Schweitzer Engineering SEL-387-0, Instruction Manual

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3.63
Date Code 20170601 Instruction Manual SEL-387-0, -5, -6 Relay
Differential, Restricted Earth Fault, Thermal, and Overcurrent Elements
Overcurrent Element
While negative-sequence overcurrent elements do not respond to balanced
load, they do detect the negative-sequence current present in unbalanced load.
For this reason, select an element pickup setting above the maximum 3I
2
current expected as a result of load unbalance.
When applied on the delta side of a delta-wye transformer, negative-sequence
relay elements also provide sensitive fault protection for ground faults on the
wye side of the transformer. This is not possible using only phase and residual
overcurrent elements.
The residual element responds to 3I
0
current, where 3I
0
= Ia + Ib + Ic.
Residual overcurrent elements detect ground faults and do not respond to
balanced load. The residual element is sensitive to unbalanced load, however,
and should be set above the maximum 3I
0
current expected as a result of load
unbalance.
When applied on the delta side of a delta-wye transformer, residual
overcurrent elements are insensitive to any type of fault on the wye side of the
transformer, and can only detect ground faults on the delta side. This
eliminates any coordination constraints with protection devices on the wye
side of the transformer, permitting very sensitive residual overcurrent element
pickup settings.
Time-Overcurrent Element Settings
The SEL-387 includes time-overcurrent elements for phase, negative-
sequence, and residual current. Each element operates using measured current
and five settings that define:
➤
Pickup current, in secondary amperes
➤
Operating time curve
➤
Operating time dial
➤
Element reset characteristic
➤
Element external torque-control
To disable a time-overcurrent element, set that element pickup setting = OFF.
When the pickup setting is OFF, the relay disables the element and you are not
required to enter any remaining settings associated with the element.
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.