408 POWER PLANT ENGINEERINGInsulating Property. It is usually unnecessary to trouble about the insulating properties of oil,
since it is always sufficiently good. A more important matter, however, is the reduction of the dielectric
strength due to the presence of moisture, which must be rigorously avoided. A very small quantity of
water in oil greatly lowers its value as an insulator, while the presence of dust and small fibers tends to
paths of low resistivity.
Flash Point. The temperature at which the vapour above an oil surface ignites spontaneously is
termed the flash point. A flash point of not less than about 160°C is usually demanded for reasons of
safety.
Fire Point. The temperature at which an oil will ignite and continue burning is about 25 percent
above the flash point, or about 200°C.
Purity. The oil must not contain impurities such as sulphur and its compounds. Sulphur when
present causes corrosion of metal parts, and accelerates the production of sludge.
Sludging. This is the most important characteristic. Sludging means the slow formation of semi-
solid hydrocarbons, sometimes of an acidic nature, which are deposited on windings and tank walls. The
formation of sludge is due to heat and oxidation. In its turn it makes the whole transformer hotter, thus
aggravating the trouble, which may proceed until the cooling ducts are blocked and the transformer
becomes unusable owing to overheating. Experience shows that sludge is formed more quickly in the
presence of bright copper surfaces. The chief remedy available is to use oil, which remains without
sludge formation after long periods of heating in the presence of oxygen, and to employ expansion
chambers to restrict the contact of hot oil with the surrounding air.
Acidity. Among the products of oxidation of transformer oil are CO, volatile water-soluble or-
ganic acids, and water. These in com-bination can attack and corrode iron and other metals. The pro-
vision of breathers not only prevents the ingress of damp air, but also helps on out-breathing to absorb
any moisture produced by oxida-tion of the oil. Oil conservators are desirable to avoid the con-densation
of water-soluble acids on the under surface of the tank lid from which acidic droplets may fall back into
the oil.
- Inhibited Oil. The deterioration of oil during its working life can be retarded by the use of
anti-oxidants, particularly oxidation “inhibitors.” The latter, which are usually of the phenolic or amino
type, convert chain-forming molecules in the oil into inactive molecules, being gradually consumed in
the process. Inhibitors greatly prolong the phase in the service life of the oil, which precedes the onset of
deterioration, and during which the acid and sludge formations are substantially zero. - Synthetic Transformer Oil. This has been developed to avoid the risk of fire and explosion,
present always with normal mineral oils. Chlorinated diphenyl, a synthetic oil suitable for transformers,
is chemically stable, non-oxidizing, rather volatile, and heavier than water. Its dielectric strength is
higher than that of mineral oil, and moisture has a smaller tendency to migrate through it. The per-
mittivity is 4.5, compared with about 2.5. This high figure is roughly the same as the permittivity of the
solid insulating material used in a transformer, so that the distribution of electric stress will differ mark-
edly from that when mineral oil is used, the stress in the oil being relieved at the expense of the solid
insulation. The oil is a powerful solvent of most varnishes, gums, binders and paints, which must conse-
quently be barred from transformers designed for synthetic oil cooling. When decomposed by electric
are, hydrogen chloride gas is the chief product: this may combine with water to form hydrochloric acid. - Temperature Rise. The temperature rises permitted in the British Standard Specification for
power and lighting transformers.