SHANTILAL SHAH GOVERNMENT ENGINEERING COLLEGE,

BHAVNAGARPostal Address : New Sidsar Campus Post : Vartej, Sidsar, Bhavnagar-364060, Gujarat, IndiaContact : 0278-2445509,2445767 Fax : 0278-2445509Website: http://www.ssgec.ac.in/

Roll No. Enrollment No. Name

3017 140430117019 JITIN J PILLAI

3010 140430117012 PARTH DODIYA

3016 140430117018 ISHAN J BHATT

Department : INSTRUMENTATION & CONTROLCourse : B.E. (2014 Batch)Semester : 3rd Batch : B1

PREPARED BY :

THREE PHASE TRANSFORMERS

Subject: ELECTRICAL MACHINES

(2131005)

Introduction

The transformers may be inherently 3-phase, having three primary windings and three secondary windings mounted on a 3-legged core.

The same result can be achieved by using three single-phase transformers connected together to form a 3-phase transformer bank.

Construction

• A three-phase transformer is constructed by winding three single-phase transformers on a single core.

• Three-phase transformers are connected in either wye or delta configurations.

The primary (input) side of a three-phase transformer can be connected in a wye or delta configuration.

The secondary (output) side of a three-phase transformer can also be connected in a wye or delta configuration.

This allows four basic connection patterns:1. Wye-Wye2. Delta-Delta3. Wye-Delta4. Delta-Wye

Primary and Secondary Connections

Wye-delta three-phase connection schematic.

Delta-wye three-phase connection schematic.

Winding identification.

In this type of connection, both the three phase primary and secondary windings are connected in delta as shown.

DELTA-DELTA Connection

The voltages on primary and secondary sides can be shown on the phasor diagram.

DELTA-DELTA Connection

Wye-Wye Connection

• When transformers are connected in wye-wye, specialprecautions have to be taken to prevent severe distortion of the line-to-neutral voltages. (1) connect the neutral of the primary to the neutral of the source, usually by way of the ground

Wye-Wye Connection

(2) provide each transformer with a third winding,called tertiary winding.

Scott Connection

Scott Connection

Scott Connection

A Scott-T transformer(also called a Scott connection) is a type of circuit used to derive two-phase electric power (2-φ, 90-degree phase rotation) from a three-phase (3-φ, 120-degree phase rotation) source, or vice versa.

The Scott connection evenly distributes a balanced load between the phases of the source.

The Scott three-phase transformer was invented by a Westinghouse engineer Charles F. Scott in the late 1890s to bypass Thomas Edison's more expensive rotary converter and thereby permit two-phase generator plants to drive Nikola Tesla's three-phase motors.

V-V Connection

The open-delta, also known as the V-V connection, is a 3-phase arrangement that makes use of only two, instead of three, single-phase transformers, as shown.

If one of the transformers is unable to operate then the supply to the load can be continued with the remaining two transformers at the cost of reduced efficiency. The connection that obtained is called V-V connection or open delta connection.

V-V Connection

Instrument transformers are high accuracy class electrical devices used to isolate or transform voltage or current levels.

The most common usage of instrument transformers is to operate instruments or metering from high voltage or high current circuits, safely isolating secondary control circuitry from the high voltages or currents.

Instrument transformers may also be used as an isolation transformer so that secondary quantities may be used in phase shifting without affecting other primary connected devices.

Instrument Transformers

Current transformer

Current transformers (CT) are a series connected type of instrument transformer. They are designed to present negligible load to the supply being measured and have an accurate current ratio and phase relationship to enable accurate secondary connected metering.

Potential transformer Potential transformers (PT) (also called voltage transformers (VT)) are a

parallel connected type of instrument transformer.

They are designed to present negligible load to the supply being measured and have an accurate voltage ratio and phase relationship to enable accurate secondary connected metering.

The capacitor voltage transformer (CVT) uses a capacitance potential divider and is used at higher voltages due to a lower cost than an electromagnetic Potential Transformer.

Capacitor Voltage Transformer

Three-phase transformer for an electric arc furnace, rated 36 MVA, 13.8 kV/160 V to 320 V, 60 Hz. The secondary voltage is adjustable from 160 V to 320 V by means of 32 taps on the primary winding (not shown). The three large busbars in the foreground deliver a current of 65,000 A. Other characteristics: impedance: 3.14%; diameter of each leg of the core: 711mm; overall height of core: 3500 mm; center line distance between adjacent core legs: 1220 mm.(Courtesy of Ferranti-Packard)

History

Core of a 110 MVA, 222.5 kV/34.5 kV, 60 Hz, 3-phase transformer. By staggering laminations of differentwidths, the core legs can be made almost circular. This reduces the coil diameter to a minimum, resulting inless copper and lower I2R losses. The legs are tightly bound to reduce vibration. Mass of core: 53,560 kg.

History

History

Same transformer with coils in place. The primary windings are connected in wye and the secondaries in delta. Each primary has 8 taps to change the voltage in steps of ±2.5%. The motorized tap-changer can be seen in the right upper corner of the transformer. Mass of copper: 15,230 kg.

Same transformer ready for shipping. It has been subjected to a 1050 kV impulse test on the HV side and a similar 250 kV test on the LV side. Other details: power rating: 110 MVA/146.7 MVA (OA/FA); total mass including oil: 158.7 t; overall height: 9 m; width: 8.2 m, length: 9.2 m.(Courtesy of ABB)

History