Oil Filled Current Transformer for Outdoor Installation

Constructional / Salient Features
AS PER IS : 2705/ IEC 185
1. Tank / Primary terminal chamber / nitrogen chamber 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.
2. Hollow porcelain insulator conforming to IS: 5621 is mounted over the ends 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 & 20 for light or normal pollution level and 25 & 31 for heavy & very heavy pollution level respectively.
3. 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.
4. Insulation with craft paper is done on primary winding having eye-bolt construction. It is divided in a series of condensers which grade the voltage stress both radically as well as axially along the hollow porcelain insulator uniformly.
5. Secondary windings on steroidal cores are mounted on eye-bolt region, i.e. lower part of the primary winding. They are housed in the tank, which forms the lower portion of the transformer. Secondary terminals are brought out in secondary terminal box on the side of the tank.
6. Secondary outputs and accuracy ratings are designed to meet the purchaser’s requirements by primary reconnection or secondary tapings or combination of both.
7. Insulation is dried out by heating in vacuum. Dried and degassed oil conforming to IS: 335 is then filled in the transformer under vacuum.
8. Di-electric strength of the oil-impregnated paper insulation is preserved by hermetic sealing of the transformer. Expansion and prevention of oxidation of oil are taken care by filling dry nitrogen above oil in the nitrogen chamber at atmospheric pressure. All the casketed joints are located below oil level to ensure positive sealing.
9. Each transformer undergoes all the routine tests before dispatch and is delivered safe at site duly packed. However, following precautions must be observed for safety:-

Transformer is to be unpacked fully and made to stand vertical before carrying out any site tests including measurement of IR value by meager. Store the transformer vertically.


Before energizing, all the secondary windings not being used are to be short circuited and grounded; Tan delta terminal is also to be grounded.

10. Primary connection is to be tightened sufficiently to avoid hot connection.
11. As the transformer is hermetically sealed, normal maintenance comprises of only visual inspection to check oil level and possible oil leaks besides cleaning insulator surface periodically depending on the pollution level existent at site.
132 KV CT
220 KV CT
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