3.2
SEL-387-0, -5, -6 Relay
Instruction Manual
Date Code 20050919
Differential, Restricted Earth Fault, Thermal, and Overcurrent Elements
Differential Element
Differential Element
Application
Description
Protect your apparatus with dual-slope percentage differential protection.
Percentage differential protection provides more sensitive and secure
protection than traditional differential protection; the dual-slope characteristic
compensates for CT ratio mismatches, CT ratio errors, CT saturation, and
errors because of tap changing.
All relays but the SEL-387-0 Relay offer the choice between harmonic
blocking and harmonic restraint to secure relay stability during transformer
inrush conditions. Even-numbered harmonics (second and fourth), augmented
by dc blocking provide security during energization, while fifth harmonic
blocking provides security for overexcitation conditions.
Operating
Characteristic
The SEL-387 has three differential elements (87R-1, 87R-2, and 87R-3).
These elements employ Operate (IOP) and Restraint (IRT) quantities that the
relay calculates from the winding input currents. The relay uses a
characteristic such as that in
. You can set the characteristic as either
a single-slope, percentage differential characteristic or as a dual-slope,
variable-percentage differential characteristic. Tripping occurs if the Operate
quantity is greater than the curve value for the particular restraint quantity. A
minimum pickup level for the Operate quantity must also be satisfied. The
four settings that define the characteristic are:
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
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
.
O87P = minimum IOP level required for operation
SLP1 = initial slope, beginning at origin and intersecting O87P at
IRT = O87P • 100/SLP1
IRS1 = limit of IRT for SLP1 operation; intersection where SLP2
begins
SLP2 = second slope, if used; must be greater than or equal to
SLP1
IOP
IRT
Operating Region
Restraining Region
087P = 0.3
IRS1 = 3
25%
60%
Slope 1
(SLP1)
Slope 2
(SLP2)
Содержание SEL-387-0
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