Vector Group of Transformer

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

Understanding Vector Group of Transformer (part1)Posted Jun 3 2012 by jiguparmar in Energy and Power, Transformers with 11 Comments

Understanding Vector Group of Transformer (part 1) | EEP http://electrical-engineering-portal.com/understanding-vector-group-trans...

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Understanding Vector Group of Transformer

Introduction

Three phase transformer consists of three sets of primary windings, one for each phase, and three sets ofsecondary windings wound on the same iron core. Separate single-phase transformers can be used andexternally 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 thedelta, 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 areconnected similarly. This means that a 3-phase transformer can have its primary and secondary windingsconnected 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 waveformswhen the primary and secondary windings are connected the same way. This condition is called “no phaseshift.”

But when the primary and secondary windings are connected differently, the secondary voltage waveforms

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will differ from the corresponding primary voltagewaveforms by 30 electrical degrees. This is called a 30degree phase shift. When two transformers areconnected in parallel, their phase shifts must beidentical; if not, a short circuit will occur when thetransformers are energized.”

Basic Idea of Winding

An ac voltage applied to a coil will induce a voltage in asecond coil where the two are linked by a magneticpath. The phase relationship of the two voltagesdepends 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 inphase or displaced as above with the coils connected in star or delta and, in the case of a star winding, havethe star point (neutral) brought out to an external terminal or not.

Six Ways to wire Star Winding:

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Six Ways to wire Star Winding

Six Ways to wire Delta Winding:


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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 magneticpath. 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 inphase or displaced as above with the coils connected in star or delta and, in the case of a star winding, havethe star point (neutral) brought out to an external terminal or not.


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Additive and substractive polarity of transformer

When Pair of Coil of Transformer have same direction than voltage induced in both coil are in same directionfrom one end to other end. When two coil have opposite winding direction than Voltage induced in both coilare in opposite direction.

Winding connection designations

First Symbol: for High Voltage: Always capital letters. D=Delta, S=Star, Z=Interconnected star, N=NeutralSecond 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 pointbrought 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 hasstated, the notation is HV-LV in sequence. For example, a step-up transformer with a delta-connectedprimary, and star-connected secondary, is not written as ‘dY11′, but ‘Yd11′. The 11 indicates the LV windingleads the HV by 30 degrees.

Transformers built to ANSI standards usually do not have the vector group shown on their nameplate and


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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, thevector group of the transformer is determined.

The transformer vector group is indicated on the Name Plate of transformer by the manufacturer. The vectorgroup indicates the phase difference between the primary and secondary sides, introduced due to thatparticular configuration of transformer windings connection.

The Determination of vector group of transformers is very important before connecting two or moretransformers in parallel. If two transformers of different vector groups are connected in parallel then phasedifference exist between the secondary of the transformers and large circulating current flows between thetwo 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 otherwindings from the reference vector, with anticlockwise rotation, is represented by the use of clock hourfigure.

IS: 2026 (Part 1V)-1977 gives 26 sets of connections star-star, star-delta, and star zigzag, delta-delta, deltastar, 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 usedconnections 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 sequenceof voltage. For example a 220/66/11 kV Transformer connected star, star and delta and vectors of 66 and 11kV windings having phase displacement of 0° and -330° with the reference (220 kV) vector will berepresented As Yy0 – Yd11.

The digits (0, 1, 11 etc) relate to the phase displacement between the HV and LV windings using a clock facenotation. The phasor representing the HV winding is taken as reference and set at 12 o’clock. Phase rotationis always anti-clockwise. (International adopted).

Use the hour indicator as the indicating phase displacement angle. Because there are 12 hours on a clock, anda circle consists out of 360°, each hour represents 30°.Thus 1 = 30°, 2 = 60°, 3 = 90°, 6 = 180° and 12 = 0° or360°.

The minute hand is set on 12 o’clock and replaces the line to neutral voltage (sometimes imaginary) of the HVwinding. This position is always the reference point.

Example


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Digit 0 =0° that the LV phasor is in phase with the HV phasorDigit 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 transmissioncircuits consisting of cables and overhead lines.

Phase Shift (Deg) Connection0 Yy0 Dd0 Dz030 lag Yd1 Dy1 Yz160 lag Dd2 Dz2120 lag Dd4 Dz4150 lag Yd5 Dy5 Yz5180 lag Yy6 Dd6 Dz6150 lead Yd7 Dy7 Yz7120 lead Dd8 Dz860 lead Dd10 Dz1030 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 TCGroup I 0 o’clock, 0° delta/delta, star/starGroup II 6 o’clock, 180° delta/delta, star/starGroup III 1 o’clock, -30° star/delta, delta/starGroup IV 11 o’clock, +30° star/delta, delta/starMinus indicates LV lagging HV, plus indicates LVleading HV

Clock Notation 0 (Phase Shift 0)


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Clock Notation 0 (Phase Shift 0)

Clock Notation 1 (Phase Shift -30)




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Clock Notation 4 (Phase Displacement -120)




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Clock Notation 6 (Phase Shift +180)




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To be continued…

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delta, phase, phase displacement, star, transformer, transformers, vector group, voltage, winding

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Energy and Power Transformers


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jiguparmar - ignesh Parmar has completed his B.E(Electrical) from Gujarat University. He has more than 12years experience in Power Transmission-Power Distribution-Electrical Theft detection-ElectricalMaintenance-Projects (Planning-Designing-coordination-Execution). He is Presently associate with one of theleading business group as a Assistant Manager at Ahmedabad,India. He has published numbers of TechnicalArticles in "Electrical Mirror", "Electrical India", "Lighting India", "Industrial Electrix"(Australian PowerPublications) Magazines. He is Freelancer Programmer of Advance Excel and design useful Excel baseElectrical Programs as per IS,NEC,IEC,IEEE codes. He is Technical Blogger and Familiar with English,Hindi, Gujarati, French languages. He wants to Share his experience & Knowledge and help technicalenthusiasts to find suitable solutions and updating themselves on various Engineering Topics.

Become EEP's Contributor and introduce yourself to 70k+ of our readers all across the web.

11 Comments

rkkatrixJul 25, 2013

simple and excellent … great work jignesh !!!!

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Vector Group of Transformer