Wednesday, August 1, 2012

Transformer Winding Connections

Winding connections
Three-phase transformers are usually operated with the high-voltage and low-voltage windings connected in Y (star), D (delta) or Z (zigzag) connection. 
The three styles are shown 

In star connection, one end of each of the three-phase windings is joined together at a neutral point N and line voltage is applied at the other end.

The advantages of star connection are:

• It is cheaper for a high-voltage winding
• The neutral point is available
• Earthing is possible, either directly or through an impedance
• A reduced insulation level (graded insulation) is possible at the neutral
• Winding tappings and tapchanger may be located at the neutral end of each phase, with low voltages to earth and between phases
• Single-phase loading is possible, with a neutral current flowing

In delta connection, the ends of the three windings are connected across adjacent phases of the supply

The advantages of a delta connection are:

A delta-connected tertiary winding is often used on large three-phase autotransformers to allow zero-phase sequence currents to circulate within the windings, or to allow triplen-frequency harmonic currents to flow in order to cancel out harmonic fluxes in the core.

The zigzag connection is used for special purposes where two windings are available on each leg and are interconnected between phases

The advantages of a zigzag connection are:

• It permits neutral current loading with an inherently low zero-sequence impedance, and it is used in ‘earthing transformers’ to create an artificial neutral terminal on the system 
• Voltage imbalance is reduced in systems where the load is not evenly distributed between phases
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