Schweitzer Engineering Laboratories SEL-700G Series, Instruction Manual, Page: 132 / 824

4.26
SEL-700G Relay Instruction Manual Date Code 20170814
Protection and Logic Functions
Group Settings (SET Command)
The variable restraint characteristic provided by SLP2 at high multiples of
TAP for a through fault accommodates transient CT error. SLP2 can be set
fairly high without jeopardizing sensitivity for low-grade partial winding
faults. The CTs should be evaluated for the likelihood of going into saturation
for a through fault, and SLP2 should be adjusted accordingly. Another
consideration for selecting the SLP2 setting is that the effectiveness of the
variable percentage depends on SLP1 and IRS1, which determine the starting
point of SLP2. If SLP1 is set very low, a higher SLP2 may be warranted.
Unrestrained Element Current Pickup.
The instantaneous unrestrained
current element is intended to react quickly to very heavy current levels that
clearly indicate an internal fault. Set the pickup level (U87P) to approximately
8 to 10 times TAP. The unrestrained differential element only responds to the
fundamental frequency component of the differential operating current. It is
not affected by the SLP1, SLP2, IRS1, PCT2, or PCT5 settings. Thus, it must
be set high enough so as not to react to large inrush currents.
Note that the U87P must be set lower than the minimum of 31 • INOMn/
TAPn, where n = 1, 2.
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 you can set
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.