Core is surrounded by winding. In this type of construction winding surrounds the
considerable portion of core. The part of which is surrounded by winding is called limb or leg
remaining part of core which is not surrounded winding, but essentially for completing the
path of flux is called yoke . Concentric cylindrical / helical /cross over windings are issued.
L.V. winding is placed closer to core involving less insulation between core and l.v. winding.
This type of construction is more common and has following advantages
1. Construction is simple.
2. cooling is better.
3. Inspection is easy
4. Flux leakage is more resulting leakage reactance is more.
Shell Type
Winding is surrounded by core. In this type of construction the core surrounds he
considerable portion of the winding. It has advantage that windings are tightly coupled
magnetically resulting leakage flux is less. Regulation of the transformer is better. Disc type /
sandwich type winding i.e alternate of LV and HV winding sections are placed on the central
limb cross section of which is double than that of side limbs so the it can accommodate the
entire flux.
Disadvantages of this type of transformers are cooling of windings is poor although cooling
of core is better and inspection of winding is difficult.
Conservator
To avoid the whole surface of the transformer oil to be exposed to the atmosphere, a
separate small metallic cylindrical tank called a conservator is fitted on the top of the
transformer tank, which ensures that entire core-coil assembly is completely immersed into
the oil. The conservator tank is vented to the atmosphere at the top and the normal oil level
is usually maintained at the middle of the conservator to allow the oil to expand and
contract with the temperature variation inside the transformer due to change of load and
atmospheric temperature.
Breather
Change in temperature within the transformer cause expansion and contraction of the
insulating oil, which then causes the air pressure change inside the conservator. Pressure
changes are balanced by a flow of atmospheric air in and out of the conservator, which is
how moisture can enter the system during air in.
Insulating oil contaminates due to ingress of moisture, it can affect the paper insulation
which may lead to affect the life and also cause internal faults. Therefore, it is necessary
that the air entering the tank is moisture-free.
The transformer's breather is a cylindrical container that is filled with silica gel or calcium
chloride absorbs moisture and allows dry air to entire the transformer tank and agent is
regenerated by heating or replaced periodically as routine maintenance. It is connected to
the top of the conservator tank through breather pipe.
Under dry condition colour of the silica gel: bluish white and under wet condition colour will
pink
The Buchholz Relay is a protective device housed on the connecting pipe from the main tank
to the conservator tank. It is used to sense the incipient faults (e.g. inter lamination, core
bolt, inter turn / layer etc. insulation failure) occurring inside the transformer. It is a simple
relay that is operated by the gases emitted during the decomposition of transformer oil
during internal faults. When fault is incipient in nature gas is gradually accumulated on the
top of the relay chamber and due to lowering down the level of oil upper float will make the
contact of alarm circuit. But when the fault is severe in nature huge amount of gas is formed
suddenly which will make a oil rush towards conservator through relay chamber resulting
tilting the lower float of the relay and the float will make the contact of trip circuit of the
circuit breaker for disconnecting the power line.
0 Comments