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Date Code 20011026
Maintenance and Testing
7-9
SEL-321/321-1 Instruction Manual
Maintenance Testing
When:
At regularly scheduled intervals, or when there is an indication of a problem with
the relay or system.
Goal:
a) Ensure that the relay is measuring ac quantities accurately.
b) Ensure that scheme logic and protection elements are functioning correctly.
c) Ensure that auxiliary equipment is functioning correctly.
What to
test:
Anything not shown to have operated during an actual fault within the past
maintenance interval.
SEL relays use extensive self-testing capabilities and feature detailed metering and event
reporting functions that lower the utility dependence on routine maintenance testing.
Use the SEL relay reporting functions as maintenance tools. Periodically verify that the relay is
making correct and accurate current and voltage measurements by comparing the relay METER
output to other meter readings on that line. Review relay event reports in detail after each fault.
Using the event report voltage, current, and relay element data you can determine that the relay
protection elements are operating properly. Using the event report input and output data you can
determine that the relay is asserting outputs at the correct instants and that auxiliary equipment is
operating properly. At the end of your maintenance interval, the only items that need testing are
those that have not operated during the maintenance interval.
The basis of this testing philosophy is simple: If the relay is correctly set and connected, is
measuring properly, and no self-test has failed, there is no reason to test it further.
Each time a fault occurs, the protection system is tested. Use event report data to point out areas
requiring attention. Slow breaker auxiliary contact operations, increasing or varying breaker
operating time, fluctuations in total communications channel delay can all be detected through
detailed analysis of relay event reports.
Because SEL relays are microprocessor-based, their operating characteristics do not change over
time. Time-current operating times and distance element characteristics are affected only by the
relay settings and applied signals. It is not necessary to verify operating characteristics as part of
maintenance checks.
At SEL, we recommend that maintenance tests on SEL relays be limited under the guidelines
provided above. The time saved may be spent analyzing event data and thoroughly testing those
systems that require more attention.
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