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.
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).