Power Transformer

1.1. Power Transformer

Power transformer is equipment that changes the voltage and current (step-up or step down) without changing the frequency.

Since a substation is mainly concerned with the changes in the levels of voltages and currents, therefore Power Transformer is an important part of the substation. The capacity of the Power Transformer is given MVA. However the capacity of the transformer is greatly depending on the type of cooling used inside it.

1.1.1.      Transformer Core

The transformer core has a great role in minimizing the losses. Therefore we shall use a high grade, non-ageing and cold-rolled electrical silicon steel. Further it is used in thin laminations and a thin uniform coating is placed over it. In this way the losses are minimized to a great extent.

There are two types of core constructions that are mainly followed:

1.1.1.1.  Core type

 The core is shaped with a hollow square through the center as shown in fig 1.

                                 Fig.1 Core Type

1.1.1.2.  Shell type

It is the most widely used type of core construction.  The core consists of E- and I-shaped sections of metal. These sections are butted together to form the laminations. The laminations are insulated from each other and then pressed together to form the core.

Fig.2 Shell Type

1.1.2.      Windings

There are two types of windings in a transformer based on the type of connections to the load or to the source. The winding that is connected to the source is called primary winding and the one connected to the load is called the secondary winding.

 The windings shall be made from copper and free from scale, burr and silvers. They are insulated from each other. The windings shall be so located that they remain balanced electrostatically and their magnetic centers remain co-incident under all conditions of operation.

1.1.3.      Cooling in transformer

There are two types of cooling in a transformer

                                                          i.          ONAN ( Oil Natural Air Natural)

If the transformer's temperature is being kept at acceptable levels by natural air from a ventilation method then it is called ONAN cooling.

                                                         ii.          ONAF ( Oil Natural Force Natural)

 If the transformer's temperature is being kept at acceptable levels by forced air from a fan then it is called ONAF cooling.

When the cooling system is ONAF, its capacity is much greater than when it is ONAN. Say if it is a 26/20 MVA rated transformer, then it means that it is generating 26MVA when it is cooling system is ONAF and 20MVA when it is ONAN.

1.1.4.      Tapings

The tapings in are provided in the high voltage winding. The center tap (principal tap) gives the rated voltage and below and above taps give lower and higher than the rated voltage, respectively.

The tap changing facility can be either ON-load or OFF-load. It means its taps can be changed either without breaking the circuit (On-load) or after breaking the circuit (off-load).