Explosion Vent

The explosion vent is used to expel out hot oil from the transformer during heavy internal

faults in order to avoid the severe explosion of the transformer. A metalized diaphragm is

fitted at bottom and during heavy faults, the diaphragm ruptures and oil rushes out from

the vent. The level of the explosion vent is normally maintained above the level of the

conservatory tank and an equaliser pipe is connected between the top portion of

conservator tank and explosion vent to equalise the pressure between them.

Transformer oil

The transformer oil insulates as well as cools the core and coil assembly. The core and

windings of the transformer must be completely immersed in the oil that normally contains

hydrocarbon mineral oils having high dielectric strength 30KV rms, high resistivity

500X1012Ω-cm, low viscosity 27cs, low dielectric dissipation factor tan δ 0.005, low moisture

content 50ppm, low acidity/oxidation stability 0.03mg / 0.4 KOH/gm, no sulphur, low sludge

0.1%, high flash point 1400C, pour point -9

0C, High specific heat.

Insulating materials

Insulating materials like papers (manila paper, craft paper), cloth, cotton tape impregnated

with varnish for enamelled copper conductor and card /press boards, synthetic resin

bonded materials are used to isolate between consecutive layers & primary and secondary

windings from each other as well as the transformer core. These windings are made of

copper due to high conductivity and ductility. High conductivity minimizes the amount of

copper needed and minimizes losses. Moreover, high ductility results in easy bending of

conductors into tight winding around the core that also minimizes the amount of copper

and volume of winding.

Tap changer

To maintain voltage variations within permissible limit ( as prescribed in I.E.Rule 1956), tap

changers are used. There are two types of tap changers – on load and off load. In on load

tap changers, tapping can be changed without isolating transformer from the supply, while

in off load, the transformer needs to be disconnected from the supply. Presently, Automatic

tap changers are used in EHV substation.

Cooling tubes

As the name suggests, cooling tubes / fins or radiator tank are used to cool the transformer

oil. The circulation of oil within the transformer may be natural or forced. In the case of

natural circulation, when the oil temperature rises, the hot oil naturally moves to the top

and cold oil moves down due to thermo symphonic effect, while in case of forced

circulation, an external pump is used.

Bushing

Connections from the transformer windings are brought out by means of bushing. It

provides insulation between conductor and main tank which is at earthed potential.

Ordinary porcelain insulators are used upto 33kV. For above voltage rating capacitor and

oil filled type bushings are used.

Concept of ideal transformer.

To understand the working of a transformer it is always instructive, to begin with the concept of

an ideal transformer with the following properties.

1. Primary and secondary windings has no resistance.

2. All the flux produced by the primary links the secondary winding i,e., there is no

leakage flux.

3. Permeability μ

r

of the core is infinitely large. In other words, to establish flux in the

core vanishingly small (or zero) current is required.

4.Core loss comprising of eddy current and hysteresis losses are neglected.