Schweitzer Engineering Laboratories SEL-421-4 - Table 6.1 System Data-230 Kv Overhead Transmission Line; Table 6.2 Secondary Impedances

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. Table 6.2 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 ondarysec
kZ
1L primary
• =
0.05 39  84• =
1.95  84=