Schweitzer Engineering Laboratories SEL-421-4, Instruction Manual

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6.55
Date Code 20171021 Instruction Manual SEL-421 Relay
Protection Applications Examples
345 kV Tapped Overhead Transmission Line Example
Convert the power system impedances from primary to secondary so you can
later calculate protection settings.
Table 6.15 lists the corresponding secondary
impedances. Convert the impedances to secondary ohms as follows:
Equation 6.19
Equation 6.20
The tapped autotransformer is rated at 500 MVA; the corresponding maximum
load current is 837 A primary at 354 kV.
Source T impedances: Z
1T
= Z
0T
0.656

87° per unit
PTR (potential transformer ratio) 345 kV:115 V = 3000.0
CTR (current transformer ratio) 1000:5 = 200
Phase rotation ABC
a
Parameter
t
is the tap point on the 345 kV line; S and R are terminals at the ends of the 345 kV
line (see
Figure 6.11
).
Table 6.15 Secondary Impedances
Parameter Value
Line impedances:
Z
1L1
= Z
1L2
Z
0L1
= Z
0L2
Z
1L3
Z
0L3
2

84.7° secondary
6.44

73° secondary
3

84.7° secondary
9.65

73° secondary
Transformer impedances:
X
HM
X
ML
X
HL
8% @ 500 MVA; 1.6% on 100 MVA
10% @ 25 MVA; 40% on 100 MVA
15% @ 25 MVA; 60% on 100 MVA
Source S impedances:
Z
1S
= Z
0S
0.67

87° secondary
Source R impedances:
Z
1R
= Z
0R
2.33

87° secondary
Source T impedances:
Z
1T
= Z
0T
0.656

87° per unit
Table 6.14 System Data—345 kV Tapped Overhead Transmission Line (Sheet 2 of 2)
Parameter Value
k
CTR
PTR
------------
200
3000
------------
0.067
= = =
Z
1L1 secondary
 
k Z
1L1 primary
 
• =
0.067 29.67

84.7
   
•
 
=
2

84.7
 
=