3-34 Protection Functions Date Code 20080110
SEL-387E Instruction Manual
Residual Current Sensitivity Threshold
The second criterion of 50GP relates to the relative sensitivity of the winding CTs compared to
the neutral CT. Use the following equation to determine the minimum second criterion for 50GP:
3CTR
maxCTR
•I05.0minGP50
nom
⋅≥
where CTR3 is the neutral CT ratio and CTRmax is the greatest CT ratio among the CTs being
used for the REF function.
The 32IOP setting defines which line CTs the relay uses for REF. For example, if 32IOP = 12,
CTRmax is the greatest of CTR1 or CTR2.
An example 50GP calculation is as follows, assuming that CTR3 = 40, CTRmax = 160,
I
nom
= 5 A, and load imbalance is 10 percent:
40
160
•A5•05.0minGP50 ≥
4•A25.0minGP50
≥
A0.1minGP50
≥
Criterion 2 minimum setting of 50GP is 1.0 A. With a 10 percent load imbalance, we can assume
the criterion 1 value to be 0.1 • 5 A, or 0.5 A. Because 50GP must be set at the greater of the two
criteria values, we would select a setting of 1.0 A.
If you attempt to save a 50GP setting that is too low, the relay will respond (for a 5 A relay),
“Minimum 50GP sensitivity is CTRmax/CTR3 • 0.25.” The relay then will prompt you for a new
setting.
The relay stores a default setting for the Residual Current Sensitivity Threshold of 5OGP = 0.5 A.
O
VERCURRENT
E
LEMENT
Application Description
The SEL-387E Relay provides numerous overcurrent elements, as many as 11 per winding, 33
total. Four levels of phase instantaneous/definite-time elements are available for single- or
three-pole feeder protection, breaker failure protection, overcurrent phase selection for targeting,
transformer backup protection, etc. Two levels of negative-sequence and residual instantaneous
elements provide protection against unbalanced conditions and ground faults. A phase, negative-
sequence, and residual time-overcurrent element are available for system backup protection.
Operating Characteristic
Each winding input of the SEL-387E Relay has 11 overcurrent elements (see Table 3.1). Nine of
these 11 are torque-controlled elements, of which there are one definite-time element, one
instantaneous element, and one inverse-time element per each of three categories. These
categories are phase, negative-sequence, and residual current. Two of the 11 overcurrent
elements, 50P
n
3 and 50P
n
4, are not torque controlled. These two elements are phase
instantaneous overcurrent elements that provide output information per phase and, through an OR
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