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6.2
SEL-421 Relay
Instruction Manual
Date Code 20171021
Protection Applications Examples
230 kV Overhead Distribution Line Example
Convert the power system impedances from primary to secondary, so you can
later calculate protection settings.
lists the corresponding secondary
impedances. Convert the impedances to secondary ohms as follows:
Equation 6.1
Equation 6.2
The maximum load current is 495 A primary.
Application Summary
This particular example is for a single circuit breaker, three-pole tripping applica-
tion with the following functions:
➤
Two zones of mho distance protection
➢
Zone 1, forward-looking, instantaneous underreaching
protection
➢
Zone 2, forward-looking, time-delayed tripping
Table 6.1
System Data—230 kV Overhead Transmission Line
Parameter
Value
Nominal system line-to-line voltage
230 kV
Nominal relay current
5 A secondary
Nominal frequency
60 Hz
Line length
50 miles
Line impedances:
Z
1L
, Z
0L
39
84° primary, 124
81.5° primary
Source S impedances:
Z
1S
= Z
0S
50
86° primary
Source R impedances:
Z
1R
= Z
0R
50
86° primary
PTR (potential transformer ratio)
230 kV:115 V = 2000
CTR (current transformer ratio)
500:5 = 100
Phase rotation
ABC
Table 6.2
Secondary Impedances
Parameter
Value
Line impedances:
Z
1L
, Z
0L
1.95
84° secondary, 6.2
81.5° secondary
Source S impedances:
Z
1S
= Z
0S
2.5
86° secondary
Source R impedances:
Z
1R
= Z
0R
2.5
86° secondary
k
CTR
PTR
------------
100
2000
------------
0.05
=
=
=
Z
1L
ondary
sec
k Z
1L primary
•
=
0.05
39
84
•
=
1.95
84
=
Summary of Contents for SEL-421-4
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