Schweitzer Engineering Laboratories SEL-700G Series, Instruction Manual

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4.7
Date Code 20170814 Instruction Manual SEL-700G Relay
Protection and Logic Functions
Group Settings (SET Command)
The four settings that define the characteristic are:
O87P = minimum IOP level required for operation
SLP1 = initial slope, beginning at the origin and intersecting O87P at
IRT = O87P • 100/SLP1
IRS1 = limit of IRT for SLP1 operation; intersection where SLP2 begins
SLP2 = second slope must be greater than or equal to SLP1
By careful selection of these settings, you can duplicate closely the
characteristics of existing differential relays that have been in use for many
years.
Figure 4.2 Percentage Restraint Differential Characteristic
Figure 4.3 to Figure 4.7 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 X are shown in Figure 4.3.
Four digital filters extract the fundamental, second, fourth, and fifth harmonics
of the input currents.
Using the transformer MVA rating as a common reference point, TAP scaling
converts all secondary currents entering the relay from the two windings to
per-unit values, thus changing the ampere values into dimensionless multiples
of TAP. Throughout the text, the term “TAP” refers to both windings, whereas
“TAPn” refers to the ampere value of a particular winding; TAPmin and
TAPmax refer to the least and greatest of the two TAPn values. This method
ensure 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 CTCX and CTCY
determine the mathematical corrections to the three-phase currents for
Winding X and Winding Y, respectively. CTCX is shown in Figure 4.3 as the
phase angle and sequence quantity adjustment for Winding X.
IRS1 IRT
(Multiples of TAP)
O87P
U87P
IOP
(Multiples of TAP)
SLP1
SLP2
87R
Operate Region
87U Operate Region
87R
Restraint Region
SLP22
SLP12
O87P2
SLP12 = min(2 • SLP1, 50)
SLP22 = max(SLP2, 50)
Sensitive Differential Characteristic
High Security Differential Characteristic