ABB Modures LZ91 - Page 18

A
1
2
3
B
IA+ I B
4 -
(
0
Fig. 3.10 - Exampel for the calcuation of k;
k
where
check the overreach for k > 1 in the event that the source
for B is out of operation:
IA + IB
::::. 1
max. possible fault current
min. possible fault current
distance relays
3.4.2 Determining the settings m. and N.
1. 1.
16
z
Ls
where
ZLp
ZL
=--p-
-=:u_
rI
rz
primary line impedance
secondary line impedance
main p.t. ratio
main c. t. ratio
impedance ratio
The secondary resistance and reactance values are calculated in the same
manner.
Calculating X.
-------------:'.!:
Before the values m. and N. can be set on the main processing unit it is
necessary to calculal:e the feactances X .. These are not exactly equal to the
line reactances XL (line impedance Z =\ + jXL), because of the inclination
of the relay reactance characteristi~ by ~he angle cl. .