6.107
Date Code 20171021 Instruction Manual SEL-421 Relay
Protection Applications Examples
EHV Parallel 230 kV Underground Cables Example
Use the following formula to determine approximately how much primary fault
resistance coverage (R
F
) that 51S1P provides on a radial basis:
Equation 6.66
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 so there is a 12-cycle (60 Hz
nominal) safety margin for phase-to-phase (high current) faults in front of the
first downstream overcurrent element. Assume the operating time of the down-
stream overcurrent element is 12 cycles for close-in phase-to-phase faults. There-
fore, set the local time-overcurrent element to operate at approximately 24 cycles
for phase-to-phase faults in front of the first downstream overcurrent element.
The fault current (3I
2
) that the relay measures for a bolted close-in phase-to-
phase fault at the remote station with the parallel cable out of service is 18.67 A
secondary. The pickup multiple is shown in Equation 6.67.
Equation 6.67
Use the parameters of 24 cycles operating time and M = 18.67 to choose the
curve and time dial settings for the 51S1 element. For curve and timing informa-
tion, see Inverse-Time Overcurrent Elements on page 5.99.
51S1C = U3. 51S1 Inverse-Time Overcurrent Curve (U1–U5)
51S1TD = 3.72. 51S1 Inverse-Time Overcurrent Time Dial (0.50–15.0)
Set the overcurrent element to emulate electromechanical reset.
51S1RS = Y. 51S1 Inverse-Time Overcurrent EM Reset (Y, N)
Torque control the overcurrent element with the forward decision from the
ground directional element.
51S1TC = 32GF. 51S1 Torque Control (SELOGIC Equation)
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; these are blocking
elements for the POTT scheme.
DIR3 := R. Zone/Level 3 Directional Control (F, R)
R
F
PTR
CTR
------------
VNOMY
3
------------------------
51S1P
---------------------------
• =
2000
200
------------
115 V
3
---------------
1.00 A
----------------•
=
664 primary=
M
I
2FAULT
51S1P
---------------------=
18.67 A
1.00 A
-------------------=
18.67=
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