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of
Electrical
Engineering
Department of Electrical Engineering Mehran UET Jamshoro
AEM Dr. MA mahar

VECTOR GROUP
of
THREE PHASE
TRANSFORMER
 M
U
E
T
JAMSHORO

Department
of
Electrical
Engineering
 Department of Electrical Engineering Mehran UET Jamshoro
AEM Dr. MA mahar
Transformer Vector Groups
The vector group determines the phase displacement between the
primary and the secondary winding.

Vector group is a connection of primary winding, secondary

winding and an electrical angle difference. This angle difference is
the difference between line voltage on each side.

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.

◼
 Department of Electrical Engineering Mehran UET Jamshoro
AEM Dr. MA mahar
Transformer Vector Groups

◼ These vector groups are especially important when two or

more transformers are to be connected in parallel.

◼ If two transformers of different vector groups are connected in

parallel then phase difference exist between the secondaries of
the transformers and large circulating current flows between
the two transformers which is very detrimental.

◼
 Department of Electrical Engineering Mehran UET Jamshoro
AEM Dr. MA mahar
Transformer Vector Groups

Group I: zero phase displacement between the primary and the

secondary line voltages.
(0 o'clock, 0°) - delta/delta, star/star)

Group II: 180◦ phase displacement.

(6 o'clock, 180°) - delta/delta, star/star)

Group III: 30◦ lag phase displacement of the secondary with

respect to the primary.
(1 o'clock, -30°) - star/delta, delta/star)
 Department of Electrical Engineering Mehran UET Jamshoro
AEM Dr. MA mahar
Transformer Vector Groups
Group IV: 30◦ lead phase displacement of the secondary with
respect to the primary.

(11 o'clock, +30°) - star/delta, delta/star

(Minus indicates LV lagging HV, plus indicates LV leading HV)

The angular displacement of secondary with respect to the

primary are shown as clock positions
 Department of Electrical Engineering Mehran UET Jamshoro
AEM Dr. MA mahar
Standard code for transformer vector groups
International Electrotechnical Commission (IEC) has devised the
standard code for determination of transformer vector group.
According to IEC the code for vector group consist of 2 or more
letters followed by one or two digits.
❖ The first letter is Capital letter which may be Y, D or Z, which
stands for High voltage side Star, Delta or interconnected star
windings respectively.

❖ The second letter is a small letter which may be y, d or z which

stands for low voltage side Star, Delta or interconnected star
windings respectively.

❖ The third is the digits which stands for the phase difference
between the high voltage and low voltage sides.
 Department of Electrical Engineering Mehran UET Jamshoro
AEM Dr. MA mahar
Phase rotation
◼ In this convention the transformer high voltage side phase
voltage (line to Neutral) represented by Minute hand is
fixed at 12 O'clock position (This position is always the
reference point) and the low voltage side phase voltage
(line to neutral) is represented by the Hour hand which is
free to move.

Phase rotation is always anti-

clockwise. (international adopted
convention)

Use the hour indicator as the

indicating phase displacement
angle.
 Department of Electrical Engineering Mehran UET Jamshoro
AEM Dr. MA mahar
Phase rotation

◼ 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.

Because rotation is anti-clockwise, 1 = 30°

lagging (LV lags HV with 30°) and 11 =
330° lagging or 30° leading (LV leads HV
with 30°)
 Yd1 vector group

M ➢ Y indicates HV star connected winding

U
➢ d indicates LV delta connected winding
➢ 1 indicates phase shift (LV lags HV by
E 30°)

T
JAMSHORO

Department
of
Electrical
Engineering
 Department of Electrical Engineering Mehran UET Jamshoro
AEM Dr. MA mahar
◼ To summarise:
Dd0
Delta connected HV winding, delta connected LV winding, no
phase shift between HV and LV.

◼ Dyn11
Delta connected HV winding, star connected LV winding with
neutral brought out, LV is leading HV with 30°

◼ Yd11
Star connected HV winding, delta connected LV winding, LV
is leading HV with 30°
 Department of Electrical Engineering Mehran UET Jamshoro
AEM Dr. MA mahar
Wye Delta connection for Yd11 vector group

A1A2, B1B2 and C1C2 are primary side

voltage phasors.
Similarly in the secondary side voltage
phasors are a1a2, b1b2 and c1c2.
Just observe that a1a2 is parallel to
A1A2, b1b2 is parallel to B1B2 and c1c2
is parallel to C1C2, this is because a1a2
and A1 A2 are in phase Similarly b1b2
and B1B2 are in phase and also c1c2 and
C1C2 are in phase.
 Wye Delta connection for Yd11 vector group

M
U
E
T
JAMSHORO

Department
of
Electrical
Engineering
 Department of Electrical Engineering Mehran UET Jamshoro
AEM Dr. MA mahar
Wye Delta connection for Yd11 vector group

◼ Now compare the primary side vector

diagram and secondary side vector
diagram. From the diagram it is clear
that as if the secondary side phasor
triad has been rotated counterclockwise
with respect to primary side. From the
geometry it can be confirmed that this
angle is 30 degree. As the phasors are
rotating counterclockwise, so the
secondary side phasor a2n (phase
voltage) lead the primary side phasor
A2N (phase voltage) by 30 degree.
 Department of Electrical Engineering Mehran UET Jamshoro
AEM Dr. MA mahar
Wye Delta connection for Yd1 vector group
 M
U
E
T
JAMSHORO

Department
of
Electrical
Engineering

Yd1 vector group Yd11 vector group

 Department of Electrical Engineering Mehran UET Jamshoro
AEM Dr. MA mahar
Delta Wye connection for Dy1 vector group
 Department of Electrical Engineering Mehran UET Jamshoro
AEM Dr. MA mahar
Delta Wye connection for Dy11 vector group
 Department of Electrical Engineering Mehran UET Jamshoro
AEM Dr. MA mahar

M
U
E
T
JAMSHORO

Department
of Yyn0 vector group
Electrical
Engineering Yy0 vector group
 Department of Electrical Engineering Mehran UET Jamshoro
AEM Dr. MA mahar

Yy6 vector group

 Dd6

M
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T
JAMSHORO

Department
of
Electrical
Engineering
 Dd0

M
U
E
T
JAMSHORO

Department
of
Electrical
Engineering
Department of Electrical Engineering Mehran UET Jamshoro
AEM Dr. MA mahar
 Department of Electrical Engineering Mehran UET Jamshoro
AEM Dr. MA mahar

➢ Undoubtedly transformers belonging to the same group can

be operated in parallel without any difficulty.

➢ It is impossible to run in parallel, transformers in Group1

and 2 with transformers in Group3 and group4.

➢ Also transformers in group1 and group2 cannot be operated

in parallel as there is 180 degree phase difference between
the two secondary windings. This can only be rectified by
changing internal connection.
 Department of Electrical Engineering Mehran UET Jamshoro
AEM Dr. MA mahar

If group3 and group4 transformers will be connected in

parallel then there will be 60 degrees phase difference
between their secondary windings. But with
transformer external connection modification the phase
difference of secondaries can be made zero. So group3
and group4 transformers can be operated in parallel
with some external modification.

.
 Department of Electrical Engineering Mehran UET Jamshoro
AEM Dr. MA mahar

Dy1 vector group

Dy11 vector group

You can only parallel Dy1 and Dy11 by crossing two incoming phases and
the same two outgoing phases on one of the transformers, so if you have a
DY11 transformer you can cross B & C phases on the primary and
secondary to change the +30 degree phase shift into a -30 degree shift which
will parallel with the Dy1, assuming all the other points above are satisfied.
If vector grouping is ignored there is an absolute certainty of creating a short
circuit if you try and parallel a DY1 and a DY11
 Department of Electrical Engineering Mehran UET Jamshoro
AEM Dr. MA mahar

Group I

◼ Example: Dd0 (no phase displacement between HV and LV)

◼ The conventional method is to connect the red phase on A/a, Yellow phase on
B/b, and the Blue phase on C/c.

◼ Other phase displacements are possible with unconventional connections (for

instance red on b, yellow on c and blue on a)

◼ By doing some unconventional connections externally on one side of the trsf,

an internal connected Dd0 transformer can be changed either to a Dd4(-120°)
or Dd8(+120°) connection. The same is true for internal connected Dd4 or
Dd8 transformers.
 Department of Electrical Engineering Mehran UET Jamshoro
AEM Dr. MA mahar

Group II

Example: Dd6 (180° displacement between HV and LV)

By doing some unconventional connections externally
on one side of the transformer, an internal connected
Dd6 transformer can be changed either to a Dd2(-60°)
or Dd10 (+60°) connection.
 Department of Electrical Engineering Mehran UET Jamshoro
AEM Dr. MA mahar

Group III

Example: Dyn1 (-30° displacement between HV and LV)

By doing some unconventional connections externally on

one side of the transformer, an internal connected Dyn1
transformer can be changed either to a Dyn5(-150°) or
Dyn9 (+90°) connection.
Department of Electrical Engineering Mehran UET Jamshoro
AEM Dr. MA mahar
Group IV

Example: Dyn11 (+30° displacement between HV and LV)

By doing some unconventional connections externally on one side of the

trsf, an internal connected Dyn11 transformer can be changed either to
a Dyn7(+150°) or Dyn3(-90°) connection.

❖ By doing some unconventional connections externally on both

sides of the transformer, an internal connected group III or group
IV transformer can be changed to any of these two groups.

❖ Thus, an internal connected Dyn1 transformer can be changed to

either a: Dyn3, Dyn5, Dyn7, Dyn9 or Dyn11 transformer, by
doing external changes on both sides of the transformer.

❖ This is just true for star/delta or delta/star connections. Changes

for delta/delta or star/star transformers between group I and
group II can just be done internally.
 M
U
E
T
JAMSHORO

Department
of
Electrical
Engineering
Department of Electrical Engineering Mehran UET Jamshoro
AEM Dr. MA mahar
Department of Electrical Engineering Mehran UET Jamshoro
AEM Dr. MA mahar
Department of Electrical Engineering Mehran UET Jamshoro
AC Machines Dr. MA mahar
Department of Electrical Engineering Mehran UET Jamshoro
AEM Dr. MA mahar
 M
U
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T
JAMSHORO

Department
of
Electrical
Engineering
 M
U
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T
JAMSHORO

Department
of
Electrical
Engineering