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L = Ae ( B / T ) ………………………………………….(2.1)
where, ‘L’ is the life of the insulation, A & B are constants determined by the reaction
rate of particular degradation, and T is the temperature in Kelvin. However, the above
relation is only valid for high operating temperatures. Every insulation material has a
threshold, below which no thermal ageing will occur. As the degradation process is an
oxidation chemical reaction, the higher the temperature, the faster the chemical
reaction, and the shorter the time to degrade the insulation.
High temperatures do not always have adverse effects in the form of thermal stress. In
certain cases, the stator windings at high temperature can be beneficial. High
temperatures will prevent moisture from settling on windings, thus reducing the risk of
electrical tracking failures. Additionally at high operating temperatures the insulation
may swell, reducing the size of any air pockets within the insulation and decreasing the
partial discharge activity (30).
2.2.2
Mechanical Stress
The stator windings of a rotating machine are subjected to a large amount of
electromagnetic forces. These forces are generated by the interaction of current carried
by the stator windings and the magnetic field in which the conductors are located. The
resultant electromagnetic forces generated vary sinusoidally at twice the supply
frequency. These cyclic forces cause the copper conductors as well as the entire coil to
vibrate at twice the supply frequency, primarily up and down in the slot. There is also a
force in the circumferential direction caused by the rotor’s magnetic field interacting
with the current in the stator bar. Figure 2.2 shows the relative movement of a stator
bar in relation to the laminated core.
These electromagnetic forces cause mechanical vibrations and is regarded as one of
the major causes of premature degradation of stator winding insulation. Maughan et al.
(32)
showed the vibration can cause fretting of insulation which can eventually lead to
mechanical failure of the stator winding. If there is any loosening of bars in the stator
slots, then the vibration forces will cause a mechanical abrasion of the coil sides and
the groundwall insulation. Such friction can cause damage to both the protective
corona shield on the coil surface and the groundwall insulation. This may result in an
increased partial discharge activity and accelerate the process of insulation
degradation. Edwards
(31)
detected the insulation abrasion in the slot region by on-line
partial discharge measurements and reported that such abrasion will lead to a
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