3.58
SEL-311C Relay Instruction Manual Date Code 20060320
Distance, Out-of-Step, Overcurrent, Voltage, Synchronism Check, and Frequency Elements
Frequency Elements
NFREQ = 60 Hz (nominal system frequency is 60 Hz)
E81 ≥ 2 (enable frequency element 2)
81D2P = 59.65 Hz (frequency element 2 pickup)
With these settings:
81D2P < NFREQ
the underfrequency part of frequency element 2 logic is enabled. 81D2 and
81D2T operate as underfrequency elements. 81D2 is used in testing only.
Frequency
Element Operation
Refer to Figure 3.34.
Overfrequency Element Operation
With the previous overfrequency element example settings, if system
frequency is less than or equal to 61.25 Hz (81D1P = 61.25 Hz), frequency
element 1 outputs:
81D1 = logical 0 (instantaneous element)
81D1T = logical 0 (time delayed element)
If system frequency is greater than 61.25 Hz (81D1P = 61.25 Hz), frequency
element 1 outputs:
81D1 = logical 1 (instantaneous element)
81D1T = logical 1 (time delayed element)
Relay Word bit 81D1T asserts to logical 1 only after time delay 81D1D.
Underfrequency Element Operation
With the previous underfrequency element example settings, if system
frequency is less than or equal to 59.65 Hz (81D2P = 59.65 Hz), frequency
element 2 outputs:
81D2 = logical 1 (instantaneous element)
81D2T = logical 1 (time delayed element)
Relay Word bit 81D2T asserts to logical 1 only after time delay 81D2D.
If system frequency is greater than 59.65 Hz (81D2P = 59.65 Hz), frequency
element 2 outputs:
81D2 = logical 0 (instantaneous element)
81D2T = logical 0 (time delayed element)
Frequency Element Voltage Control
Refer to Figure 3.33 and Figure 3.34.
Note that all six frequency elements are controlled by the same undervoltage
element (Relay Word bit 27B81). Relay Word bit 27B81 asserts to logical 1
and blocks the frequency element operation if any voltage (V
A
, V
B
, or V
C
)
goes below voltage pickup 27B81P. This control prevents erroneous frequency
element operation following fault inception.
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