Date Code 20020129
Differential, Restricted Earth Fault,
3-27
Overcurrent, Voltage, and Frequency Elements
SEL-387E Instruction Manual
3. Select the remaining CT ratios (e.g., CTR2–CTR3) by considering the maximum continuous
secondary current, I
LS
, for each winding. Typically, the CT ratio is based on the rated
maximum MVA of the particular winding. If this rating is much smaller than the rating of
the largest winding, you may violate the tap ratio limit for the SEL-387E Relay (see steps 4
and 5). As before, for wye-connected CTs I
REL
equals I
LS
. For delta-connected CTs I
REL
equals
»
3 • I
LS
. Select the nearest standard ratio such that I
REL
is between 0.1 • I
N
and 1.0 • I
N
A
secondary.
4. The SEL-387E Relay calculates settings TAP1 through TAP3 if the ratio TAP
MAX
/TAP
MIN
is
less than or equal to 7.5. When the relay calculates the tap settings, it reduces CT mismatch
to less than 1 percent. Allowable tap settings are in the range (0.1–31) • I
N
.
5. If the ratio TAP
MAX
/TAP
MIN
is greater than 7.5, select a different CT ratio to meet the above
conditions. You can often do this by selecting a higher CT ratio for the smallest rated
winding, but you may need to apply auxiliary CTs to achieve the required ratio. Repeat steps
2 through 5.
6. Calculate the maximum symmetrical fault current for an external fault, and verify that the CT
secondary currents do not exceed your utility standard maximum allowed CT current,
typically 20 • I
N
. If necessary, reselect the CT ratios and repeat steps 2 through 6.
7. For each CT, multiply the burdens calculated in step 1 by the magnitude, in secondary
amperes, of the expected maximum symmetrical fault current for an external fault. Select a
nominal accuracy class voltage for each CT that is greater than twice the calculated voltage.
If necessary, select a higher CT ratio to meet this requirement, then repeat steps 2 through 7.
This selection criterion helps reduce the likelihood of CT saturation for a fully offset fault
current signal.
Please note that the effective C voltage rating of a CT is lower than the nameplate rating if a tap
other than the maximum is used. Derate the CT C voltage rating by a factor of ratio used/ratio
max.
R
ESTRICTED
E
ARTH
F
AULT
E
LEMENT
Application Description
Use the Restricted Earth Fault (REF) element to provide sensitive protection against ground faults
in your wye-connected transformer winding. The element is “restricted” in the sense that
protection is restricted to ground faults within a zone defined by neutral and line CT placement.
Operating Characteristic
Restricted Earth Fault (REF) protection is a technique for sensitive detection of ground faults in a
grounded wye-connected transformer winding. Because it employs a neutral CT at one end of the
winding and the normal set of three CTs at the line end of the winding, REF protection can detect
only ground faults within that particular wye-connected winding. For REF to function, the line-
end CTs must also be connected in wye, because the technique uses comparison of zero-sequence
currents. Delta-connected CTs cancel out all zero-sequence components of the currents,
eliminating one of the quantities the REF element needs for comparison.
The REF implementation in the SEL-387E Relay uses a directional element (32I) that compares
the direction of an operating current, derived from the line-end CTs, with the polarizing current,
