instruction manual - BHEL Bhopal

Oil-filling of the tank is done through valve (12) at a low level on the
transformer and at a maximum rate of 4-5 kL/hour. The pressure in the oil
pipes shall be kept positive and shall be checked by a manometer 13 (if
provided). When the oil-level has reached about 50 mm below the cover/the
vacuum pump is stopped it is preferred that dry nitrogen is introduced in the
tank by opening valve (8). Valve (7) is then opened .and valve (6) closed and
the filling is continued in tank and conservator until the correct oil- level has
been reached in the conservator.
Conservator supplied with rubber air cell shall be filled according Clause
2.6. For oil filling in diverter switch assembly of a OLTC refer leaflet (Vol.1).
Separate vacuum-proof cooler system/radiators can be evacuated for
about 1-2 hours and filled separately with purified and degassed oil. The oil is
then circulated through the vacuum filter at least twice via drain valves as
near as possible to the transformer. When the oil circulation has been completed
the valves between coolers and main tank shall be opened.
Care should be taken to keep all air release plugs and valves open to
allow escape of trapped air during oil filling operation. These valves/plugs
should be closed after completion of oil filling.
2.5.7 HOT OIL CIRCULATION
To facilitate oil-penetration and absorption of possible gas bubbles, the
temperature of transformer shall-after completed oil-filling-be increased by
circulating the oil through the vacuum filter and with circulation direction
according to Fig. 2.4.
The oil will be circulated through a vacuum filtration machine till the
parameters are attained as per table 2.3 below.
TABLE 2.3
kV Class of
Transformer
Recommended Permissible Limits
Electric Moisture Resistivity * Tan delta*
Strength Content at 90oC at 90oC (BDV) in
kV (min)
ppm (Max) (Ohm-cm)
Upto 72.5 40 25 1x1012 0.05
145 50 20 1x1012 0.05
245 and 420 60 15 1x1012 0.05
* Subject to availability of testing facility at site.
Method of test for Electric Strength and moisture content shall be as
per IS:335.
CAUTION:
The temperature during oil circulation should not increase beyond 70 o C
otherwise this may cause oxidation of oil.
Prepared By Samvet Lahari, Bhopal. Phone- 5277412