Mehran University College Of Engineering & Technology,

Khairpur Mir’s

ENGR. AHSANULLAH MEMONLECTURER DEPARTMENT OF ELECTRICAL ENGINEERING MUCET KHAIRPUR MIRS

THREE PHASE TRANSFORMERS AND VECTOR GROUPS

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 TRANSFORMERThe 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°)