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…
Dear Sir ,
Please send Vector Group Condition Yd1 for 3 Ph. Step down Transformer
Dear All this is not connected to vector group, i could not find the place for request, Can we get circuit diagram/ connection drawing for the protection relays like,3OC/EF,Differenctial relay, master trip, motor protection, etc. otherwise any hand book is available please let me know. This is for we want to teach every protection relays working principle and connection to our trainee engineers.
Please support us thank you
This is very help full article; but what my question is what is the effect of vector group transformer on it’s function.
In situations of parallel operation, where more than one transformer has to be connected together, both or all of the connected transformers have to have same vector groups, else this results in circulating currents and other issues that could cause adverse operation of the system and subsequent failure.
Hi Sir,
This is the very clear explanation about the vector groups, very nice,
And what is my question is why we needs to follow multiple vector groups, Commonly can follow only one vector group for each star and delta, what is the advantage of these multiple vector groups? if doing synchronization the vector group must be same, this is the condition, so we can follow any one group for delta and stat, why multiple phase shift used? what is the use of these?
Hi everyone, this article is really good and explain everything very detailed, but certainly there is something that I have tried many times to find without success about what is the criteria to select a group connection for a transformer, for instance a Step-up transformer could be YNd1 or YNd11 what would be the criteria to select whichever of this and how does affect the system (no transformer in parallel). There is something to take account for selecting a group connection?
Yd1 will be used in transmission at generating station to neutral the load angle of alternator which is in between 30 deg lag.
Dy11 will be used in that same system for distributing to balance the voltage so that user will get the same phase shift as gerating voltage and also in Dy11 the secondary side wil also eliminate zero sequence current or earth fault current by connecting the neutral to ground
Hello Fellows.
This the best explanation of the vector grouping.
Thanks.
is it possible to parallel YNyno and YNd11 Transformers. What will be happen if it paralled
In case we have step up transformer 400v/33KV 2,5 MVA is it correect the vector group Ynd-11
If HV side (Secondary side i.e. 33KV) is star winding and primary is in delta then YnD11 is OK.
One error though: “S=Star” is not correct. Instead, the symbol “Y” is used, aka “Wye”. Note that this configuration resembles a star, that is correct.
This is very helpful article for understanding and making of transformer.
One thing need to know.
If we need to connect two transformers in parallel with 2 different vector groups , what would be the sequence?
Very useful and helpful article, I am also an Electrical engineer and doing job in electrical maintenance in steel industries from last more than 18 years.
Dear Sir
Good morning.Your article is very good.
Sir I am intrested in panel designing and testing,can u send details regarding this on mail id.
Thank you, Jitendra. Try searching EEP for the terms “panel design”, I’m sure you will find a lot of resources (guides and articles).
dear let me ask some quarry about vector group of transformer.
lets assume a transformer having YNd11 VECTOR GROUP.
other one having YNd5 vector group .
so tell me where we will prefer to YNd11 at YNd5 ?
Hello,
Is is possible to connect only two transformers with two different vector group to each other?
There would be no other transformers on the grid. Only to transformer connected to eachother but different vector groups…
Thank you
Sir,
If generator transformer vector group is Dyn1 and Distribution Transformer vector group is also Dyn1 i.e. phase displacement exist between generator and load voltage. Is there any problem in power flow if transformer paralleling is not required? What problems may arise from this phase displacement except transformer paralleling?