6.20
SEL-421 Relay Instruction Manual Date Code 20171021
Protection Applications Examples
500 kV Parallel Transmission Lines With Mutual Coupling Example
Convert the power system impedances from primary to secondary so you can
later calculate protection settings. Table 6.8 lists the corresponding secondary
impedances. Convert the impedances to secondary ohms as follows:
Equation 6.6
Equation 6.7
The maximum load current is 1302 A primary and occurs when the parallel line
is out of service.
Application Summary
This application is for two circuit breakers, single-pole tripping application with
the following functions:
➤ POTT (permissive overreaching transfer tripping) scheme
➤ Three zones of phase (mho) and ground (mho and quadrilateral)
distance protection
➢ Zone 1, forward-looking, instantaneous underreaching
protection
➢ Zone 2, forward-looking, communications-assisted and time-
delayed tripping
➢ Zone 3, reverse-looking, prevents unwanted tripping during
current reversals
➤ Inverse-time directional zero-sequence overcurrent backup
protection
➤ SOTF protection, fast tripping when the circuit breaker closes
Relay settings that are not mentioned in this example do not apply to this applica-
tion example.
Table 6.8 Secondary Impedances
Parameter Value
Line impedances:
Z
1L1
= Z
1L2
Z
0L1
= Z
0L2
3.98 87.6° secondary
14.48 82.1° secondary
Zero-sequence mutual coupling:
Z
0M
7.86 76.6° secondary
Source S impedances:
Z
1S
= Z
0S
4.45 88° secondary
Source R impedances:
Z
1R
= Z
0R
1.78 88° secondary
k
CTR
PTR
------------
400
4500
------------ 0.089===
Z
1L1 ondarysec
kZ•
1L1 primary
=
0.089 • (44.78 87.6=
3.98 87.6=
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