3-50 Protection Functions Date Code 20080110
SEL-387E Instruction Manual
The residual overcurrent elements are automatically disabled if the CT for the associated winding
is connected in delta (W
n
CT = D). The residual element is disabled because the delta connected
CT cannot deliver any residual operating current.
Time-Overcurrent Pickup
Set the phase time-overcurrent element to provide sensitive detection and coordinated time-
overcurrent protection for balanced and unbalanced fault conditions. Use the negative-sequence
time-overcurrent element to provide sensitive detection and coordinated time-overcurrent
protection for unbalanced fault conditions including phase-to-phase, phase-to-ground and phase-
to-phase-to-ground faults. Set the residual time-overcurrent element to provide sensitive
detection and coordinated time-overcurrent protection for phase-to-ground faults.
Time-Overcurrent Curve and Time-Dial
Select the element curve and time-dial settings individually to coordinate with downstream phase,
negative-sequence, and residual time-overcurrent elements. Refer to the time-overcurrent
equations and curves in this section for the specific time-overcurrent characteristic equations.
Time-Overcurrent Element Reset Characteristic
You can set the relay time-overcurrent element to emulate an induction disk relay reset charac-
teristic by setting the 51
xn
RS (
x
= P, Q, or N;
n
= winding number) setting equal to Y. With this
setting, the relay emulates the spring-torque-governed disk reset action of an induction time-
overcurrent unit. Make this setting when the SEL-387E Relay time-overcurrent element must
coordinate with upstream electromechanical time-overcurrent relays during trip-reclose cycles.
When you set 51
xn
RS = N, the relay fully resets the time-overcurrent element one cycle after
current drops below the element pickup setting. Make this setting when the relay time-
overcurrent element must coordinate with upstream static or microprocessor-based time-
overcurrent elements that have fast reset characteristics.
Instantaneous and Definite-Time Overcurrent Elements
The SEL-387E Relay includes instantaneous and definite-time overcurrent elements for phase,
negative-sequence and residual current. There are three separate phase instantaneous elements,
which can be used for various tripping or supervisory functions defined by the user. Each
element operates using measured current, a pickup setting and, for the definite-time elements, a
time delay setting.
Instantaneous and Definite-Time Element Pickup and Time Delay Settings
Use the instantaneous overcurrent elements to provide fast tripping for heavy internal transformer
faults. Set the element pickup settings high enough to prevent tripping for faults outside the
protection zone. Both definite-time and instantaneous phase overcurrent elements are sensitive to
load, but should, based on other setting constraints, be set well above the maximum expected
loading.
Use the definite-time delayed overcurrent elements to provide fast backup protection for down-
stream instantaneous elements. Allow sufficient time delay to coordinate with downstream
breakers, reclosers, or other protection.
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