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Mehran University College
Of Engineering & Technology,
Khairpur Mir’s
THREE PHASE TRANSFORMERS
AND VECTOR GROUPS
ENGR. AHSANULLAH MEMON
LECTURER
DEPARTMENT OF ELECTRICAL ENGINEERING MUCET KHAIRPUR MIRS
THREE PHASE TRANSFORMERS
 A three phase transformer has three sets of primary and secondary
windings.
 Depending upon how these sets of windings are interconnected, determines
whether the connection is a star or delta configuration.
 The available voltage which are each displaced from the other by 120
electrical degrees
H.W:- Advantages and disadvantages of core type construction
and shell type construction
CONNECTIONS OF THREE PHASE TRANSFORMERS
 A three-phase transformer is made of three sets of
primary and secondary windings.
 Those sets of primary and secondary windings will be
connected in either Δ or Y configurations to form a
complete unit.
 Y connections provide the opportunity for multiple
voltages,
 while Δ connections enjoy a higher level of reliability
(if one winding fails open, the other two can still
maintain full line voltages to the load).
Primary - Secondary
Y
Y
Y
Δ
Δ
Y
Δ
Δ
Having both primary and secondary winding sets connected in “Y” formations
allows for the use of neutral conductors (N1 and N2) in each power system.
Such a configuration would allow for the provision of multiple
voltages (line-to-line or line-to-neutral)
When there is no need for a neutral conductor in the secondary
power system, Δ-Δ connection schemes are preferred because of
the inherent reliability of the Δ configuration.
Considering that a Δ configuration can operate satisfactorily
missing one winding, some power system designers choose to
create a three-phase transformer bank with only two transformers,
representing a Δ-Δ configuration with a missing winding in both
the primary and secondary sides:
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 provides a simple way of indicating how the
internal connections of a transformer are arranged.
 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.
 The vector group is indicated by a code consisting
of two or three letters, followed by one or two
numeric digits. The letters indicate the winding
configuration as follows:
 D or d: Delta winding, also called a mesh winding.
 Y or y: Wye winding, (also called a star).
 Z or z: Zigzag winding, or interconnected star
winding. Similar to a wye winding, but two
windings form each phase are arranged so that the
three legs are "bent" when the phase diagram is
drawn. Zigzag-wound transformers have special
characteristics and are not commonly used where
these characteristics are not needed.
ZIGZAG CONNECTION OF TRANSFORMER
 The zigzag connection of tranformer is also called
the interconnected star connection.
 This connection has some of the features of the Y
and the ∆ connections, combining the advantages
of both.
 The zigzag transformer contains six coils on three
cores.
 Its applications are for the deviation of a neutral
connection from an ungrounded 3-phase system
and the grounding of that neutral to an earth
reference point and harmonics mitigation.
 It can cancel triplet (3rd, 9th, 15th, 21st, etc.)
harmonic currents.
 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°.
Example:
 Digit 0 =0° that the LV phasor is in phase with the HV phasor
 Digit 1 =30° lagging (LV lags HV with 30°)
 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°)

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