Chapter 3 Transformer - Electrical and Computer … · Three phase transformer’s Winding...

Chapter 3 Transformer

By Sintayehu Challa

Three phase transformer’s Winding

connection

Three phase transformer’s primary and secondary windings basically have four types of connection and these are:

1. Star - star:

• This is the most economical connection for small, high voltage transformers, as the number of turns per phase and the amount ofinsulation is minimum.

• The possibility of utilizing both star pints for a fourth wire may be useful. Third harmonics is absent from the line voltage; unless there is a fourth wire, no third-harmonic current will flow.

• If the transformer is worked at the normal flux density, however, the neutral potential will oscillate, while the third-harmonic phase voltage may be high for shell type three phase units.

• The connection is almost satisfactory with the three-phase core type while for other types, the provision of a tertiary winding stabilizes the neutral conditions.

Contd.2. Delta – delta:

• This is an economical connection for large, low-voltage transformers in which insulation is not a problem cause

• This connection increases the number of turns per phase and reduces the necessary sectional area of conductors.

• In delta-delta connection, large unbalance of load can be met without difficulty; the closed mesh will damp out third-harmonic voltages. Also, should one of the phases develop a fault,

• it is possible to operate the transformer at 58 % of its rating in delta connection.

• In the contrary, the absences of a star- point may be disadvantageous.

Contd.

3. Star - delta: • The star/delta arrangement is very common for

power supply transformers. • It has the advantage of a star-point for mixed

loading, and delta winding to carry third harmonic currents, which stabilizes the star-point potential.

• If the H.V. winding is the star-connected side, there is some saving in cost of insulation.

• A delta-connected H.V. winding is almost universal.

Contd.

4. Delta-star:

• In this connection, there is a reduction in the third harmonic voltage and at the same time unbalanced loading is permitted;

• on account to the type of connection, however, the delta has to be confined to a fairly low voltage winding.

• The delta/star connection has been employed where delta connections were mechanically weak (because of large number of turns and small copper sections) in high voltage transformers

By Sintayehu Challa

Chapter 3 Transformer - Electrical and Computer … · Three phase transformer’s Winding...

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