6.71
Date Code 20171021 Instruction Manual SEL-421 Relay
Protection Applications Examples
345 kV Tapped Overhead Transmission Line Example
Zone/Level Direction
Zone 1 and Zone 2 distance element directions are fixed in the forward direction.
You can select the other zones independently as forward-looking (F) or reverse-
looking (R). Set Zone 3 distance elements reverse-looking because these are
blocking elements for the DCB trip scheme.
DIR3 := R. Zone/Level 3 Directional Control (F, R)
Directional Control
The SEL-421 uses an array of directional elements to supervise the ground-
distance elements and residual directional overcurrent elements during ground
fault conditions. Internal logic automatically selects the best choice for the
ground directional element (32G) from among the negative-sequence voltage-
polarized directional element (32QG), zero-sequence voltage-polarized direc-
tional element (32V), and the zero-sequence current-polarized directional ele-
ment (32I).
The relay setting ORDER determines the order in which the relay selects direc-
tional elements to provide ground directional decisions. You can set ORDER
with any combination of Q, V, and I. The listed order of these directional ele-
ments determines the priority in which these elements operate to provide the
ground directional element. Only one specific directional element operates at any
one time. Directional element classification is as follows:
➤ Q—Negative-sequence voltage-polarized directional element
➤ V—Zero-sequence voltage-polarized directional element
➤ I—Zero-sequence current-polarized directional element
Set ORDER to QV. The first listed directional element choice, Q, is the first pri-
ority directional element to provide directional control for the ground-distance
elements and residual directional overcurrent elements. If Q is not operable, the
second listed directional element choice, V, provides directional control for the
ground-distance elements and residual directional overcurrent elements. A polar-
izing quantity was not available for choice I, so I is not selected for this particular
application example.
ORDER := QV. Ground Directional Element Priority (combine Q, V, I)
SEL
OGIC control equation E32IV must assert to logical 1 to enable V or I for
directional control of the ground-distance elements and residual directional over-
current elements. Set E32IV to logical 1.
E32IV := 1. Zero-Sequence Voltage and Current Enable (SELOGIC Equation)
Reverse Ground Directional Checks
32QG and 32V makes forward and reverse directional decisions during unbal-
anced faults based upon the following four settings:
➤ Z2F—Forward Directional Z2 Threshold
➤ Z2R—Reverse Directional Z2 Threshold
➤ Z0F—Forward Directional Z0 Threshold
➤ Z0R—Reverse Directional Z0 Threshold
For 32QG, if the apparent negative-sequence impedance measured by the relay
(z2) is less than Z2F, the unbalanced fault is declared forward. If z2 is greater
than Z2R, the unbalanced fault is declared reverse.
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