Date Code 20020129
Differential, Restricted Earth Fault,
3-1
Overcurrent, Voltage, and Frequency Elements
SEL-387E Instruction Manual
6(&7,21
DIFFERENTIAL, RESTRICTED EARTH FAULT,
OVERCURRENT, VOLTAGE, AND FREQUENCY
ELEMENTS
I
NTRODUCTION
This section describes general applications and operating characteristics for the current
differential, restricted earth fault (REF), overcurrent, over- and undervoltage, frequency, and
volts/hertz protection elements. The section also contains application guidelines for the
differential and overcurrent elements and setting calculation information for the differential
elements and restricted earth fault elements.
D
IFFERENTIAL
E
LEMENT
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.
The relay provides a choice between harmonic blocking and harmonic restraint, both providing
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 overexitation conditions.
Operating
Characteristic
The SEL-387E Relay 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. Figure 3.1 shows the relay characteristic. 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:
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, if used; must be greater than or equal to SLP1
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.
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