Schweitzer Engineering SEL-387-0, Instruction Manual

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3.2
SEL-387-0, -5, -6 Relay Instruction Manual Date Code 20170601
Differential, Restricted Earth Fault, Thermal, and Overcurrent Elements
Differential Element
By careful selection of these settings, the user can duplicate closely the
characteristics of existing differential relays that have been in use for many
years.
Figure 3.1 Percentage Restraint Differential Characteristic
Figure 3.2 , Figure 3.3 , Figure 3.4 , and Figure 3.5 illustrate how input currents
are acquired and used in the unrestrained and restrained differential elements.
Data acquisition, filtering, tap scaling, and transformer and CT connection
compensation for Winding 1 are shown in
Figure 3.2 .
Four digital band-pass filters extract the fundamental, second, fourth (all but
SEL-387-0), and fifth (not shown) harmonics of the input currents. A dc filter
(not shown) forms one cycle sums of the positive and negative values.
Using the transformer MVA rating as a common reference point, TAP scaling
converts all secondary currents entering the relay from the four windings to
per-unit values, thus changing the ampere values into dimensionless multiples
of TAP. Throughout the text, the term “TAP” refers to the per-unit value
common to all four windings, whereas “TAP
n ” refers to the ampere value of a
particular winding(s); TAPmin and TAPmax refer to the least and greatest of
the four TAP
n values. This method ensures that, for full-load through-current
conditions, all incoming current multiples of TAP sum to 1.0 and all outgoing
current multiples of TAP sum to –1.0, with a reference direction into the
transformer windings.
Transformer and CT connection compensation adjusts the sets of three-phase
currents for the phase angle and phase interaction effects introduced by the
winding connection of the transformer and CTs. Settings W1CTC through
W4CTC determine the mathematical corrections to the three-phase currents
for Winding 1 through Winding 4, respectively. CTC1 is shown in
Figure 3.2
as the phase angle and sequence quantity adjustment for Winding 1.
I1W1C1, I2W1C1, and I3W1C1 are the fundamental frequency A-phase,
B-phase, and C-phase compensated currents for Winding 1. Similarly,
I1W1C2, I2W1C2, and I3W1C2 are the second-harmonic compensated
currents for Winding 1. The dc, fourth-harmonic, and fifth-harmonic
compensated currents use similar names. The I1 compensated currents are
used with differential element 87-1, I2 with element 87-2, and I3 with element
87-3.
IOP
IRT
Operating Region
Restraining Region
087P = 0.3
IRS1 = 3
25%
60%
Slope 1
(SLP1)
Slope 2
(SLP2)