Siemens SIPROTEC 7UM62 - Page 170

Functions
2.20 Out-of-Step Protection (ANSI 78)
SIPROTEC, 7UM62, Manual
C53000-G1176-C149-7, Release date 03.2010
170
Impedance Values
The measured impedances perceived by the protection device are decisive for the settings. For the direction
to the machine (as viewed from the location of the voltage transformers), the power swing reactance of the
machine must be considered, which is approximately the transient reactance X
d
' of the machine. Consequently,
you will calculate the secondary transient reactance and use it for Z
b
≈ X
d
' (see the figure below).
Figure 2-71 Power swing polygon
X
d
' can be calculated from the per unit reactance x
d
' as follows:
with
X
d
Transient reactance of the generator
X
d
Transient per unit reactance
U
N, Gen
Primary rated voltage - generator
I
N, Gen
Primary rated current - generator
N
CTR
Current transformer ratio
N
VTR
Voltage transf. transform. ratio
Dependent on the type of generator and on the secondary current, a secondary voltage U
N
= 100 V or 120 V
leads to approximately the reactance ranges listed in the table below.