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BE1-11m Frequency (81) Protection
9 • Frequency (81) Protection
Four frequency (81) elements monitor the frequency of the sensing voltage applied to the BE1-11m. An
element can be configured to protect against underfrequency, overfrequency, or the rate of frequency
change.
The four, identical frequency protection elements are designated 81-1, 81-2, 81-3, and 81-4. Element
logic connections are made on the BESTlogic™Plus screen in BESTCOMSPlus® and element operational
settings are configured on the Frequency settings screen in BESTCOMSPlus. A summary of the logic
inputs and outputs and operational settings appears at the end of this chapter.
BE1-11m protection systems enabled for IEC-61850 communication
(style Mxxxx5xxxxxxxx) have their frequency protection elements fixed
at two underfrequency elements and two overfrequency elements.
BESTCOMSPlus Navigation Path: Settings Explorer, Protection, Frequency, Frequency (81)
HMI Navigation Path: Settings Explorer, Protection, Settings Group x (where x = 0 to 3), Frequency
Protection 81
Frequency Measurement
For four-wire or single-phase sensing connections, the power system frequency is measured through the
BE1-11m A-phase sensing voltage input. For a three-wire connection, the power system frequency is
measured through the A- and B-phase sensing voltage connections. Refer to the Typical Connections
chapter for information on voltage connections. Power system frequency is also measured through the
BE1-11m auxiliary voltage (Vx) sensing input. Refer to the Typical Connections chapter for information on
voltage connections.
To measure frequency, the voltage sensed by the BE1-11m must be greater than 10 Vac. The measured
frequency is the average of two cycles of voltage measurement.
Underfrequency and Overfrequency Protection
Underfrequency and overfrequency protection can be useful for detecting load shedding or islanding. For
example, when a source of distributed generation (DG) is suddenly separated or isolated from the electric
utility, the frequency will change quickly from the nominal value (except for the improbable case of a
perfect load-to-generation match). This makes frequency measurement an excellent method for detecting
an island condition.
Any of the four 81 elements can be configured for underfrequency or overfrequency protection.
Mode
Underfrequency or overfrequency protection is selected through the Mode setting. A setting of Under
selects underfrequency protection and a setting of Over selects overfrequency protection.
Sensing Source
Underfrequency or overfrequency protection can be applied to the voltage sensed at the BE1-11m phase
VT input or auxiliary VT (Vx) input. A Source setting of Phase VT selects the voltage sensed at the phase
VT input and a Source setting of Aux VT selects the voltage sensed at the Vx input.
The Auxiliary VT (Vx) input can be connected to a voltage transformer on the motor side of the breaker to
determine if the motor is spinning when the breaker is open.
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