Date Code 20001006 Overcurrent, Voltage, Synchronism Check, Frequency, and Power Elements 3-55
SEL-351 Instruction Manual
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 3.29 and Figure 3.30.
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.
Other Uses for Undervoltage Element 27B81
If voltage pickup setting 27B81P is applicable to other control schemes, Relay Word bit 27B81
can be used in other logic at the same time it is used in the frequency element logic.
If frequency elements are not being used, Relay Word bit 27B81 can still be used in other logic,
with voltage setting 27B81P set as desired. Enable the frequency elements (setting E81 ³ 1) and
make setting 27B81P. Apply Relay Word bit 27B81 in desired logic scheme, using SELOGIC
Control Equations. Even though frequency elements are enabled, the frequency element outputs
(Relay Word bits 81D1T through 81D6T) do not have to be used.
Frequency Element Uses
The instantaneous frequency elements (81D1 through 81D6) are used in testing only.
The time-delayed frequency elements (81D1T through 81D6T) are used for underfrequency load
shedding, frequency restoration, and other schemes.
Courtesy of NationalSwitchgear.com
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