Technical Specification As Per IS : 3156
|Highest System Voltage( H.S.V.)||12 KV ( r.m.s.)||36 KV (r.m.s.)|
|Power frequency dry/ wet withstand voltage on primary winding for one minute||28 KV ( r.m.s.)||70 KV ( r.m.s.)|
|Lightning Impulse withstand voltage (1.2 x 50 μs||75 KV ( r.m.s.)||170 KV ( r.m.s.)|
Constructional / Salient Features
Tank and top chambers are made of mild steel. Their external surface is painted with one coat of primer and two coats of synthetic enamel paint or hot dip galvanized as per customer requirement. Their internal surface is painted with two coats of an oil-insoluble paint.
Hollow porcelain insulator conforming to IS: 5621 is mounted over the live terminal end of primary winding and is bolted to the tank. Maximum creep age factor of insulator shall be 3.5 for light or normal pollution level and 4.0 for heavy or very heavy pollution level whereas the minimum creep age distance in mm / kv of HSV shall be 16 or 20 for light or normal pollution level and 25 or 31 for heavy or very heavy pollution level respectively.
Primary winding consists of coated synthetic enamel round copper wire in suitable alternate layers of wire and insulating craft paper. It is wound in pyramid construction to have the voltage stress graded uniformly. In unearthed transformer, primary winding is done in two halves and in earthed transformer, it is done in single unit. The live end of the primary winding is connected to top chamber fitted to the top of insulator.
The secondary winding is placed over the core limb prior to the primary winding and the core coil assembly is housed in the tank forming the lower part of the transformer. Secondary terminals are brought out in the secondary terminal box on the side of the tank.
Insulation of VT is dried out by heating in vacuum. Dried and degassed oil conforming to IS:335 is then filled in the transformer under vacuum.
Di-electric strength of the oil-impregnated paper insulation is preserved by hermetic sealing of the transformer. Oil expansion is taken care by filling dry air above oil in the top chamber at atmospheric pressure. All the casketed joints are located below oil level to ensure positive sealing.
Insulation is dried out by heating in vacuum. Dried and degassed oil conforming to IS:335 is then filled in the transformer under vacuum
Di-electric strength of the oil-impregnated paper insulation is preserved by hermetic sealing of the transformer. Oil expansion is taken care by filling dry air above oil, at atmospheric pressure. All the casketed joints are located below oil level to ensure positive sealing.
Each transformer undergoes all the routine tests before dispatch and is delivered safe at site in duly packed condition. However, few precautions must be observed for its intended performance, which are as follows : •Transformer is to be unpacked fully and made to stand vertical before carrying out site tests. •Transformers are to be stored vertically. Before energizing, one terminal of all the secondary windings not being used is required to be grounded.
As the transformer is hermetically sealed, normal maintenance comprises of only visual inspection to check oil level and possible leaks besides cleaning insulator surface periodically depending on the pollution existent at site.