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# Understanding Vector Group of Transformer (Part 1)

Home / Technical Articles / Understanding Vector Group of Transformer (Part 1)

## 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).

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.

## 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 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.

#### 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 11 (Phase Shift +30)

To be continued…

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### Jignesh Parmar

Jignesh Parmar has completed M.Tech (Power System Control), B.E (Electrical). He is member of Institution of Engineers (MIE), India. He has more than 13 years experience in transmission & distribution-energy theft detection and maintenance electrical projects.

1. Rajasekhar Reddy Adala
Dec 02, 2019

Their is also a vector group YNd9, but not told about this.

2. Keyuri R Kariya
Nov 14, 2019

which vector group apply in 24 pulse furnace transformer

3. THAMBI GNANABALAN K
Oct 13, 2019

Dear sir,

#1 In Dy11 condition you have mention 330 deg lagging and 30 deg lead. How it is possible and when.?

#2 In practice Dy11 is connected to load which is mostly lagging power factor how the 30 deg lead is possible ?

• Engr. Mustafizur Rahman
Dec 21, 2019

Vector group has no relation with load power factor. Better you draw a clock marking with 1,2…upto12 and then highlight the point 11. Now if you count how far the Point 11 from 12. You will find that its 330 deg far but in clock wise direction and again only 30 deg far but this time in Anti clock wise direction. Thus 330 deg Lag = (360-330=) 30 deg Lead. Likewise 300 deg Lag= (360-300 =) 60 deg Lead and so on.

• Harihar Behera
Jun 12, 2020

why neutral point not available in induction furnace transformer. The vector group is dy11. Ratio of transformer is 6000 KVA 11/0.690-0.690 kv

4. JOSE PINEDA
Oct 09, 2019

Dear Sir.

I would like to know if someone know about the conection of Ynad .
Please let me know to my email [email protected]

Many thanks.

5. ENGR A A MAHMUD
Aug 23, 2019

Recently in one project we have found that system design as below

Source Power supply by 4nos. 1.5MW Generator ( output 415V, 50Hz) and generator are in star connection, for that use 4nos. step up transformer they use vector group YNd1 ( 415V / 11KV) and after 400M distance where they need power do Step down by 4nos. Transformer use vector group Dyn11 ( 11KV / 415V )

We ask them why they design like this, they informed us that at the time of step up it will be in lagging and at the time of step down it will be leading, so their will be no phase angel. it will work perfectly

Can any one guide us or clarify us more ?

Thanking you in advance for kind support

Best Regards

Mahmud

• Shaukat Amin
Aug 30, 2019

How you protect the earth fault at 11 KV cable.you should use any one side star Y connection for earth.if your 11KV cable ground then append. because you use delta connection on both side of 11 KV.

• Lalremruatkima Khawlhring
Mar 29, 2020

YNd1 is 30°lagging while Dyn11 is 330°lagging (which is similar to 30°leading according to the lecture above). So the resultant angle will be in phase…

6. RatnaKumar
Aug 09, 2019

Nice information…

7. Madan Lal Sachdeva
Aug 08, 2019

Delta connection is provided on the generator side to avoid flow of 3rd harmonics on generator side. The other side may be Star or delta. The star side provides facility for detection of earth fault and operate ground fault protection.In case of delta connection, for ground fault detection ,Zig-zag transformer is required to be provided for creating artificial neutral for passage of current to earth.

In case of delta connection , each phase winding full insulation over the complete length of coil where as in case of Star connection for system above 66kV, the insulation on neutral side is reduced ans cost of transformer is reduced.
As regards selection of vector connection. the delta connection is always connected to generator side and other side connection is why for ground fault detection otherwise, additional Zig -Zag transformer is to be provided with delta connection for ground fault detection.
On load side, the phase angle of the existing bus is to be ascertained and then select the vector arrangement of new transformer. Under unavoidable circumstances, phase shifting transformer would be required to be provided to avoid unbalance voltages.
Neutral connection of star connected winding shall not be left unconnected/ floating from earth as this will cause unbalance phase voltages, avoid detection of ground fault current and may adversely effect the safety of operator/ public.

8. Tansukh
Feb 26, 2019

Dear Sir,

we are doing one power project in abrod middle east and capacity of 4Mwe*4. we choose the each generator 1250kva, 415v to 3.3kv transformer. we interest to avoid NGR. Please suggest transformer

9. Tansukh
Feb 26, 2019

Dear Sir,

We are going on one gas base power project of 6 MWe. power generation is LT 415V with synch. After step up 415 to 11KV. we plan to installation 5 transformer to each generator. we choose transformer capacity of 2000kva*2 and 1600*3. Please suggest vector group. we are not interest to use NGR.

10. Tony Hazouri
Jan 24, 2019

Dear Sir’s

I need to synchronize a steam turbine 6.3 KV With the bus 11 KV
I need to request transformer step up 6.3 KV TO 11 KV
So what is the connection of the transformer should be Dd0 or Yy0 i don’t want any phase shifting.

any idea ?

Thank you

11. Naren
Jan 21, 2019

First of all Thanks for your Article on Vector Groups. All the Points are clear to understand. I’m little bit confused with Dy5 and Dy7 vector group diagrams. Can you please explain that alone for me.

• Ken T
Jul 28, 2019

Have a look at the Delta side, It’s different. The Star side is the same.

Aug 11, 2018

What is principal purpose/necessity of vector grouping for 3 phase transformer ?
Because I didn’t think only for the parallel operation is the main reason behind it but I can’t conclude it properly, so please help me to understand this matter…..

Apr 05, 2018

Thanks to the writer for his wonderful article.

14. Sugandh
Mar 14, 2018

How can I tell “Yd11″(which you have mentioned as a step-up) is not a step down transformer.Please explain

15. Dhimant Gondaliya
Mar 13, 2018

sir how can i connect the auto transformer at phase shifting of 40 degree for -20 and +20 degree for deferential delta connection

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