ABB RELION REB500 Technical Manual page 48

Section 5
Bay protection functions
where:
• Z1, Z2, Z3, Z4 are the impedance reach of the various zones (Ω/ph).
• ZOR is the impedance reach of the overreaching zone (Ω/ph).
• k ≥ 1 is the factor to take the apparent increase of line impedance seen by a relay due to an
intermediate infeed into account.
• a, b are the impedance of the corresponding section of line (Ω).
Example for calculating k:
Check the overreach for k > 1 if the infeed B is not in operation.
IEC19000409 V1 EN-US
Figure 8:

k
where:
• IA' is the maximum fault current possible
• IB' is the minimum fault current possible
• 1...5 are the distance relays
Calculating the secondary line impedances
The primary values calculated from the grading table for the line impedances have to be
converted to secondary values. These are obtained by applying the following relationship:

Z
LS
where:
• Z
Lp
• Z
Ls
• K
U
• K is the main CT ratio.
I
• K
Z
The same applies to the conversion of the resistances and reactances.
42
A
1 2
~
I '
A
Calculation of k

' '
I I

A B
1
I
A
Z Z

LP LP
 
K
K
Z
U
 
 
K
I
is the primary positive-sequence line impedance.
is the secondary positive-sequence line impedance.
is the main VT ratio.
is the impedance ratio.
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C
B
I ' + I '
A B
3 4
5
~
18000007-IEC19000409-1-en.vsdx
1MRK 505 406-UEN B
D
Bay protection functions REB500
Technical manual