Schweitzer Engineering Laboratories SEL-351-5, Instruction Manual

Page: 130 / 706

3.50
SEL-351-5, -6, -7 Relay Instruction Manual
Date Code 20070117
Overcurrent, Voltage, Synchronism Check, Frequency, and Power Elements
Frequency Elements
Note that all six frequency elements are controlled by the same undervoltage
element (Relay Word bit 27B81). For example, when group setting VNOM
≠
OFF, and Global setting PTCONN = WYE, 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.
However, if group setting VNOM = OFF, Relay Word bit 27B81 is only
affected by the voltage applied to the
VA-N terminals. This is useful in
applications where there is only single-phase voltage available to the relay.
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
≥
1) and make setting 27B81P. Apply Relay
Word bit 27B81 in desired logic scheme, using SEL
OGIC
control equations.
Even though frequency elements are enabled, the frequency element outputs
(Relay Word bits 81D1T–81D6T) do not have to be used.
Frequency Element
Uses
The instantaneous frequency elements (81D1–81D6) are used in testing only .
The time-delayed frequency elements (81D1T–81D6T) are used for
underfrequency load shedding, frequency restoration, and other schemes.