The most frequent failure in electrical equipment is the
degradation and breakdown (flashover) of the insulation. Hence, it is necessary
to mention in this module n basic electrical theory the effect of operating
environment on electrical machine insulation.
Electrical insulation can be liquid or solid, organic or
inorganic. Organic insulation material consists of enamels, varnishes, resins,
or polymers that are applied to the steel surface to provide high inter-laminar
(between windings) resistance as found on most air-cooled machinery and some
oil-immersed transformers.
Larger transformers are oil-filled with pure mineral oil to
provide higher insulation capability and more effective heat dissipation when
equipped with external radiators, fans and pumps. Physical insulation inside
these transformers is often in the form of oil-impregnated paper wrapped around
the conductors.
Inorganic insulation material can include a combination of
magnesium oxide, silicates, phosphates, and ceramic powder. This type of
insulation is usually heat-treated into the surface of the steel and is less
common than organic insulation. No matter what the type of insulation, the two
most common contributing factors in insulation failure are moisture and heat.
Excessive Moisture
On air-cooled electrical machinery, the moisture content of
the air is very important. With aging of the insulation, small hairline cracks
will appear in the insulation. Moisture will seep into these cracks and allow
an electrical path to short-circuit between adjacent turns of wire.
Although the voltage between the turns is quite small, when
they short together, a closed loop to the magnetic flux is provided, and this
causes tremendous currents to flow in the shorted loop. This usually destroys
the electrical machine, and it has to be removed and re-wound/replaced.
On oil-cooled machinery (i.e., transformers), moisture can
only be detected by regular oil samples. Moisture will be sucked into the oil
via the oil expansion air vent, through the continuous process of transformer
heating and cooling cycles. Special air dryers (i.e., Drycol) and absorbents
can assist in decreasing the rate the moisture is absorbed into the oil.
Excessive Temperature
On air-cooled electrical machinery, prolonged high
temperature causes thermal aging. This causes the insulation to become brittle.
Eventual failure can occur due to moisture penetration as just discussed, or by
physical contact of conductors.
In oil-filled transformers the effect is called insulation
aging. Chemical aging occurs more rapidly at high temperatures, with the loss
of insulation life being almost exponential with temperature. As an example,
for a standard 65 deg C (temperature rise) rated insulation the loss of life
increases from 0.001% per hour at 1000C to 0.05% per hour at 140 deg C and 1.0%
per hour at 180 deg C.
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