6.11
Date Code 20171021 Instruction Manual SEL-421 Relay
Protection Applications Examples
230 kV Overhead Distribution Line Example
Use the following formula to determine approximately how much primary fault
resistance coverage (RF) is provided by 51S1P on a radial basis:
Equation 6.5
Use the following as a guide to set the curve and time dial; for secure backup pro-
tection, perform a coordination study. Set the local overcurrent element to coordi-
nate with the downstream overcurrent element such that there is an 18-cycle (60
Hz nominal) safety margin for ground faults in front of the first downstream
overcurrent element. Assume the operating time of the downstream overcurrent
element is 12 cycles for a close-in ground fault. Therefore, set the local time-
overcurrent element to operate approximately 30-cycles for ground faults in front
of the first downstream overcurrent element.
51S1C := U3. 51S1 Inverse-Time Overcurrent Curve (U1–U5)
51S1TD := 1.96. 51S1 Inverse-Time Overcurrent Time Dial (0.50–15)
Set the overcurrent element to emulate electromechanical reset, so the overcur-
rent element coordinates properly with electromechanical relays.
51S1RS := Y. 51S1 Inverse-Time Overcurrent Electromechanical Reset (Y, N)
Torque control the overcurrent element with the forward decision from the
ground directional element.
51S1TC := 32GF. 51S1 Torque Control (SELOGIC Equation)
Directional Control
The SEL-421 uses an array of directional elements to supervise the ground-dis-
tance elements and residual ground 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 equal to QV. The first listed directional element choice, Q, is the first
priority directional element to provide directional control for the ground-distance
elements and residual ground directional overcurrent elements. If Q is not opera-
ble, the second listed directional element choice, V, provides directional control
R
F
PTR
CTR
------------
VNOMY 3
51S1P
-----------------------------------
• =
2000
100
------------
115V 3
1.72A
------------------------•
=
722 =
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