Capacitive leakage currents in the
insulating material within bushings cause dielectric losses.
Dielectric losses within a bushing can be calculated by the following
equation using data directly from the nameplate or test report:
Pd = 2 pi f C V2 tan ��
where
Pd = dielectric losses, W
f = applied frequency, Hz
C = capacitance of bushing (C1), F
V = operating voltage, rms V
tan �� = dissipation
factor, p.u.
A bushing operating at rated voltage
and current generates both ohmic and dielectric losses within the
conductor and insulation, respectively. Since these losses, which
both appear in the form of heat, are generated at different locations
within the bushing, they are not directly additive.
However, heat generated in the
conductor influences the quantity of heat that escapes from within
the core. A significant amount of heat generated in the conductor
will raise the conductor temperature and prevent losses from escaping
from the inner surface of the core.
This causes the dielectric losses to
escape from only the outer surface of the core, consequently raising
the hottest-spot temperature within the core.
Most insulating materials display an
increasing dissipation factor, tan ��, with higher
temperatures, such that as the temperature rises, tan ��
also rises, which in turn raises the temperature even more. If this
cycle does not stabilize, then tan �� increases
rapidly, and total failure of the insulation system ensues.
Bushing failures due to thermal
instability have occurred both on the test floor and in service. One
of the classic symptoms of a thermal-stability failure is the high
internal pressure caused by the gases generated from the
deteriorating insulation.
These high pressures cause an
insulator, usually the outer one because of its larger size, either
to lift off the flange or to explode. If the latter event occurs with
a porcelain insulator, shards of porcelain saturated with oil become
flaming projectiles, endangering the lives of personnel and causing
damage to nearby substation equipment.
Note from Equation that the operating
voltage, V, particularly influences the losses generated within the
insulating material. It has been found from testing experience that
thermal stability only becomes a factor at operating voltages 500 kV
and above.
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