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Petroleum Analysis
The dielectric constant of petroleum and petroleum products may be used to indicate the presence of various constituents, such as asphaltenes, resins, or oxidized materials. Furthermore, the
dielectric constant of petroleum products that are used in equipment, such as condensers, may actually affect the electrical properties and performance of that equipment (ASTM D877).
The dielectric constant of hydrocarbons and hence most crude oils and their products is usually
low and decreases with an increase in temperature. It is also noteworthy that for hydrocarbons,
hydrocarbon fractions, and products the dielectric constant is approximately equal to the square
of the refractive index. Polar materials have dielectric constants greater than the square of the
refractive index.
10.5.3 dIeleCtrIC strengtH
The dielectric strength, or breakdown voltage (ASTM D877), is the greatest potential gradient or
potential that an insulator can withstand without permitting an electric discharge. The property
is, in the case of oils as well as other dielectric materials, somewhat dependent on the method of
measurement; that is, on the length of path through which the breakdown occurs, the composition, shape, and condition of the electrode surfaces, and the duration of the applied potential
difference.
The standard test used in North America is applied to oils of petroleum origin for use in cables,
transformers, oil circuit breakers, and similar apparatus. Oils of high purity and cleanliness show
nearly the same value under standard conditions, generally ranging from 30 to 35 kV. For alkanes,
dielectric strength has been shown to increase linearly with liquid density, and the value for a
mineral oil fits the data well. For n-heptane a correlation was found between the dielectric strength
and the density changes with temperature. There are many reasons that the dielectric strength of
an insulator may fail. The most important appears to be the presence of some type of impurity,
produced by corrosion, oxidation, thermal or electrical cracking, or gaseous discharge; invasion by
water is a common trouble.
10.5.4 dIeleCtrIC loss And PoWer FACtor
A condenser insulated with an ideal dielectric shows no dissipation of energy when an alternating
potential is applied. The charging current, technically termed the circulating current, lags exactly
90° in phase angle behind the applied potential, and the energy stored in the condenser during each
half-cycle is completely recovered in the next. No real dielectric material exhibits this ideal behavior; that is, some energy is dissipated under alternating stress and appears as heat. Such a lack of
efficiency is broadly termed dielectric loss.
Ordinary conduction comprises one component of dielectric loss. Here the capacitanceheld charge is partly lost by short circuit through the medium. Other effects in the presence
of an alternating field occur, and a dielectric of zero conductivity may still exhibit losses.
Suspended droplets of another phase undergo spheroidal oscillation by electrostatic induction
effects and dissipate energy as heat as a consequence of the viscosity of the medium. Polar
molecules oscillate as electrets and dissipate energy on collision with others. All such losses
are of practical importance when insulation is used in connection with alternating-current
equipment.
The measure of the dielectric loss is the power factor. This is defined as the factor k in the
relation
k
W
EI
=
where W is the power in watts dissipated by a circuit portion under voltage E and passing current, I.
Petroleum Analysis
The dielectric constant of petroleum and petroleum products may be used to indicate the presence of various constituents, such as asphaltenes, resins, or oxidized materials. Furthermore, the
dielectric constant of petroleum products that are used in equipment, such as condensers, may actually affect the electrical properties and performance of that equipment (ASTM D877).
The dielectric constant of hydrocarbons and hence most crude oils and their products is usually
low and decreases with an increase in temperature. It is also noteworthy that for hydrocarbons,
hydrocarbon fractions, and products the dielectric constant is approximately equal to the square
of the refractive index. Polar materials have dielectric constants greater than the square of the
refractive index.
10.5.3 dIeleCtrIC strengtH
The dielectric strength, or breakdown voltage (ASTM D877), is the greatest potential gradient or
potential that an insulator can withstand without permitting an electric discharge. The property
is, in the case of oils as well as other dielectric materials, somewhat dependent on the method of
measurement; that is, on the length of path through which the breakdown occurs, the composition, shape, and condition of the electrode surfaces, and the duration of the applied potential
difference.
The standard test used in North America is applied to oils of petroleum origin for use in cables,
transformers, oil circuit breakers, and similar apparatus. Oils of high purity and cleanliness show
nearly the same value under standard conditions, generally ranging from 30 to 35 kV. For alkanes,
dielectric strength has been shown to increase linearly with liquid density, and the value for a
mineral oil fits the data well. For n-heptane a correlation was found between the dielectric strength
and the density changes with temperature. There are many reasons that the dielectric strength of
an insulator may fail. The most important appears to be the presence of some type of impurity,
produced by corrosion, oxidation, thermal or electrical cracking, or gaseous discharge; invasion by
water is a common trouble.
10.5.4 dIeleCtrIC loss And PoWer FACtor
A condenser insulated with an ideal dielectric shows no dissipation of energy when an alternating
potential is applied. The charging current, technically termed the circulating current, lags exactly
90° in phase angle behind the applied potential, and the energy stored in the condenser during each
half-cycle is completely recovered in the next. No real dielectric material exhibits this ideal behavior; that is, some energy is dissipated under alternating stress and appears as heat. Such a lack of
efficiency is broadly termed dielectric loss.
Ordinary conduction comprises one component of dielectric loss. Here the capacitanceheld charge is partly lost by short circuit through the medium. Other effects in the presence
of an alternating field occur, and a dielectric of zero conductivity may still exhibit losses.
Suspended droplets of another phase undergo spheroidal oscillation by electrostatic induction
effects and dissipate energy as heat as a consequence of the viscosity of the medium. Polar
molecules oscillate as electrets and dissipate energy on collision with others. All such losses
are of practical importance when insulation is used in connection with alternating-current
equipment.
The measure of the dielectric loss is the power factor. This is defined as the factor k in the
relation
k
W
EI
=
where W is the power in watts dissipated by a circuit portion under voltage E and passing current, I.