Schweitzer Engineering SEL-387-0, Instruction Manual

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3.19
Date Code 20170601 Instruction Manual SEL-387-0, -5, -6 Relay
Differential, Restricted Earth Fault, Thermal, and Overcurrent Elements
Differential Element
frequency charging current of a long cable run on the transformer secondary
terminals could “dilute” the level of second harmonic seen at the primary to
less than 15 percent.
Fourth-Harmonic Blocking (All But SEL-387-0 Relay)
Transformer magnetizing inrush currents are generated during transformer
energization when the current contains a dc offset due to point-on-wave
switching. Inrush conditions typically are detected using even harmonics and
are used to prevent misoperations due to inrush. The largest even-harmonic
current component is usually second harmonic followed by fourth harmonic.
Use fourth-harmonic blocking to provide additional security against inrush
conditions; set PCT4 less than PCT2.
Fifth-Harmonic Blocking
Fourier analysis of transformer currents during overexcitation indicates that a
35 percent fifth harmonic setting is adequate to block the percentage
differential element. To disable fifth harmonic blocking, set PCT5 to OFF.
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. At full load, a TH5P setting of 0.1 corresponds
to 10 percent of the fundamental current. A delay, TH5D, that can be set by
the user prevents the relay from indicating transient presence of fifth-
harmonic currents.
You may consider triggering an event report if transformer excitation current
exceeds the fifth harmonic threshold.
There are two criteria for setting TH5P:
➤
TH5P • TAP
MIN

0.05 • I
NOM
, and
➤
TH5P • TAP
MAX

31 • I
NOM
where TAP
MIN
and TAP
MAX
are the least and greatest of the tap settings.
Independent Harmonic Blocking Element (IHBL)
When a three-phase transformer is energized, inrush harmonics are present in
at least two phase currents. In traditional single-phase relays, each relay
performs a comparison of the harmonic current flowing through its phase. The
SEL-387 can perform harmonic blocking two ways:
➤
Independent Harmonic Blocking (IHBL = Y) blocks the
percentage differential element for a particular phase if the
harmonic (second or fifth) in that phase is above the block
threshold. Other differential elements are not blocked.
➤
Common Harmonic Blocking (IHBL = N) blocks all of the
percentage differential elements if any one phase has a
harmonic magnitude above the blocking threshold.
Common Harmonic Blocking is a more secure scheme but may slightly delay
percentage differential element operation since harmonics in all three phases
must drop below their thresholds.