ABB RELION RET670 - Page 458

fact that all settings are performed in primary values. The impedance transformation
factor is presented for orientation and testing purposes only.
1200 0.115
0.069
5 400
p
s
s p
I
V
KIMP
I V
= × = × =
EQUATION1735 V1 EN-US (Equation 370)
The minimum load impedance at minimum expected system voltage is equal to
equation 371.
2 2
min
min
max
380
144.4
1000
L
V
Z
S
= = = W
EQUATION1736-ANSI V1 EN-US (Equation 371)
The minimum load resistance R
Lmin
at maximum load and minimum system voltage is
equal to equation
372.
( )
min min max
cos 144.4 0.95 137.2
L L
R Z
j = × = × = W
EQUATION1338 V1 EN-US (Equation 372)
The system impedance Z
S
is determined as a sum of all impedance in an equivalent
two-machine system, see figure 228. Its value is calculated according to equation 373.
( )
1 1 1
17.16 154.8
S SA L SB
Z Z Z Z j= + + = + W
EQUATION1339 V1 EN-US (Equation 373)
The calculated value of the system impedance is of informative nature and helps
determining the position of oscillation center, see figure 229, which is for general case
calculated according to equation 374.
1
1
S
CO SA
B
A
Z
Z Z
E
E
= -
+
EQUATION1340 V1 EN-US (Equation 374)
In particular cases, when
Section 8 1MRK 504 163-UUS A
Impedance protection
452 Transformer protection RET670 2.2 ANSI
Application manual