Loading of Transformer Tertiary winding

The short answer is 'utilizing the tertiary transformer for the purpose of why it is used for'.

The tertiary transformer has 3 windings: one primary, 1 secondary and the other is tertiary. It is also called 3-winding transformer. The difference between the regular transformer and the tertiary transformer is the additional third winding. For a tertiary winding transformer, the sum of the capacities of secondary and the tertiary winding is equal to the primary winding. The tertiary transformer is used to serve some special purposes. The most common purposes are:

(1) To provide additional terminal with different power voltage. The third winding is of smaller capacity compared to the secondary winding. This helps avoiding the requirement of additional auxiliary transformer in the substation or power station.

(2) To provide path for zero sequence current. A small short circuited third winding is used for circulating the zero-sequence current for Y-side winding and terminal of the third winding is not brought out side.

(3) To reduce harmonic current for some nonlinear loads, such as UPS input transformer. In this type of transformer, the second and third windings are of same capacity.

The tertiary transformers are more expensive than the regular one. So, when the tertiary winding transformer is used in an electrical system, there should be specific reason of using it, otherwise you are wasting money and transformer capability.

- MS

Some points have already been given and let me also add some others.

There are many reasons and some of them are:

1- If two winding transformer has to supply an additional load which has to be insulated from the secondary windings for some reasons then three winding transformer may be used with additional load carried by tertiary winding.

2- The phase compensating devices can be supplied with three winding transformer which are not operating at either primary or secondary voltage but at some different voltage.

3- The tertiary winding can be used as a voltage coil in a
testing transformer.

4- Three supply systems operating at different votlages can be interconnected using three winding transformer

5- The three winding transformer can be used to load large winding generators.

6- The substation requirements can be met using three winding transformer which requires a votlage different from that primary and secondary windings

7- The tertiary winding connected in delta reduces the impedance offered to the zero sequence curretns so a larger earth fault current flows for proper operation of protective equipment. For unbalanced load it limits the imbalance in voltage.

For some more information please see

http://www.answerbag.com/q_view/381729