Specifications
182
of metal surfaces. If acidity content becomes excessive in service, the oil should be
replaced with new oil.
vi)
’
Flash
’
point and ‘Pour
’
point should be as per specifications laid down.
vii) It should be chemically stable, i.e. it should not react to oxygen in the air even at high
temperatures.
16.7 Different methods of purifying and drying-out transformer oils :
There are primarily three types of oil purifiers in common use:
i) Centrifugal purifiers such as the
‘
De Laval
’
type.
ii) Filter pack type such as the
‘
Streamline
’
purifiers.
iii) Filters using activated earth media.
The purpose of oil purification is to remove from it, all contaminants such as water, carbon
deposits, dirt, sludge, dissolved moisture and gases. In transformer oils the most important
quality to be preserved is the di-electric strength. This is severely affected by the presence of
water either in the free state or as dissolved moisture. Transformer oils and the insulating materials
used in the transformer windings are hygroscopic, i.e. tend to absorb moisture, which may
enter into the transformer tank due to defective breathers, gaskets or by addition of untreated
make up oil. Circuit breaker and switch, oils get carbonised on account of the tremendous heat
produced in the electric arc before it is interrupted. Dissolved air and excessive heating (due to
arcing) cause oxidation of the oil and formation of heavy sludge deposits at the bottom of the
tank and on the surface of the windings. It is essential to remove all these impurities at least
once in two years by filtering the oil, in order to maintain the equipment in a healthy state.
16.8 Checking out dielectric strength for oil :
Dielectric breakdown strength of transformer oil is one of the most reliable tests for proving
the condition of the oil, and therefore good care is essential in conducting the test, since the
slightest trace of contamination or presence of moisture brings down the breakdown value very
sharply. For example, if finger tips are immersed for a moment or two into an oil cup containing
transformer oil which has been tested for a breakdown value of say 45 kV and test again, it is
astonishing to note that it may now breakdown at 20 kV or even less. So if low values are
recorded in a test, a probable cause may be careless handling. Unless staff are specially trained
and they fully appreciate the importance of perfect cleanliness at every stage of the test, the
results will be completely misleading.
Some of the points to be considered are detailed below :
i) The breakdown values (BDV) given refer to the RMS voltage when tested as per IS
335:1993, using a standard test cell with two 13 mm dia. polished spheres, and a test
gap of 2.5 mm. The shape and spacing of the electrodes has a great influence on the
test values