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Most manufacturers publish energy
capability values of arresters. However, there are no specified tests
either in IEEE/ANSI, or IEC standards. Hence, there is confusion in
using these important application
parameters. Both IEEE and IEC are
contemplating writing standardized tests for the arrester energy
handling capability.
When metal oxide arresters are
energized, valve elements of the arrester will absorb energy which
results in a temperature increase of the valve elements. Under normal
operating conditions (i.e. absence of overvoltage) there is a balance
between the heat generated by the valve elements and the heat
dissipated by the arrester through conduction, convection and
radiation such that a stable operating condition is maintained.
Overvoltage events disturb this stable
condition by causing the valve elements to absorb increased levels of
energy for some limited amount of time. The subsequent response of
the arrester depends greatly on the magnitude and rate of energy
input and on the specific design of the arrester.
For simple applications where
overvoltages are well defined, the resulting energy absorbed by the
arrester can be determined by calculation (use arrester minimum
voltage characteristics for energy calculation).
If the temperature rise of the valve
elements due to energy absorption is too high, the arrester can be
driven into a state of thermal runaway, a condition in which heat
generated exceeds heat dissipated, resulting in further increase in
valve element temperature. It is possible for temperature to reach a
high enough level to cause damage to the valve element material,
leading to an electrical breakdown and failure of the arrester.
If the energy density is sufficiently
high or if the distribution of energy density within the valve
element is non-uniform to cause locally high temperature gradients,
thermomechanical damage in the form of valve element cracking or
puncture may occur. This is possible even if the overall temperature
rise of the valve elements would not have been high enough to drive
the arrester into thermal runaway.
The energy handling capability of metal
oxide arresters is often expressed in terms of kilojoules per kV of
arrester MCOV or per kV of arrester rating (duty cycle). First, the
users must be aware to compare the right parameters, since kJ/kV MCOV
can be 25% higher than kJ/kV of duty-cycle rating. Some manufacturers
also publish the kJ/kV values that are applicable for single shot
energy discharge, and another (higher) value for multi-shot (usually
three) discharges within one-minute period. Spacing between the shots
gives time for the heat to distribute throughout the disk.
The energy handling capability is
dependent on the specific form (magnitude, wave shape and duration)
of the current discharged through the arrester, hence it cannot be
expressed by a single value of kJ/kV. Arrester energy ratings are
typically specified by the manufacturers based on transmission line
discharge (switching surge) tests. Manufacturers should be consulted
for specific types of discharges that the published kJ/kV values
apply.
The energy capability of a MOSA is also
dependent on the operating voltage present subsequent to the
discharge current. The arrester can potentially absorb more energy
than it's rating, without going into thermal instability if the post
event voltage is less than MCOV. Published arrester energy withstand
data do not provide the necessary information to assess the influence
of post-event voltage on arrester energy capability.
1 comment:
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