Schweitzer Engineering SEL-387-0, Instruction Manual

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3.13
Date Code 20170601 Instruction Manual SEL-387-0, -5, -6 Relay
Differential, Restricted Earth Fault, Thermal, and Overcurrent Elements
Differential Element
You may use the presence of fifth-harmonic differential current to assert an
alarm output during startup. This alarm indicates that the rated transformer
excitation current is exceeded. You may also consider triggering an event
report if fifth-harmonic current exceeds the fifth-harmonic threshold that you
set.
Fifth-Harmonic Alarm Time Delay Pickup (TH5D)
Range: 0–8000 cycles, in 0.125-cycle steps
With this pickup, you can delay assertion of an alarm for excessive fifth-
harmonic differential current.
DC Ratio Blocking (DCRB) (All But SEL-387-0)
For all relays but the SEL-387-0, set E87W x = Y1 to make the fourth-
harmonic (PCT4), dc ratio blocking (DCRB), and harmonic restraint
(HRSTR) settings available. Some magnetizing inrush cases contain very little
harmonic content but contain a dc offset. The SEL-387 can detect the dc offset
and use it in the blocking (not restraint) logic. Enable this function by setting
DCRB = Y.
Harmonic Restraint (HRSTR) (All But SEL-387-0)
For all relays but the SEL-387-0, set E87W x = Y1 to make the fourth-
harmonic (PCT4), dc ratio blocking (DCRB), and harmonic restraint
(HRSTR) settings available. Even harmonics (second and fourth) can be used
to provide security against magnetizing inrush currents during transformer
energization. Choose between harmonic blocking and harmonic restraint.
Harmonic blocking treats the second and fourth harmonics independently and
blocks the relay when the second- or fourth-harmonic content (harmonic
current as a percentage of the fundamental current) exceeds the PCT2 or
PCT4 setting, respectively. For example, assume the following:
PCT2 = PCT4 = 20 percent, and the harmonics in the differential current
are:
second harmonic = 15 percent, fourth harmonic = 7 percent
In this case, the relay does not block because neither harmonic content
exceeds its particular setting. But when the second-harmonic content increases
to 21 percent, the relay blocks, regardless of the value of the fourth-harmonic
content present in the differential current. Increasing the fourth-harmonic
content to exceed the PCT4 setting while the second-harmonic content
remains lower than the PCT2 setting yields the same result.
Harmonic restraint is more secure than harmonic blocking since it effectively
raises the relay restraint slope characteristic by the sum of the scaled second
and fourth harmonic values. See
Figure 3.4 and Figure 3.5 for details.
Set HRSTR = Y to select the harmonic restraint function. The relay
automatically applies harmonic restraint to each phase independently. When
HRSTR = Y, the relay setting IHBL is not used.
NOTE:
The relay permits the setting
of values smaller than TH5P •
TAP
MIN

0.05 • I
NOM
, but alerts the
user with the message "Settings times
minimum TAP should be >= 0.25." If
the setting is not changed to a value
within the limits, the relay
performance may be outside the
element specification.