Schweitzer Engineering Laboratories SEL-701, Instruction Manual

Page: 91 / 468

Date Code 20010719
701 Motor Protection Relay
Settings Calculation
Basic Motor Protection
4.27
When the relay is equipped with optional voltage inputs, the load-loss detection
function depends on the power used by the motor. The relay arms the load-loss
detection logic a settable time after the motor starts, as defined by the Load Loss
Starting Time Delay setting. Set this delay to allow pumps or compressors to reach
normal load. Once armed, the function issues an alarm or trip if three-phase power
drops below the alarm or trip threshold for the specified time delay. Set the Load Loss
Trip and Alarm thresholds greater than the expected motor no load power but less than
the minimum power expected when the motor is operating normally.
If you expect the motor to operate at no load normally, disable this function by
setting LLAPU equal to OFF. The relay automatically hides the remaining load-loss
settings.
Current Unbalance Elements
Table 4.19 Current Unbalance Element Settings
Unbalanced motor terminal voltages cause unbalanced stator currents to flow in
the motor. The negative-sequence current component of the unbalance current causes
significant rotor heating. While the SEL-701 Relay motor thermal element models the
heating effect of the negative-sequence current, many users desire the additional
unbalance and single-phasing protection offered by a current unbalance element.
The SEL-701 Relay calculates percent unbalance current in one of two ways
depending on the magnitude of the average current. When the average current, Iav, is
greater than the motor-rated full load current, the relay calculates percent unbalance:
Equation 4.2
Table 4.19 Current Unbalance Element Settings
Setting Prompt
Setting Range Setting Name =
Example Settings
Current Unbalance Alarm Pickup OFF, 2%–80% 46UBA = 15
Current Unbalance Alarm Delay 0.10–400.00 s 46UBAD = 10.00
Current Unbalance Trip Pickup OFF, 2%–80% 46UBT = 20
Current Unbalance Trip Delay 0.10–400.00 s 46UBTD = 5.00
U B% 100%
Im Iav –
( )
Iav
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•
=