THREE PHASE TRANSFORMER

3 Phase Power is More Efficient Than Single Phase

Three phase electricity powers large industrial loads more efficiently than single-phase electricity. When single-phase electricity is needed, It is available between any two phases of a three-phase system, or in some systems , between one of the phases and neutral.

By the use of three conductors a 3 phase system can provide 173% more power than the two conductors of a single-phase system.

Three-phase power allows heavy duty industrial equipment to operate more smoothly and efficiently.

3 phase power can be transmitted over long distances with smaller conductor size.

Transfer of 3 Phase Power

Using bank of 3 Single phase transformers Costly Requires large space

Using 1 Three-Phase Transformer Economical Less weight

Construction of Transformer

Core Type Concentric windings are used

Shell Type Sandwich windings are used

Core Type Single Phase Transformer

Shell Type Single Phase Transformer

Core Type Three Phase Transformer

Shell Type Three Phase Transformer

View of Three Phase Transformer

Three-Phase Transformer Connections:

1) Delta to Delta – used for industrial applications

2) Delta to Wye – used most commonly; commercial and industrial

3) Wye to Delta – used for high voltage transmissions

4) Wye to Wye – used rarely, causes harmonics and balancing problems.

5) Open delta- Open delta – not used

Delta Connection

Star Connection

Open Delta

Nameplate Rating of Transformer

Service Number of phases Frequency Type of cooling Rating Ratio Impedance Duty Overload rating Maximum temperature rise Vector Group Tap changer HV, LV & Neutral Bushings   Minimum Clearance (mm) in AIR Size and weight Type of insulation used

Vector Group Provides a simple way of indicating the internal connections

of a transformer, Indicated by a code consisting of two or three letters,

followed by one or two digits. Delta winding: D, d Wye winding: Y, y Zigzag winding: Z, z The digits following the letter codes indicate the difference in phase

angle between the windings, with HV winding is taken as a reference. The number is in units of 30 degrees. Phase rotation is always anti-clockwise.

For example, a transformer with a vector group of Dy1 has a delta-connected HV winding and a wye-connected LV winding. The phase angle of the LV winding lags the HV by 30 degrees.

11/3.3 kV Transformers• 3.15, 4, 5 and 6.3

11/6.6 kV Transformers• 3.15 and 4

33/11 kV Transformers• 1.0, 1.6, 3.15, 4.0, 5.0, 6.3 and 10.0

66/11 kV Transformers• 6.3, 8.0, 10.0, 12.5 and 20.0

Standard Rating of Power Transformer in MVA

11/0.433 kV Transformers• 160, 200, 250, 315, 400, 500, 630,

800, 1000, 1250, 1600, 2000, 2500 and 3000

33/0.433 kV Transformers• 315, 400, 500, 630, 800, 1000, 1250,

1600, 2000, 2500 and 3000

Standard Rating of Distribution Transformer in

kVA

Autotransformer

Sometimes called autoformer An electrical transformer with only one winding.

The winding has at least three electrical connection points called taps.

One tap at the end of the winding is a common connection to both circuits (source and load).

A portion of the same winding acts as part of both the primary and secondary winding

Autotransformer

Applications of Autotransformer

Frequently used in power applications to interconnect systems operating at different voltage classes, for example 138 kV to 66 kV for transmission.

On long rural power distribution lines, special autotransformers with automatic tap-changing equipment are inserted as voltage regulators, so that customers at the far end of the line receive the same average voltage as those closer to the source.

In Labs to provide variable supply to various instruments/devices

Other type of Transformers

Three winding Transformer Instrument Transformer

• Current Transformer• Voltage Transformer

Instrument Transformers

Used extensively for measurement and monitoring.

Protection devices and revenue metering may use separate CTs & PTs.

Safely isolates measurement and control circuitry from the high voltages typically present on the circuit being measured.