Schweitzer Engineering Laboratories SEL-387E - Residual Current Sensitivity Threshold (50 GP); Setting Calculation; Operating Quantity

3-32 Protection Functions Date Code 20080110
SEL-387E Instruction Manual
For the relay to enable REF, the zero-sequence current at Winding n must be greater than a0
times the positive-sequence current at that input, or |IRW
n | > a0 • |I1W n |. This supervision
provides security against “false I0” that may occur because of CT saturation during heavy three-
phase faults.
Residual Current Sensitivity Threshold (50GP)
Range:
1 A: 0.05–3 A, in 0.01 A steps
5 A: 0.25–15.00 A, in 0.01 A steps
You can set the residual current sensitivity threshold to as low as 0.05 times nominal current
(0.25 A for 5 A nominal CT current), the minimum residual current sensitivity of the relay.
However, the minimum acceptable value of 50GP must meet two criteria:
1. 50GP must be greater than any natural residual current (IR) imbalance caused by load
conditions.
2. 50GP must be greater than a minimum value determined by the relationship of the CTR
n
values used in the REF function.
You must set the threshold setting, 50GP, at the greater of the two criteria values. Determine
criterion 1 for load imbalance. The second criterion relates to the relative sensitivity of the
winding CTs compared to the neutral CT.
Setting Calculation
Operating Quantity
Figure 3.14 depicts how to determine the Operate quantity, 32IOP, setting.