3-24 Protection Functions Date Code 20080110
SEL-387E Instruction Manual
Second-Harmonic Blocking
Transformer simulations show that magnetizing inrush current usually yields more than 30
percent of IF2/IF1 in the first cycle of the inrush. A setting of 15 percent usually provides a
margin for security. However, some types of transformers, or the presence within the differential
zone of equipment that draws a fundamental current of its own, may require setting the threshold
as low as 7 percent. For example, the additional fundamental 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
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-387E Relay can perform harmonic blocking two
ways:
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