Applications of Insulating Materials

All electrical systems require some kind of insulation to prevent short circuits and leaking currents.
3 forms of insulators: solid, liquid and gaseous
Performance of these insulators depend on the temperature
Classification according to their temperature rating.

Impregnation: Letting the solid material absorb some liquid
With the advent of new materials, the classification has been modified by International Electrotechnical Commission:

The transformer insulation:
(a) conductor or turn-to-turn insulation,
(b) coil-to-coil insulation,
(c) low voltage coil-to-earth insulation,
(d) high voltage coil-to-low voltage coil insulation, and
(e) high voltage coil-to-ground insulation.

Transformer oil
-- provides the required dielectric strength and insulation
-- cools the transformer by circulating itself through the core and the coil structure.
-- should be in the liquid state over the complete operating range of temperatures between -40°C and+50°C.
-- gets oxidized when exposed to oxygen at high temperatures
-- formation of peroxides, water, organic acids and sludge.
-- chemical deterioration of the paper insulation and the metal parts of the transformer.
-- sludge being heavy, reduces the heat transfer capabilities of the oil, and also forms as a heat insulating layer on the
coil structure, the core and the tank walls.
-- the effects of oxidation are minimized by designing them such that access to oxygen itself is limited.
-- sealed transformers
-- filling with nitrogen gas,
-- providing oxygen absorbers like activated clay or alumina
-- arc discharge inside a transformer decomposes the oil and causes explosions
Application in Rotating machines
-- low voltage machines: up to 6,600 V , class E or F insulation
-- high voltage machines: 6,600 V and up, Class F insulation
-- machines above 22 kV rating are not built except under special conditions.

Application in Circuit Breakers
A circuit breaker: a switch which automatically opens the circuit when a critical current or voltage rating
is exceeded.
-- AC currents are considerably easier to interrupt than DC currents.
-- AC current interruption sequence:
1) an arc for part of the metallic circuit
2) its deionization when the current goes through zero, so that the arc will not form again.
Circuit breakers categories: the low voltage and high voltage types.
Low voltage breakers
-- use synthetic resin moldings to carry the metallic parts.
-- for higher temperatures ceramic parts are used.
-- if the arc is likely to come into contact with molded parts, melanine or some special kind of
alkyd resins are used because of their greater arc resistance.
High voltage breakers: air circuit breakers and oil circuit breakers.
-- many insulating fluids are suitable for arc extinction
-- the choice depends on the rating and type of the circuit breaker.
-- commonly used insulating fluids:
-- atmospheric air,
-- compressed air,
-- high vacuum,
-- SF6 and
-- oil (transformer oil) (interrupts the arc)
-- Askarels produce large quantities of toxic and corrosive products.
The circuit breaker bushings of lower voltage ratings may consist of solid cylinders of porcelain and
shellac or resin treated paper wrapped on the current carrying electrode.
High voltage bushings of voltages of 66 kV and above are filled with oil. The constructional details vary
widely.
Application in Cables
-- Synthetic rubbers and plastics are used as cable insulation.
-- The insulator should have
-- good elongation and tensile strength and toughness to withstand handling during installation and
service.
-- low dielectric constant and power factor
-- high dielectric strength and insulation resistance.
-- excellent resistance to ageing at high temperatures.
-- resistance to long exposure to sunlight and various chemicals.
-- High voltage cables also give rise to ozone and the insulation will deteriorate in its presence. This is
particularly severe for the insulation near the conductors.
--Underwater applications require very low water absorption for the insulator.
--At low temperature the insulation should not become stiff and brittle.
--The partial discharges in the cable insulation should also be kept as low as possible.

Application in Cables
-- Synthetic rubbers and plastics are used as cable insulation.
-- The insulator should have
-- good elongation and tensile strength and toughness to withstand handling during installation and
service.
-- low dielectric constant and power factor
-- high dielectric strength and insulation resistance.
-- excellent resistance to ageing at high temperatures.
-- resistance to long exposure to sunlight and various chemicals.
-- High voltage cables also give rise to ozone and the insulation will deteriorate in its presence. This is
particularly severe for the insulation near the conductors.
--Underwater applications require very low water absorption for the insulator.
--At low temperature the insulation should not become stiff and brittle.
--The partial discharges in the cable insulation should also be kept as low as possible.

Applications in Power Capacitors
power capacitors are used for
-- voltage regulation of power transmission systems
-- the improvement of power factor of power distribution networks
-- power factor correction in high frequency heaters and induction furnaces
5
also used in
-- DC applications such as
-- impulse voltage generators
-- energy storage
-- welding
-- high intensity flash x-ray and light photography.
Power capacitors:
-- voltage ratings from 220 to 13800 V
-- power ratings from 0.5 to 25 kVAR.
Power capacitors are made of
-- several layers of insulation paper of adequate thickness
-- aluminium foil of 6 microns thickness as electrodes interleaved and wound.
-- single units are connected in parallel to increase capacity
-- placed in containers hermetically sealed, thoroughly dried, and then impregnated with insulating oil:
-- mineral oil (high cost)
-- chlorinated diphenyl oil (low cost, non-inflammable; hence, preferred)
Properties required for the insulation paper:
-- high dielectric strength
-- low dielectric loss
-- high dielectric constant
-- uniform thickness, and
-- minimum conducting particles.
Compared to paper, polypropylene film has considerable power dielectric loss and higher operating
voltage. However, paper is still widely used partly, mainly due to the reason that paper after impregnation
offers many desirable properties required for use at high voltages in addition to economy.
Arrhenius Equation
Temperature is important in the performance of insulators because certain reaction rates (for chemical
reactions that degrade the insulation) are strong functions of temperature. The dependence of some
chemical reactions on temperature can be modeled by the Arrhenius Equation:
RR = Ce−Eth /(kT ) where C is a constant with no or a weak dependence on temperature, Eth is the threshold
energy fort he reaction, k is the Boltzman’s constant, T is the temperature in °K
In the Arrhenius equation, note that when T is small, the exponential term is small and when
T is large the exponential term is large. Note also that the threshold energy for the reaction
determines the meaning of "small" and "large" temperature. Thus we see that degradation of insulation by
chemical reactions can be controlled to some extent by controlling the operating temperature range.
Loss Tangent
Since temperature is very important for electrical insulation, it is important that electrical
losses within the insulation be quantified. This is done with the loss tangent. In the notes
shown below, a tilde (~) is used to indicate phasor voltage and phasor current. Phasors allow us to
represent sinusoidal quantities with complex quantities.
To treat losses in a dielectric, consider that we construct a parallel plate capacitor from the

dielectric and energize it with ac voltage:

Phasor Diagram: