Schweitzer Engineering Laboratories SEL-701, Instruction Manual

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701 Motor Protection Relay
Date Code 20010719
Settings Calculation
Basic Motor Protection
4.28
When the average current is less than motor-rated full load current, the relay
calculates percent unbalance:
Equation 4.3
where:
UB% = Current unbalance percentage
Im = Magnitude of the phase current with the
largest deviation from average
Iav = Magnitude of the average phase current
FLA = Motor-rated full load current
In either case, the function is disabled if the average phase current magnitude is
less than 25% of the Full Load Amps setting.
A 1% voltage unbalance typically causes approximately 6% current unbalance in
induction motors. If a 2% voltage unbalance can occur in your location, set the current
unbalance alarm greater than 12% to prevent nuisance alarms. A 15% current
unbalance alarm pickup setting corresponds to approximately 2.5% voltage unbalance
and a 20% current unbalance trip setting corresponds to approximately 3.3% voltage
unbalance. A 10-second alarm delay and 5-second trip delay should provide adequate
performance in most applications.
Phase Reversal Tripping
Table 4.20 Phase Reversal Tripping Setting
The SEL-701 Relay uses phase currents or phase voltages (if available) to
determine that the phase rotation of signals applied to the relay matches the phase
rotation setting, PHROT. When you set E47T equal to Y, the relay trips 0.5 seconds
after incorrect phase rotation signals are applied to the relay. For relays equipped with
current inputs only, the trip will occur approximately 0.5 seconds after the motor start
is initiated. When the relay is equipped with voltage inputs, the trip will occur
approximately 0.5 seconds after ac voltages are applied to the relay.
NOTE:
When the relay is applied with a single voltage input
(SINGLEV = Y) phase reversal detection is based on phase
current measurement only.
U B% 100%
Im Iav –
( )
FLA
-----------------------------
•
=
Table 4.20 Phase Reversal Tripping Setting
Setting Prompt
Setting Range Setting Name =
Factory Default
Enable Phase Reversal Tripping Y, N E47T = Y