Schweitzer Engineering SEL-387-0, Instruction Manual

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3.62
SEL-387-0, -5, -6 Relay Instruction Manual Date Code 20170601
Differential, Restricted Earth Fault, Thermal, and Overcurrent Elements
Overcurrent Element
the off-fundamental-frequency content of the magnetizing inrush current. The
SEL-387 instantaneous, definite-time, and time-overcurrent elements need
only be set with regard to expected load and fault conditions.
Where the SEL-387 is applied to a distribution substation transformer serving
load centers, expected load conditions include steady-state load as well as
transient conditions caused by hot and cold load pickup.
Hot load pickup inrush occurs when a distribution circuit is energized shortly
after being de-energized, such as in a feeder trip-reclose cycle. Hot load
pickup inrush current that the SEL-387 may see consists primarily of starting
current from motor loads, incandescent and fluorescent lighting load inrush,
and resistive heating element inrush. The overall effect is an inrush current
several times the normal load current that can last for several seconds.
Cold load pickup inrush occurs when a distribution circuit is energized after
being de-energized for a relatively long period of time. The cold load pickup
includes many of the same inrush characteristics as hot load pickup but is
usually more severe and longer lasting because more thermostatically
controlled systems need to satisfy their heating or cooling requirements after
the prolonged outage.
For these reasons, overcurrent protection must be tailored to meet the
protection requirements for the specific transformer, avoid tripping for various
types of nonfault transient conditions, and coordinate with upstream and
downstream protection devices. These factors constrain the selection of
settings and characteristics for the applied overcurrent protection.
Overcurrent Element Operating Quantities
The SEL-387 phase overcurrent elements respond to the maximum phase
current magnitude, Ip, where Ip is the largest value of |Ia|, |Ib|, and |Ic|. Set
phase overcurrent element pickup settings above the highest expected load
current to avoid tripping on normal load current. You can set the pickup lower
if you use torque control.
Since you can use the negative-sequence overcurrent elements to detect phase-
to-phase faults, you can set the phase overcurrent elements for three-phase
fault detection only. This setting selection improves the ratio of the minimum
phase fault current to maximum load current required for secure phase
overcurrent relay application.
The negative-sequence elements respond to |3I2| current, where 3I
2
= Ia + Ib •
(1

240) + Ic • (1

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:
Equation 3.46
where I
P
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.
I
2
3
3
-------
 
 
I
P
• =
3 I
2
•
3 I
P
• =
3I
2
I
P
----------

1.73
=