9424200996 84-13
BE1-11m Settings Calculation Examples
Select three-phase mode and set the Pickup setting to 71.77 Secondary A as shown in Figure 84-15.
Figure 84-15. Protection, Current, Instantaneous Overcurrent (50-1) Screen
Instantaneous Overcurrent (50-2) - Ground Fault Protection
The system is solidly grounded so that a ground fault draws current in the magnitude of a three-phase
fault per IEEE C37.96-2000, IEEE Guide for AC Motor Protection. To determine the three-phase fault
current, consider that the motor subtransient reactance of 0.15 is the limiting impedance. The pickup
setting is calculated using Equation 84-7. A pickup setting of 1/3 of the instantaneous three-phase fault
current is recommended per IEEE C37.96-2000, IEEE Guide for AC Motor Protection.
=
1
"
1
3
=
1
0.15
363
80
1
3
= 10.08
Equation 84-7. 50-2 Pickup
Select 3I0 (residual) mode and set the Pickup setting to 10.08 Secondary A as shown in Figure 84-16.
Figure 84-16. Protection, Current, Instantaneous Overcurrent (50-2) Screen
Instantaneous Overcurrent (50-3) - Unbalance Protection
The NEMA MG-1 standard states that the existence of unbalanced voltage feed results in four to 10 times
the current imbalance. The negative-sequence voltage unbalances should not exceed 5 percent. This
application uses negative-sequence current as the input to an overcurrent element to sense unbalance
conditions. For conservative protection, choose 4 • 5%, which is 20 percent imbalance. A 10 second
delay is appropriate because the heating is slow (motor mass is large). The pickup setting calculation is
shown in Equation 84-8.
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