This document provides information about three-phase transformers, including: 1) Three-phase transformers can have three primary and secondary windings on a three-legged core or be constructed from three single-phase transformers connected together. 2) Primary and secondary windings can be connected in wye, delta, wye-delta, or delta-wye configurations. 3) Scott and V-V connections are also discussed as variations of three-phase transformer connections.

1. SHANTILAL SHAH GOVERNMENT
ENGINEERING COLLEGE, BHAVNAGAR
Postal Address : New Sidsar Campus
Post : Vartej, Sidsar, Bhavnagar-364060, Gujarat, India
Contact : 0278-2445509,2445767 Fax : 0278-2445509
Website: 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 & CONTROL
Course : B.E. (2014 Batch)
Semester : 3rd
Batch : B1
PREPARED BY :

2. THREE PHASE
TRANSFORMERS
Subject:
ELECTRICAL MACHINES
(2131005)

3. 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.

4. 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.

5.  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-Wye
2. Delta-Delta
3. Wye-Delta
4. Delta-Wye
Primary and Secondary Connections

6. Wye-delta three-phase connection
schematic.

7. Delta-wye three-phase connection
schematic.

8. Winding identification.

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

10. The voltages on primary and secondary sides can be shown on the phasor diagram.
DELTA-DELTA Connection

11. Wye-Wye Connection
• When transformers are connected in wye-wye, special
precautions 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

12. Wye-Wye Connection
(2) provide each transformer with a third winding,
called tertiary winding.

13. Scott Connection

14. Scott Connection

15. 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.

16. 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.

17. 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

18.  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

19. 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.

20. 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.

21. 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

22. 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 65,000A. Other
characteristics: impedance: 3.14%;
diameter of each leg of the core: 711
mm; overall height of core: 3500 mm;
center line distance between adjacent
core legs: 1220 mm.
(Courtesy of Ferranti-Packard)
History

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

24. 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.

25. 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