3-56 Overcurrent, Voltage, Synchronism Check, Frequency, and Power Elements Date Code 20001006
SEL-351 Instruction Manual
VOLTAGE SAG, SWELL, AND INTERRUPTION ELEMENTS (AVAILABLE IN FIRMWARE
VERSIONS 3 AND 4)
The SEL-351 Relay has three types of elements to detect voltage quality problems. These elements
detect voltage sags, swells, and interruptions.
Voltage Sag Elements:
If the magnitude of a phase voltage drops below the voltage sag percentage setting of the positive-
sequence memory voltage magnitude for one cycle, the corresponding SAG Relay Word bit for that
phase asserts (SAGA, SAGB, or SAGC). If all three phase SAG elements assert for one cycle, an
additional Relay Word bit asserts - SAG3P. The SAG elements remain asserted until the
magnitude of the corresponding phase voltage rises and remains 5% above the sag pickup
threshold.
Figure 3.31: Voltage Sag Elements
Voltage Swell Elements:
If the magnitude of a phase voltage rises above the voltage swell percentage setting of the positive-
sequence memory voltage magnitude for one cycle, the corresponding SW Relay Word bit for that
phase asserts (SWA, SWB, or SWC). If all three phase SW elements assert for one cycle, an
additional Relay Word bit asserts - SW3P. The SW elements remain asserted until the magnitude
of the corresponding phase voltage drops and remains 5% below the swell pickup threshold.
Courtesy of NationalSwitchgear.com
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