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QUICK REFERENCE GUIDE
12.9
Motor Protection
469 MOTOR MANAGEMENT RELAY – QUICK REFERENCE GUIDE
Ground CT:
For high resistive grounded systems, sensitive ground detection is possible with the
50:0.025 CT. On solidly grounded or low resistive grounded systems where the fault current
is much higher, a 1A or 5A secondary CT should be used. If residual connection is used,
pickup levels and timers must be set with respect to the acceleration time. The chosen zero
sequence CT needs to be able to handle all potential fault levels without saturating. In this
example, 50:5A CT is selected.
Motor FLC:
Set the Motor Full Load Current to 348A, as specified by the data sheets.
The above data will be used to set the relay System Parameters, such as CT and VT
connections, VT secondary voltage, and CT and VT primary to secondary ratios.
Overload Pickup:
The overload pickup is set to the maximum allowed by the service factor of the motor.
Since this motor has RTDs and the relay will be using the RTD bias feature for enhanced
protection. In this case, it would be set to the highest setting of 1.25 x FLC for the motor
service factor of 1.15. If service factor is unknown we must assume 1.0.
Overload Curve:
The standard overload curve to be just below the cold thermal limit to give maximum
process uptime, without compromising protection. The best fitting curve is curve 7.
Short Circuit Trip:
The short circuit trip should be set above the maximum locked rotor current but below the
short circuit current of the fuses. The data sheets indicate a maximum locked rotor current
of 630% FLC or 6.3 x FLC. A setting of 7 x FLC with an instantaneous time delay will be ideal
but nuisance tripping may result due to the asymmetrical starting currents and DC offset.
If asymmetrical starting currents limits the starting capability, set the S/C level higher to a
maximum of 11 x FLC to override this condition (1.7x6.3=11.7 where 1.7 is the maximum
DC offset for an asymmetrical current).
Ground Fault:
Unfortunately, there is not enough information to determine a ground fault setting. These
settings depend on the following information:
1.
The Ground Fault current available.
2.
System Grounding - For example high resistive grounding, or solidly grounded.
3.
Ground Fault CT used.
4.
Ground Fault connection, which could be zero sequence or Residual connection.
For the purpose of this example, we will assume a fault current of 10 Amps or 10/50 = 0.2 x
CT, no intentional time delay.
SYSTEM DATA
49
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