Schweitzer Engineering SEL-387-0, Instruction Manual

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3.30
SEL-387-0, -5, -6 Relay Instruction Manual Date Code 20170601
Differential, Restricted Earth Fault, Thermal, and Overcurrent Elements
Restricted Earth Fault Element
typical in ring-bus or breaker-and-a-half configurations. With 32IOP set at 12,
the relay sums the residual currents from the Winding 1 and Winding 2 inputs
to create the Operate quantity.
If you are protecting an autotransformer with two breakers and CT sets on one
line end of the winding, set 32IOP at 123 and connect all three sets of CT
inputs to the relay as indicated. The relay sums residual currents from the
three winding inputs to create the Operate quantity. From the standpoint of the
REF function, it does not matter which of the CT sets goes to which relay
winding input. However, it is best to connect the two sets at the same
transformer winding terminal to inputs 1 and 2, so that you have the option of
using the Combined Overcurrent elements.
Calculation of the residual current at each relay winding input is as follows:
IRW n = IAW n + IBW n + ICW n ( n = 1, 2, 3, or 4)
For the neutral CT connection, the relay uses only one of the three ABC
inputs, e.g.,
IBW4 , so the residual current for input 4 would be IRW4 = 0 +
IBW4 + 0 = IBW4.
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:
Equation 3.11
where CTR4 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, CTR2, and CTR4.
An example 50GP calculation is as follows, assuming that CTR4 = 40,
CTRmax = 160, I
NOM
= 5 A, and load imbalance is 10 percent:
Equation 3.12
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),
Out of Range
. 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.
50GPmin 0.05 I
NOM
CTRmax
CTR4
----------------------- • •

50GPmin 0.05 5 A
160
40
--------- • •

50GPmin 0.25 A 4
•

50GPmin 1.0 A
