Technical Specification As Per IS : 2705
|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 / Primary terminal chamber / base 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 ends of primary winding and is bolted to the tank in case of dead tank design. In live tank design, it is mounted under the ends of secondary windings / tank and is bolted to the base. Maximum creep age factor of insulator shall be 3.5 for light / normal pollution level and 4.0 for heavy / 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 flexible rope of copper and has the required number of turns duly insulated in one or more sections. Their ends are connected to the primary terminals in primary terminal chamber fitted to the top of insulator in dead tank design. In live tank design, they are connected to the primary terminals in the tank fitted to the top of insulator.
Insulation with craft paper is done on primary winding in dead tank design, while it is done on secondary winding in live tank design.
In dead tank design, the secondary windings on steroidal cores are mounted in the lower part of the primary winding and housed in the tank which forms the lower part of the transformer. In live tank design, they are mounted inside the primary winding and housed in the tank which forms the upper part of the transformer. Secondary terminals are brought out in secondary terminal box on the side of tank in dead tank and base in live tank design.
Secondary outputs and accuracy ratings are designed to meet the purchaser's requirements by primary reconnection or secondary tapings or combination of both.
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 duly packed. However, following precautions must be observed for its intended performance : •Transformer is to be unpacked fully and made to stand vertical before carrying out site tests. Store the transformer vertically •Before energizing, all the secondary terminals not being used are short circuited and grounded; Tan delta terminal in 66 KV is also to be grounded. Primary connection is to be tightened sufficiently to avoid hot connection.
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.