68 6LE008146B
Advanced protection functions
5.4 Underfrequency protection (UF-81L)
The underfrequency protection (UF-81L), ANSI 81L permanently monitors the installation
frequency. When the frequency exceeds acceptable limits, either the circuit breaker trips and a
trip alarm is generated, or only an advanced protection alarm is generated.
The frequency of an electrical installation must be maintained within a strict operating range to
limit the risk of damaged caused to the motor loads or sensitive electronic equipment and to
maintain a good performance level of the loads.
Underfrequency protection may be used in an energy production installation (generator,
photovoltaic, etc.).
In addition, continuous monitoring of the frequency allows the appropriate action to be
performed to secure the electrical installation, for example load-shedding, switching the power
source, starting a backup generator.
Operating principle
Underfrequency protection allows the frequency to be monitored.
It launches at the end of the activation time delay when the measured frequency reaches the
activation threshold.
Fmin
tFmin
Adjustment parameters
UF-81L
ANSI 81L
Config.
Configuration of the protection
Inhibit Inhibition of the protection
Threshold Activation threshold Fmin
Time Activation time delay tFmin
Configuration
The configuration defines the protection behaviour when the activation threshold and the time
delay are reached.
Trip Circuit breaker tripping
Alarm Alarm without tripping the circuit breaker
O Protection deactivated (default setting)
Fmin: activation threshold of the
underfrequency protection
tFmin: activation time delay of the
underfrequency protection
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