The fundamental performance or reliability criterion is the
acceptable failure rate. This criterion is based on the consequence of failure
and on the expected life of the equipment. Therefore, the failure rate of
transmission lines and substation equipment may be different.
The basis for the increase
is the consequence of failure in the gas-insulated station that may require
significant outage and repair times.
TRANSMISSION LINES
The performance/reliability criterion for lightning is
normally specified as the number of flashovers per 100 km-years. For switching
surges, the flashover rate is normally specified in terms of flashovers per
number of switching operations.
However, the highest magnitude switching surges normally
occur when reclosing, which is normally caused by a fault associated with
lightning. Thus, the two separate criteria may not be appropriate in specifying
the line reliability.
Another criterion, denoted as the storm outage rate, is the
number of unsuccessful reclosures per year and is obtained by multiplying the
lightning flashover rate for the line in units of flashovers per year by the
switching surge flashover rate in terms of flashovers per switching oper—ation.
For example, assuming the lightning flashover rate to be two
per year and the switching surge
flashover rate to be one per 100 switching operations, the
storm outage rate is two per 100 years assuming one reclosing operation per
year.
Both the storm outage rate and the lightning flashover rate
may be important since the lightning fault creates voltages “dips” or
depressions in power frequency voltage, which may affect customer power
quality.
For transmission lines, lightning flashover rates vary with
system voltage and may range from 0.5 for EHV systems to 20 per 100 km-year for
HV systems. Although lines are being designed for switching surge flashover
rates between 1 and 10 flashovers per 100 switching operations, due to other
conservative assumptions, switching surge flashovers are relatively rare.
SUBSTATION
The commonly used reliability criterion for lightning is the
MTBF.
Because the consequence of failure within the station is
greater than the consequence of a flashover of a single line, the station
reliability criterion is higher than the line by a factor of 10. In addition,
transformers, other wound devices, and equipment with non self-restoring insulation
may be arrester protected due to the consequence of failure.
Different criteria are usually applied to air- and
gas-insulated stations. For example, the MTBF for air-insulated stations varies
between 50 and 200 years, whereas for the gas-insulated station, the MTBF has
been set as high as 800 years.
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