1.3.
2 Transformer Oil: Oil used for insulation in transformers is mineral oil and it is obtained
by refining crude petroleum. Animal oils and vegetable oils are not used for this purpose as these
form fatty acids on heating which are corrosive for the cellulosic paper used in insulation.
Mineral oils were in use as liquid dielectrics in electrical equipment for over hundred years now.
Despite the availability of a variety of synthetic oils, with far more superior properties, mineral
oils held its way, due to their abundant availability and economy. Three properties that are
fundamental to use of mineral oil as dielectric are:
A. High insulating property,
B. Good oxidative and ageing stability and good heat transferability.
The reliable performance of mineral insulating oil in an insulation system depends upon
certain basic oil characteristics, which can affect the overall performance of the electrical
equipment.
A. High electric strength to withstand the stresses in service
B. Sufficiently low viscosity so that its ability to circulate and transfer heat is not impaired
C. Adequate low temperature properties down to the extreme of climatic conditions expected at
the installation site
D. Proper oxidation resistance to ensure long life in service
E. Good resistance to emulsion to prevent holding water in suspension in it
F. Free from inorganic acid, alkali and corrosive sulphur which causes corrosion of metal parts
and insulation and accelerate the production of sludge
G. Free from sludging under normal separating conditions
H. High flash/fire points
I. Low pour point
J. Low Moisture Content
The main requirements of transformer oil as listed in various national and international
standards only state the minimum requirements for transformer oil (table 1.1), whereas many
transformer producers and electricity companies and boards have their own stringent
specifications based on the required conditions. With advanced refining techniques, it is possible
to produce transformer oil tailor-made to suit the exact requirements for such specification.
1
Table 1.1 Characteristic requirements of IS, IEC and BS specification for uninhibited transformer oil
S.No. Characteristics IS-335 IEC-296 IEC-296 BS-148
(1993) (Class I) (Class II) (Class I)
1 Appearance The oil shall be clear, transparent and free from suspended
matter or sediments.
2 Density(g/cm³) 0.89 @ 0.895 @ 0.895 @ 0.895@
29.5°C(max) 20°C(max 20°C(max) 20°C(max)
3 Kinematic 27 max. @ 16.5 max. 11.0 max. 16.5 max.
Viscosity (cSt) 27°C @ @40°C @40°C 800
40°C 800 max. @ 15°C
max. @-
15°C
4 Interfacial 0.04 - - -
tension
N/m (min.)
5 Flash point°C 140 140 130 140
(min.)
6 Pour point, °C -6 -30 -45 -30
(max)
7 Electric
Strength
(BDV), kV 30(min) 30 (min.) 30 (min.) 30 (min)
i. New (As
unfiltered 60(min) 50 (min.) 50 (min.) delivered)
oil
ii. After
filtration
8 Tan delta @ 0.002 (max) 0.005 (max) 0.005 (max) 0.005 (max)
90°C
9 Resistivity @ 35x10¹²(min) - - -
90°C @ 27°C, 1500x10¹²
Ωcm (min)
10 Corrosive Non- Non- Non- Non-corrosive
sulphur corrosive corrosive corrosive
11 Oxidation 0.4 (max.) 0.4 (max.) 0.4 (max.) 1.5 (max.)
Stability 164
hrs.
i. Neutralization
value mg.
KOH/gm
ii. Sludge 0.1 (max.) 0.1 (max.) 0.1 (max.) 0.1 (max.)
content
2
% by wt.
12 Neutralization 0.03 0.03 0.03 0.03
value
i. Total acidity
mg. Nil - - -
KOH/gm
ii. Sludge
content % by
wt.
13 Oxidation 0.05% max. Non Non Non
inhibitor is detectable detectable detectable
treated as
absence of
inhibitor
14 Water content 50 (max.) 40 (max.) 40 -
(ppm)
15 S.K. Value Under - - -
consideration
16 Accelerated - - -
Ageing Test 2.5x10¹²Ωcm
(open beaker .
method with (min)
copper catalyst) 0.2x10¹²Ωcm
i. Resistivity @ .
27°C (min)
ii. Resistivity @
90°C 0.2 (max.)
iii. Tan Delta at 0.05 (max.)
90°C
iv. Total 0.05% by wt.
Acidity (max.)
mg. KOH/gm
v. Total Sludge
17 Gassing - - - +5(max.)
tendency
