Rectangular Plain Tank

The transformer tank shall be of robust construction rectangular in shape and shall be built up of tested MS sheet.

The internal clearance of tank shall be such that it shall facilitate easy lifting of core with coils from the tank without dismantling LV bushings.

All joints of tank and fittings shall be oil tight and no buldging should occur during service.

The tank design shall be such that the core and windings can be lifted freely. The tank plate shall be of such strength that the complete transformers when filled with oil may be lifted bodily by means of lifting lugs. Inside of tank shall be painted with varnish / hot oil resistant paint.

The four walls of the tank shall be made of two “L” shaped sheets (without joints) fully welded at the corners from inside and outside of the tank for withstanding a pressure of 0.8 kg/cm² for 10 minutes.

The tank shall be reinforced by welded angle on all the outside walls on the edge of the tank to form two equal compartments. Permanent deflection when the tank without oil is subject to a vacuum of 525 mm of mercury for rectangular tank and 760 mm of mercury for round tank shall not be more than 5 mm upto 750 mm length and 6 mm upto 1250 mm length. The tank shall further be capable of withstanding a pressure of 0.8 kg/sq cm (g) and a vacuum of 0.3 kg/sq cm (g) without any deformation.

Pressed steel radiators / circular cross section cooling tubes/ elliptical tubes shall be used for cooling. The transformer shall be capable of giving continuous rated output without exceeding the specified temperature rise. 4 Nos. welded heavy duty lifting lugs of MS plate 8 mm thick (min) suitably reinforced by vertical supporting flat welded edgewise below the lug shall be provided on the side wall.

Top cover fixing shall be with galvanised iron bolts and 6 mm Neoprene bonded cork gasket conforming to IS 4253 part-II shall be placed between tank and cover. The bolts outside tank shall have 2 flat washers & one spring washer.

10.2 Corrugated Tank

The transformer tank shall be of robust construction corrugated in shape and shall be built up of CRCA sheets of 1.2 mm thickness.

The internal clearance of tank shall be such that, it shall facilitate easy lifting of core with coils from the tank with CCA (core-coil assembly), HV & LV bushings mounted on Top cover.

All joints of tank and fittings shall be oil tight and no bulging should occur during service.

The tank design shall be such that the core and windings can be lifted freely. The tank plate shall be of such strength that the complete transformers when filled with oil may be lifted bodily by means of lifting lugs. Inside of tank shall be painted with varnish / hot oil resistant paint.

Corrugated panel shall be used for cooling. The transformer shall be capable of giving continuous rated output without exceeding the specified temperature rise.

2 Nos. welded heavy duty lifting lugs of MS plate 8 mm thick (min) suitably reinforced shall be provided.

Top cover fixing shall be with GI (Galvanised Iron) bolts and 6 mm Neoprene bonded cork gaskets conforming to IS 4253 part-II shall be placed between tank and cover. The bolts outside tank shall have 2 flat washers & one spring washer.

Tanks with corrugations and without conservator shall be tested for le akage test at a pressure of 0.15kg/cm² measured at the top of the tank.

10.3 Sealed Transformer with Radiators

In this type of construction, tank is designed to have cover welded to the curb of tank. Space is provided above the core coil assembly where inert gas cushion system accommodates the oil expansion under variable pressure. The tank should be of stiff construction, able to withstand pressure of 2 atmospheres.

10.4 Conservator

The provision of conservator is obligatory for plain Tanks mentioned in clause 10.1 above i.e., where circular tubes or elliptical tubes or pressed steel radiators are used for oil circulation. Conservator is not required for corrugated tanks.

When a conservator is provided, oil gauge and the plain or dehydrating breathing devise shall be fixed to the conservator, which shall also be provided with a drain plug and a filling hole (M30 normal size thread) with cover. The capacity of a conservator tank shall be designed keeping in view the total quantity of oil and its contraction and expansion due to temperature variations.

In addition, the cover of main tank shall be provided with an air release plug to enable air trapped within to be released, unless the conservator is so located as to eliminate the possibility of air being trapped within the main tank.

The inside diameter of the pipe, connecting the conservator to the main tank, should be within 20 to 50 mm and it should be projected into the conservator so that its end is approximately 20 mm above the bottom of the conservator so as to create a sump for

collection of impurities. The minimum oil level (corresponding to -5 deg.C.) should be above the sump level.

10.5 Surface Preparation and Painting

For surface preparation refer to section A of this Manual.

11.0 TERMINALS

The terminals arrangement alternatives are given below : kVA Voltage Details of Terminals

All ratings 11000 17.5 kV porcelain Bushings as per IS 3347 for normally polluted atmosphere, or 3p 1G air or compound filled cable box suitable for 3core XLPE /PILC aluminium cables.

All ratings 33000 36 kV porcelain Bushings as per IS3347 for normally polluted atmosphere, or 3p 1G air or compound filled cable box suitable for 3core XLPE /PILC aluminium cables.

160/200 433 4p1G air filled cable box suitable for3 1/2core 135mm² PVC aluminium cable 250 433 4p1G air filled cable box suitable for3 1/2core 400mm² PVC aluminium cable 315,400,500 433 4p2G air filled cable box suitable for3 1/2core 400mm² PVC aluminium cable 630,800 433 4p4G air filled cable box suitable for3 1/2core 400mm² PVC aluminium cable 1000,1250 433 4p6G air filled cable box suitable for3 1/2core 400mm² PVC aluminium cable 1600 433 4p21G air filled cable box suitable for 1 core 1000mm² PVC aluminium cable 2000,2500

3000

433 4p28G air filled cable box suitable for 1 core 1000mm² PVC aluminium cable Note : (a) Alternatively 433V terminal could be provided wth 1.1kV bushings as per IS:3347 for normally

polluted atmosphere.

(b) Alternatively 433V terminal could be provided wth 1.1kV bushing suitable for bus duct connections.

(c) Alternatively 433V terminal could be provided wth 1.1kV epoxy bushings in cable box or bus duct.

(d) P & G denote ‘Pole’ and ‘Gland’ respectively.

(e) Epoxy may be used as the filling medium instead of bitumen compound.

(i) The bushings shall conform to the relevant standards specified and shall be outdoor type. The bushing rods and nuts shall be made of brass material 12 mm diameter for both HT & LT. The bushings shall be fixed to the transformers on side with straight pockets and in the same plane or on the top cover. The tests as per IS 2099 / IS 7421 shall be conducted on the transformer bushings.

(ii) For 0.433/11 kV/33 kV service voltage, 1.1/17.5/36 kV class bushings shall be used.

Bushings of plain sheds as per IS-3347 shall be mounted on the tank/cover. For 1.1 kV class indoor transformers, 1.1 kV class epoxy busbar bushings /porcelain bushings can be used. Bushings in HV cable box as per BS2562 may be used for compound filled cable box or termination in air with boots covering the live terminals.

(iii) Dimensions of the bushings shall conform to the Standards specified.

(iv) A minimum phase to phase clearance of 75 mm for LV (upto 1.1 kV bushings) and 255 mm for HV bushings shall be obtained with the bushings mounted on the transformer.

(v) The bushings shall be fixed on the sides with pockets in the same plane or on the top cover.

(vi) Brazing of all inter connections, jumpers from winding to bushing shall have cross- section larger than the winding conductor.

(vii) The design of the cable box internal bushing for LV shall be such as to provide adequate earth clearance and creepage distance as stipulated in the standards specified.

All other tests as per relevant standards shall be applicable.

(viii) The terminal arrangement shall not require a separate oil chamber not connected to oil in the main tank.

(ix) HV and LV bushings shall be mounted on top cover in case of corrugated tank.