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Metal oxide arresters protect the equipment from high voltage surges by absorbing the energy from the surge. Hence, the energy handling capability (or also called "energy absorption capability" or "energy withstand capability") is an important consideration in the design and application of metal oxide arresters.

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.  

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1 comment:

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