer 415V/30V with Open Delta configuration for both Primary & Secondary Sides. Phase shift shall be 0 Deg.
Jerry Kwabla Agbo
Am very happy and enjoy your thesis and details.
I work with power company ,please I will like to know how load balancing.On 500kva transformer if Red phase is 400A ,Yellow phase is 345A and Blue phase is 258A lastly Natural current is 211A. Please what do I do on such load balancing.I would be glad if I will get some magazine’s and small tools (instruments) to assist me on my work.
Thanks for your blessing.
My address is ECG Accra west , P.O.Box 6348.North Accra,West African Ghana.
Darren
Really appreciated this article helped me greatly. Explained clearly. looking forward to getting more transformer theory explained
Saadat Ullah
Can i connect Dyn11yn11 parallel with Dy11y11 – DeltaHV side Dual LV winding star connected? I have 7 dry transformers with Dyn11yn11 configration but we didn’t connect neutral with the transformer. Now i am thinking to order a new oil transformer of Dy11y11 configuration. According to my understanding, there won’t be any problem because the difference is just having neutral connection out or not?
Khan
When you want to go for parallel you have to check follows.
1)both transformer’s same vector group.
2)impedance volts % variation should be within 0.5 %of both transformer’s.
3) ‘0’ voltage should be measured in same phase of one transformer to the second transformer.
Example:TR-1 secondary ‘r’ phase to TR-2 secondary ‘r’ phase voltage should be ‘0’. and repeat for all phases.
Jacqueline Mukangala
ATT: SALES
Kindly quote us for the following;
Transformer
Voltage: 11KV/3.3KV
Rating: 1250KVA
Vector Group: DYn11
Regards,
Jacqueline
PLAGGMAN INVESTMENTS LIMITED
Plot # 1280 Timo Building
Freetown Road, Second Class
Light Industrial Area
Kitwe, Zambia
Office: +260 212 221128
Mobile: +260 963 465173
kiko vargas
my colleague needed to fill out a form recently and located a web service that has a searchable forms database . If people are wanting it as well , here’s a https://goo.gl/5auDai
GOURAV
why we need phase shift at secondary side is it necessary if yes why?
B Raj Thakur
Thanks a lot sir
Md.Siddique Shariful Islam.
Thanks Sir,
We need more …..
ashok
on what vector group a transformer is selected for a particular application.
Fabrice de Lapparent
I have a set up with one stepup transformer 1250 kVA 415 V/22 kV and then another step down transformer 600 m away 650 kVA 22 kV / 415 V. How should I choose the vector group for each transformer. The cos phi is >= .8. The application is for a resort hotel.
Thank you
K.V.RAO
Nice explanation on vector groups of transformers. Pl comment on doubts raised on the content as the explanation taken in right earnest by many engineers. Also pl explain the significance of 30 degree multiple. Thanks a lot to the author for helping many engineers in learning Electrical Engineering in a layman’s style. My best wishes for many more contributions
Mihai
Very good explanation, but one question rises: what’s the reason for all that variety of vector groups (I mean, why is it required to have that many phase shifts combinations, for what use)? Thanks.
Saha
When we try connect different transformer in parallel
Abdelazeem
thanks a lot to your your discussion very good thanks a lot
raghubir singh khokher
How to confirm which vector group one three phase transformer belongs to, please explain calculations & connections made for
different vector groups for testing
Deepak
Can we have common grounding/earthing for neutral of two or more power transformers and DG sets (at industrial units). If not then why. Also can we have common earthing for neutral and body of transformer.
sajjad
dear sir, i want to know why it is so important to build vector group in a transformer, why we not use a transformer without any vector group
Rabiul
I will convert 360-380v to 415 v. What will be vector group for that. Both windings are Star.
Aqeel
If you say the rotation anti-clock then TR. Dyn11 LV lags HV by 30 degree and Dyn1 LV leads HV by 30 degree . I think clock – wise is correct
jay
Generally direction is taken as anticlockwise.
phaneendra kumar
Why the transformers are not designed at Higher impedance (>20%) even though current density suits the short circuit temperature(Asper IEC—180 degrees )?Please clarify the doubt ?
Shayaque Safi
As far as you increase the impedance, short circuit current reduces but at the same time efficiency decreases.
Bob Robinson
My question is ‘why did we need different types in the first place?’
Francisco Gomez
Thanks for your explanation about vector group. I would like that you verify this affirmation. If i have a transformer Dy11 step down (33/0.415kV) connected in series with a transformer Dy11 step up (11/0.415kV), we are going to have 0 degrees of difference between 33kV and 11kV sides.
Thanks
mojtaba
thank you very much for your preamble
mustafa
Hi
I was working in a project and I have seen that there are two different power transformers has been connect in parallel even they were have different primary and secondary connection.
one was Dy11 and the other was Yd5.
any one can explain how this performed.
Chet Ram Yadav
Dear Sir,
I have faceting problem in testing of Transformer Vector Group DY11Y11Y11Y11,(380VX4/33000V for solar Plant) I have ask to client for factory test report but he is not providing me.
You are requested please help me, I am electrical engineer and working for solar power plant first time.
vijay
Hello sir,
I have design 480kva trx.Dry type.On primary side Delta connection & on secondary side Star connection.
& both have 2 layers of winding.
I want vector group Dy1 then what is connection (both delta & star)plz guide me.my email idvijaychavan100@gmail.comThank you
s.balasubramanian
Vector diagram is least understood by many. Most important is how you draw vector diagram when trafo are connected in any manner and how to draw conclusion on phase shift. can u develop some OR if it is OK I can try my hand at it
YOGESH SHARMA
Sir
what in the mean of Lio vector group and why we can you the other vector group in place of this vector group.
bharat baravaliya
sir, we have to need ans of diff between vector group of dy1 and dy11 and als0 dy1 is used instead of dy11?
Ananda kumar J
your explanation is very good
Elumalai Ramasamy
Dear Sir We need Phase Shifting Transformer Design details
jonathan
very good explanation and great mind
Ezeh Doris Oluchi
From the Vector group YznII of a particular transformer, how would you know that the Ratio test on the HV side of a transformer has failed? ie from which range(the input to the output should be before you can say it has failed )
shail
what is the use of vector group (phase displacement) in distribution transformer. can i use that transformer which have no phase displacement in voltage b/w praimary and secendry w/d.
NANJAPPA
it is a good collection and enjoy reading and for working practical experience in international market.
Muralikrishnan
3ph ac magnetic mutual indexon vector groups and clock position super
shivani
This info is realy useful because the connections of vector groups r confusing me very much but now i understand
Heri
What i need if will install synchronizer generator to bus if VT sensing at before and after transformer ?. The transformer vector is Yd11.
imamhussain
thanks for giveing above electricle information for vecter group
Chris
Very useful, but often these articles remind me of my university days where the real world application of the information is missed.
Can you please explain why and when you would use these different vector groups on a transformer?
Thanks
Rohan Thurstans
Mostly a good article except Winding Connection Designation for star is Y not S. But more importantly most of the clock notation vector diagrams are wrong. Please fix these I have had two people this week give me wrong vector diagrams because they copied your errors.
Gobinda
Kindly tell which diagrams are wrong & explain.
devadath g
How to find out Different vector group ?
masood
Thank you. when it will be continued?
aziz
whats the difference between Dyn11 and Dyn5 in 3-phase transformers, which connection better.
thx
Cookiess
Why it does not have Yy4 (120 lagging) ?
High voltage winding; use Y0
and Low voltage winding; use Y8
after i read your article. i think my above connection we can get Yy4 (120 lagging)
Please advise.
feras mahfoud
thanks a lot for that article.
al amin osman omer
very nice
Srikant
Why do dd and yy winding cannot have 30 or 60 degree phase shift in vector group formation.
Beny pardede
Very nice article
but I think your Additive and Substractive diagram is not right.
:)
Radhakrishnan K A
Dear Jignesh
This is an article well explained in common mans language.I have been an avid reader of many an article published by you and every article is just great.Thanks for sharing all this.
Radhakrishnan K A
mdburanou
Please check the additive polarity symbolic daigram
vasanthu
I have a Delta/Star Step down transformer with a vector group Dyn11 written on the name plate. From the article here I understood that secondary side winding is phase shift by 30 degrees with respect to primary side winding. They also say that this configuration eliminates third harmonics in the system. I don’t clearly understand how it is helping to eliminate the third harmonics. I would appreciate if some one could give a better explanation on this with graph or equations if possible. Thanks a lot..
rkkatrix
simple and excellent … great work jignesh !!!!
khuram388
realy great effort sir
yogesh parmar
dear jignesh sir
your are really a great contributor in EEP.your all articles are mindblowiing.
thanking you
from yogesh parmar
saad1966
really it is wonderful effort…..thanks so much
rachidakoum
really it is wonderful effort.
thanx
Edvard
This is the far best explanation of vector group meaning and usage! Thank you very much Jignesh!