Sel SEL-787 - Table 4.4 Differential Element Settings

4.13
Date Code 20150130 Instruction Manual SEL-787 Relay
Protection and Logic Functions
Basic Protection
harmonic restraint logic and the HBLK := Y setting to enable the harmonic
blocking logic.
Common harmonic blocking provides superior security against tripping on
magnetizing inrush during transformer energization, yet allows faster
differential element tripping for an energized transformer fault. Differential
tripping through the harmonic restraint logic is slightly slower, but provides a
dependable tripping function when energizing a faulted transformer that might
otherwise have the differential tripping element blocked by common harmonic
blocking logic.
Differential Element Settings in SEL-787, SEL-387, and SEL-587
The SEL-787 restraint quantity IRTn calculation differs from the SEL-587 and
SEL-387 by a factor of 2. To achieve the same characteristics for the
differential elements in the SEL-787, SEL-387, and SEL-587, we must
account for this factor of 2. Find below the settings relationships among the
three products.
Convert SEL-387 and SEL-587 Relay Settings to the SEL-787 Relay
O87P
787
= O87P
387/587
SLP1
787
= 1/2 • SLP1
387/587
SLP2
787
= 1/2 • SLP2
387/587
IRS1
787
= 2 • IRS1
387/587
U87P
787
= U87P
387/587
Convert SEL-787 Relay Settings to the SEL-387 and SEL-587 Relays
O87P
387/587
= O87P
787
SLP1
387/587
= 2 • SLP1
787
SLP2
387/587
= 2 • SLP2
787
IRS1
387/587
= 1/2 • IRS1
787
U87P
387/587
= U87P
787
Setting Calculation
General Discussion of Connection Compensation
The general expression for current compensation is as follows:
where IAWn, etc., are the three-phase currents entering terminal “n” of the
relay; I1WnC, etc., are the corresponding phase currents after compensation;
and [CTC(m)] is the three-by-three compensation matrix.
Setting WnCTC = m specifies which [CTC(m)] matrix the relay is to use. The
setting values are 0, 1, 2, ..., 11, 12. These are discrete values “m” can assume
in [CTC(m)]; the values physically represent the “m” number of increments of
30 degrees that a balanced set of currents with ABC phase sequence will be
rotated in a counterclockwise direction when multiplied by [CTC(m)].
If a balanced set of currents with ACB phase rotation undergoes the same
exercise, the rotations by the [CTC(m)] matrices are in the clockwise
direction. This is because the compensation matrices, when performing phasor
I1WnC
I2WnC
I3WnC
CTC m
IAWn
IBWn
ICWn
=