ABB Modures LZ91 - Page 19

jX
R
x =1
R
/X =2
R
Fig. 3.11 - Line impedance z entered in the relay characteristic
L
Thus to obtain the reactances X., the line reactances have to be corrected by
l.
the factor kx:
The correction factor kx is given by:
tan a
k =l+"---
x tan <f>
If the phase-angle V(_ is less than 40°, as can be the case in cable systems, there
are two possibilities for grading as follows:
grading in relation to XL
grading in relation to RL
Grading with XL:
If the setting R/X is greater than 1, e.g. R/X
kx according to the table 3.12 is used.
2 (see Fig. 3.11), then the factor
Grading with RL:
If the setting of the polygon is made in relation to the line resistance RL with
the arch resistance compensation R/X = 1 (see Section 3.5), then RLi is used
instead of XLi and kR instead of kx for grading (see Table 3.12).
Where for ii L < 40°:
Xi = kR . RLi ; with kR = 1 tancc'. . tan 'f
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