Understanding Vector Group of Transformer (Part 1)
Understanding Vector Group of Transformer (Part 1)


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).

It’s important to remember that the secondary voltage waveforms are in phase with the primary waveforms when the primary and secondary windings are connected the same way. This condition is called “no phase shift.”

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 Star Winding
Six Ways to wire Star Winding

Six Ways to wire Delta Winding:

Six Ways to wire Delta Winding
Six Ways to wire Delta 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 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.

Additive and substractive polarity of transformer
Additive and substractive polarity of transformer

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 transformer vector group is indicated on the Name Plate of transformer by the manufacturer. The vector group indicates the phase difference between the primary and secondary sides, introduced due to that particular configuration of transformer windings connection.

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.

Hardly any power system adopts such a large variety of connections. Some of the commonly used connections with phase displacement of 0, -300, -180″ and -330° (clock-hour setting 0, 1, 6 and 11).

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.


  • 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
30 lagYd1Dy1Yz1
60 lagDd2Dz2
120 lagDd4Dz4
150 lagYd5Dy5Yz5
180 lagYy6Dd6Dz6
150 leadYd7Dy7Yz7
120 leadDd8Dz8
60 leadDd10Dz10
30 leadYd11Dy11Yz11

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 I0 o’clock, 0°delta/delta, star/star
Group II6 o’clock, 180°delta/delta, star/star
Group III1 o’clock, -30°star/delta, delta/star
Group IV11 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 0 (Phase Shift 0)
Clock Notation 0 (Phase Shift 0)

Clock Notation 1 (Phase Shift -30)

Clock Notation 1 (Phase Shift -30)
Clock Notation 1 (Phase Shift -30)

Clock Notation 2 (Phase Shift -60)

Clock Notation 2 (Phase Shift -60)
Clock Notation 2 (Phase Shift -60)

Clock Notation 4 (Phase Displacement -120)

Clock Notation 4 (Phase Displacement -120)
Clock Notation 4 (Phase Displacement -120)

Clock Notation 5 (Phase Displacement -150)

Clock Notation 5 (Phase Displacement -150)
Clock Notation 5 (Phase Displacement -150)

Clock Notation 6 (Phase Shift +180)

Clock Notation 6 (Phase Shift +180)
Clock Notation 6 (Phase Shift +180)

Clock Notation 7 (Phase Shift +150)

Clock Notation 7 (Phase Shift +150)
Clock Notation 7 (Phase Shift +150)

Clock Notation 11 (Phase Shift +30)

Clock Notation 11 (Phase Shift +30)
Clock Notation 11 (Phase Shift +30)

To be continued…

About Author //


Jignesh Parmar

jiguparmar - Jignesh Parmar has completed M.Tech (Power System Control) ,B.E(Electrical). He is member of Institution of Engineers (MIE),India. Membership No:M-1473586.He has more than 13 years experience in Transmission -Distribution-Electrical Energy theft detection-Electrical Maintenance-Electrical Projects (Planning-Designing-Technical Review-coordination -Execution). He is Presently associate with one of the leading business group as a Deputy Manager at Ahmedabad,India. He has published numbers of Technical Articles in "Electrical Mirror", "Electrical India", "Lighting India", "Industrial Electrix"(Australian Power Publications) Magazines. He is Freelancer Programmer of Advance Excel and design useful Excel base Electrical Programs as per IS, NEC, IEC,IEEE codes. He is Technical Blogger and Familiar with English, Hindi, Gujarati, French languages. He wants to Share his experience & Knowledge and help technical enthusiasts to find suitable solutions and updating themselves on various Engineering Topics.


  1. Rabiul
    Sep 16, 2015

    I will convert 360-380v to 415 v. What will be vector group for that. Both windings are Star.

  2. Aqeel
    Sep 02, 2015

    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

  3. phaneendra kumar
    Aug 02, 2015

    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 ?

  4. Bob Robinson
    Jul 09, 2015

    My question is ‘why did we need different types in the first place?’

  5. […] delta-connected windings the single and three-terminal testings can be combined by applying the impulse to two line […]

  6. Francisco Gomez
    Jun 03, 2015

    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.

  7. mojtaba
    Jun 01, 2015

    thank you very much for your preamble

  8. mustafa
    May 21, 2015

    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.

  9. Chet Ram Yadav
    May 16, 2015

    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.

  10. vijay
    Apr 22, 2015

    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

  11. s.balasubramanian
    Mar 23, 2015

    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

    Mar 08, 2015

    what in the mean of Lio vector group and why we can you the other vector group in place of this vector group.

  13. […] If these connections are made using an internal terminal board, check to ensure that these connections are made properly according to the chart on the transformer nameplate. […]

  14. bharat baravaliya
    Feb 23, 2015

    sir, we have to need ans of diff between vector group of dy1 and dy11 and als0 dy1 is used instead of dy11?

  15. Ananda kumar J
    Feb 16, 2015

    your explanation is very good

  16. Elumalai Ramasamy
    Feb 06, 2015

    Dear Sir We need Phase Shifting Transformer Design details

  17. jonathan
    Jan 20, 2015

    very good explanation and great mind

  18. Ezeh Doris Oluchi
    Jan 09, 2015

    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 )

  19. shail
    Jan 06, 2015

    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.

    Nov 29, 2014

    it is a good collection and enjoy reading and for working practical experience in international market.

  21. Muralikrishnan
    Nov 03, 2014

    3ph ac magnetic mutual indexon vector groups and clock position super

  22. shivani
    Oct 21, 2014

    This info is realy useful because the connections of vector groups r confusing me very much but now i understand

  23. Heri
    Sep 26, 2014

    What i need if will install synchronizer generator to bus if VT sensing at before and after transformer ?. The transformer vector is Yd11.

  24. imamhussain
    Sep 02, 2014

    thanks for giveing above electricle information for vecter group

  25. Chris
    Aug 21, 2014

    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?


  26. Rohan Thurstans
    Aug 01, 2014

    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
      Sep 12, 2014

      Kindly tell which diagrams are wrong & explain.

  27. devadath g
    Jul 29, 2014

    How to find out Different vector group ?

  28. masood
    Jul 17, 2014

    Thank you. when it will be continued?

  29. aziz
    Jun 04, 2014

    whats the difference between Dyn11 and Dyn5 in 3-phase transformers, which connection better.

  30. Cookiess
    May 14, 2014

    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.

  31. feras mahfoud
    May 14, 2014

    thanks a lot for that article.

  32. al amin osman omer
    May 14, 2014

    very nice

  33. Srikant
    Apr 17, 2014

    Why do dd and yy winding cannot have 30 or 60 degree phase shift in vector group formation.

  34. Beny pardede
    Apr 16, 2014

    Very nice article
    but I think your Additive and Substractive diagram is not right.

  35. […] learn more about phase difference you need to learn about vector group. This link will be useful http://electrical-engineering-po…Embed QuoteUpvote • Comment Loading… • 1m ago Loading… […]

  36. […] the induced voltage reading into the applied voltage value. When ratio tests are being made on three-phase transformers, the ratio is taken on one phase at a time with a three-phase TTR until the ratio measurements of […]

  37. Radhakrishnan K A
    Dec 10, 2013

    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

  38. […] Understanding Vector Group of Transformer (part 1) How AM and FM Works Building a wireless energy transfer circuit step-by-step Five Lightweight Linux Distros For Your Old PC Puppy Linux This is the smallest Linux distro at just 132MB download. The distro loads itself entirely into RAM on bootup, so is incredibly quick and responsive. Top 7 Certifications In Programming Languages 1) Ruby on RailsThis is essentially a good certification program and its certification helps you confirm the skills of IT professionals and is controlled by ITOCHU Techno-Solutions Corporation. This certification is granted by the Ruby Association LLC. This exam involves questions related to execution environment in Ruby, command line option, incorporated classes, grammar, incorporated functions, and object orientation. 2) JavaScriptThe CIW Web Development Series features three dedicated exams that you need to qualify so as to get the Web Development Professional title, which are Perl, Database Design and Javascript. A Cool Experiment: The IceCube Neutrino Observatory A traditional neutrino detector Up until the turn of the 21st century, neutrino detectors were pretty much unheard of. This made the hunt for the plentiful (but elusive) subatomic particle a lot more difficult than it needed to be. However, we recently created an observatory that will revolutionize our understanding of the universe. This observatory just happens to be located in one the coldest, most inhospitable place on Earth. fullnews 1. Build Your Own PCby Morris Rosenthal – McGraw-Hill, 2004As the introduction of the book says, "This visual how-to manual is loaded with photographs and images to help you see how to assemble from scratch your PC easily. This guide will lead you through making preparations to build your PC, which includes guidance selecting the right parts." 2. The Secret Guide to Computersby Russ Walter – Russell Walter Pub, 2003As the introduction of the book says, "The book explains everything important about computers — simply, candidly, and as wisely as possible, with a cynical laugh. 25 Best Linux Apps 1. Nagios for Monitoring – This app is for free. Its license status is GPL. The app lies inactive on your computer system monitoring host resources and network services with the aim of notifying the admin in the event a problem arises. You can download the app from http://www.nagios.org 2. How to Get into FPGA The FPGA business is growing at a rapid pace and so is the technology. A bachelor’s degree in electronics, electrical or computer science and, of course, a good understanding of the topic is all it takes to launch your career in FPGA view_article view_article The wireless security hub by Maven Systems is a security solution for homes and small offices that can detect and report fire, gas leakage or intrusion. The alerts are almost real-time (within minutes of the detection), and the information reaches the owner, neighbours and security guards in the housing colony or workplace almost instantaneously. FlashbackSuhas Jain, project manager, Maven Systems, says, “Safety and security is a must-have in today’s world. However, most of today’s systems require advanced planning to install such systems. This means existing houses need chiseling of walls for laying additional cables. Temperature is the most often measured environmental quantity and many biological, chemical, physical, mechanical and electronic systems are affected by temperature. Some processes work well only within a narrow range of temperatures. So proper care must be taken to monitor and protect the system. When temperature limits are exceeded, electronic components and ciruits may be damaged by exposure to high temperatures. Temperature sensing helps to enhance circuit stability. view_article Robotics […]

  39. mdburanou
    Nov 06, 2013

    Please check the additive polarity symbolic daigram

  40. vasanthu
    Oct 22, 2013

    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..

  41. rkkatrix
    Jul 25, 2013

    simple and excellent … great work jignesh !!!!

  42. […] Voltage = Secondary Side Line Voltage.Primary side Phase Voltage= Secondary Side Phase Voltage.No phase shift between primary and secondary voltages. Advantages of Delta-Delta Transformer ConnectionSinusoidal […]

  43. […] While the HV end shows 12 (0), the LV end shows 11 o’clock (after 330°).Determining the connection (vector) group is valid only in three phase transformers. The high voltage winding is shown first (as reference) […]

  44. khuram388
    Feb 20, 2013

    realy great effort sir

  45. yogesh parmar
    Nov 20, 2012

    dear jignesh sir

    your are really a great contributor in EEP.your all articles are mindblowiing.
    thanking you
    from yogesh parmar

  46. […] = 280; Circuit switchers have been developed to overcome some of the limitations of fusing for substation power transformers. Circuit switchers have SF6 gas interrupters and are designed to provide three-phase interruption […]

  47. saad1966
    Aug 07, 2012

    really it is wonderful effort…..thanks so much

  48. […] Zigzag Grounding Transformer (Post Glover Resistors, Inc.) The zigzag connection of tranformer is also called the interconnected star connection.  This connection has some of the features of […]

  49. rachidakoum
    Jun 05, 2012

    really it is wonderful effort.

  50. […] 220kV Class 110kV Class Single Phase Traction Transformer Continued from first part Understanding Vector Group of Transformer (part 1) Points to be consider while Selecting of Vector GroupVector Groups are the IEC method of […]

  51. Edvard
    Jun 03, 2012

    This is the far best explanation of vector group meaning and usage! Thank you very much Jignesh!

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