Introduction & Basic Principle

• Transformer is a static piece of apparatus which transfers electrical energy from one circuit to another, which are electrically disconnected by means of mutual induction.

• A Transformer consists of two or more stationary electric circuits where the source supplies the power to the primary winding and the coil delivers power to the Load through Secondary winding.

• The two electrical circuits are electrically disconnected but interlinked by a common magnetic circuit to transfer electrical energy from the Primary winding to the Secondary Winding without a change in frequency.

Schematic diagram showing the Magnetic Flux path & Current flow of a single phase Transformer

Working Principle of a single phase Transformer

Coil 1

i1(t) i2(t)

Coil 2

M

e1(t) e2(t)

S1 S2

i1(t) i2(t)

(Primary has N1 turns) (Secondary has N2 turns)

V2

3- Transformer Construction.

A three phase transformer is constructed by winding three single phase transformers on a single core. These transformers are put into an enclosure which is then filled with dielectric oil. The dielectric oil performs several functions. Since it is a dielectric, a nonconductor of electricity, it provides electrical insulation between the windings and the case. It is also used to help provide cooling and to prevent the formation of moisture, which can deteriorate the winding insulation.

Three phase transformer Connections to be discussed:-

• Star Connection ( Y )

• Delta Connection ( )

• Zigzag Connection ( Z )

Star (Y) connection

• Line current is same as phase current .

• Line-Line voltage is 3 phase-neutral voltage.

• Power is given by 3 VL-LI Lcos or 3VphIphcos.

Delta () connection

• Line-Line voltage is same as phase voltage.

• Line current is 3 phase current.

• Power is given by 3 VL-LI Lcos or 3VphIphcos.

Interconnected Star or Zigzag (Z) connection

• The zigzag winding provides an easy path for in-phase currents but does not allow the flow of currents that are 120° out of phase with each other.

• A zigzag transformer is more effective for grounding purposes because it has less internal winding impedance going to the ground than when using a wye-type transformer.

Three phase transformer Connection Combinations:

• Y - Y ( Star - Star)

• Y - ( Star - Delta)

• - Y ( Delta - Star)

• - ( Delta - Delta)

• Y – Z ( Star - Zigzag)

• - Z ( Delta - Zigzag)

Transformer Vector Groups• The three phase transformer windings can be connected several ways. Depending upon the windings' connection & the phase shift of line-neutral voltages between primary and secondary; the vector group of the transformer is determined. The phase angle between H.V & L.V line emf phasors , depends upon the orientation of the windings i.e. the direction in which the windings are wound and the methods of connecting the three phases.

Importance of Transformer Vector Groups• The Determination of vector group of transformers is very important before connecting two or more transformers in parallel. Usually transformers between two different groups should not be paralleled. 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.

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.

Phase displacement0Group 1

Phase displacement180Group 2

Lag phase displacement30Group 3

Lead phase displacement30Group 4

Yy0Dd0Dz0

Yy6Dd6Dz6

Yd1Dy1Yz1

Yd11Dy11Yz11

Different Transformer Vector Groups

Group 1: (0 degree phase displacement)

Yy0

Dd0

Dz0

Group 2: (180 degree phase displacement)

Yy6

Dd6

Dz6

Group 3: (-30 degree phase displacement)

Yd1

Dy1

Yz1

Group 4: (+30 degree phase displacement)

Yd11

Dy11

Yz11

Advantages of Star Connection :-

•The primary side is star connected. Hence fewer numbers of turns are required. This makes the connection economical for large high voltage step down power transformers.•The neutral available on the primary can be earthed to avoid distortion.•The neutral point allows both types of loads (single phase or three phases) to be met.•Required Few Turns for windingDue to star connection, phase voltages is (1/√3) times the line voltage. Hence less number of turns is required. Also the stress on insulation is less. This makes the connection economical for small high voltage purposes.•Required Less Insulation LevelIf the neutral end of a Y-connected winding is grounded, then there is an opportunity to use reduced levels of insulation at the neutral end of the winding. A winding that is connected across the phases requires full insulation throughout the winding.

•Handle Heavy LoadDue to star connection, phase current is same as line current. Hence windings have to carry high currents. This makes cross section of the windings high. Thus the windings are mechanically strong and windings can bear heavy loads and short circuit current.•Eliminate Distortion in Secondary Phase VoltageThe connection of primary neutral to the neutral of generator eliminates distortion in the secondary phase voltages by giving path to triple frequency currents toward to generator.•Sinusoidal voltage on secondary sideNeutral give path to flow Triple frequency current to flow Generator side thus sinusoidal voltage on primary will give sinusoidal voltage on secondary side.•Used as Auto TransformerA Y-Y transformer may be constructed as an autotransformer, with the possibility of great cost savings compared to the two-winding transformer construction•Use for Three phases Four Wires SystemAs neutral is available, suitable for three phases four wire system.

Advantages of Delta Connections :-

A delta system is a good short-distance distribution system. It is used for neighborhood and small commercial loads close to the supplying substation. Only one voltage is available between any two wires in a delta system. The delta system can be illustrated by a simple triangle. A wire from each point of the triangle would represent a three-phase, three-wire delta system. The voltage would be the same between any two wires.

Advantages of Zigzag Transformer connection :-

•Less Costly for grounding Purpose

It is typically the least costly than Y-D and Scott Transformer. The ability to provide a path for in-phase currents enables us to use the zigzag connection as a grounding bank, which is one of the main applications for this connection.

•Third harmonic suppression

The zigzag connection in power systems to trap triple harmonic (3rd, 9th, 15th, etc.) currents. Here, We install zigzag units near loads that produce large triple harmonic currents. The windings trap the harmonic Currents and prevent them from traveling upstream, where they can produce undesirable effects.

•Ground current isolation

If we need a neutral for grounding or for supplying single-phase line to neutral loads when working with a 3-wire, ungrounded power system, a zigzag connection may be the better solution. Due to its composition, a zigzag transformer is more effective for grounding purposes because it has less internal winding impedance going to the ground than when using a Star type transformer.

•No Phase Displacement

There is no phase angle displacement between the primary and the secondary circuits with this connection; therefore, the ∆-zigzag connection can be used in the same manner as Y-Y and ∆- ∆ transformers without introducing any phase shifts in the circuits.