3-48
Differential, Restricted Earth Fault,
Date Code 20020129
Overcurrent, Voltage, and Frequency Elements
SEL-387E Instruction Manual
The negative-sequence elements respond to |3I
2
| current, where 3I
2
= Ia + Ib • (1
µ
240) + Ic •
(1
µ
120). The negative-sequence overcurrent elements are uniquely suited to detect phase-to-
phase faults and are not sensitive to balanced load.
For a phase-to-phase fault:
(
)
73
.
1
I
/
I
3
I
•
3
I
•
3
I
•
3
/
3
I
P
2
P
2
P
2
=
\
=
=
where Ip is the maximum phase current
Thus, the negative-sequence element is 1.73 times more sensitive to phase-to-phase faults than a
phase overcurrent element with the same pickup setting.
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 due to 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 due to 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-387E Relay 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.
