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There are two losses in a transformer. 

a. Iron losses. 

b. Copper losses. 

Iron losses – Iron losses constitutes of two losses. 

a. Eddy current losses, these are due to the induced emf in the core, which constitutes a current in the core. These will heats up the core. 

Eddy current losses can be minimised by using laminated core immersed in varnish. This provides a high resistance between the laminations and thus eddy current in reduced. 

b. Hysterisis losses, these are due to the magnetic reversal of current by which there is friction between molecules of core and heat is generated. 

Hysterisis losses can be minimised by selecting proper magnetic material, like silicon steel. 

Copper losses – these losses are due to the resistance of the winding, which is equal to I² rt (calories). These losses are depends on load. That is the losses are increased to the square of the load current.

I – current through winding. 

r – resistance of winding. 

t – time duration.

Properties of transformer oil are,

a) Colour – pale yellow. 

b) Moisture content - <10 ppm. 

c) Acidity (KOH/gramme of oil) – <0.1. 

d) Dielectric strength – limit value is 45 kV and preferable value is 60 kV. 

e) Flashpoint - 141º C. 

f) Inter-surface tension – 30 to 40 dynes / cm or 0.3 to 0.4 Newton. 

g) Resistivity – 

Conservator: This allows for shrinkage and swelling of transformer oil. When the oil is heated up it swells and rises to the conservator. When cools down it goes back to main tank. Conservator reduces sludge formation of oil because only the oil surface in conservator is exposed to atmosphere where oil in the main tank is not exposed to atmosphere.

Breather: It provides dry sir to conservator when transformer breathes. That is when there is shrinkage of oil atmospheric air enters conservator through breather. The moisture is absorbed in breather by silica gel. 

Buchholz relay (gas operated): If there is an initial fault, heating up of core, high resistance joints heating up by conduction through insulation and supports. There is heating up of oil, which breaks down and gases are released. This gas actuates the mechanics in the relay, There by closing contacts of mercury switches for alarm. Also if there is a short circuit, the buchholz relay will trip the transformer. Also if there is any leakage of oil through bushing etc and oil level comes down the relay will give alarm and also will trip the transformer if transformer oil level comes down the point. Gases can be taken from the relay to identify nature of fault.

Explosion vent: It provided on transformer main tank, provided with two Bakelite diaphragm which break when the pressure exceeds 5 psi in the transformer tank and relieve the pressure.

Core: To provide low reluctance path for the magnetic lines of force. It carries both the HV and LV windings. 

HV Winding: High voltage is given to HV winding and low voltage is taken from the LV winding.

LV Winding: Low voltage is given to LV winding and high voltage is taken from the HV winding.

Cooling tubes: These are provided to cool the transformer oil so that the heat of oil will be given to the atmosphere.

HT bushing: Carries the HV terminals. 

LT bushing: Carries the LV terminals. 

Tap changer: this is provided so that we can get the required voltage out put. There are two types of tap changer. Online tap changer and off line tap changer