Siemens SIPROTEC 7UM62 - Out-Of-Step Protection Logic

Functions
2.20 Out-of-Step Protection (ANSI 78)
SIPROTEC, 7UM62, Manual
C53000-G1176-C149-7, Release date 03.2010
167
Figure 2-68 Impedance Trajectory at Measurement Location m
2.20.2 Out-of-Step Protection Logic
The following figure shows the power swing polygon in greater detail. For transparency purposes the inclination
angle ϕ
P
is assumed to be 90°. The setting parameters of impedances Z
a
, Z
b
, Z
c
and (Z
d
–Z
c
) determine the
power swing polygon. The polygon is symmetrical about its vertical axis. Z
b
is measured in reverse direction
into the generator, in the forward direction (Z
c
) into the unit transformer, and the second stage (Z
d
) into the
power network. The power swing polygon is divided into two parts. Characteristic 1 (i.e. the non-hatched area)
represents the lower section of the rectangle. Characteristic 2 covers the upper hatched area. Depending on
the electrical centre of the power swing, or in the vicinity of the power station, the impedance vector progresses
through the range of characteristic 1 or that of characteristic 2. The point of crossing of the symmetry (imagi-
nary) axis is decisive for the characteristic assignment.
Power swings are three-phase symmetrical occurrences. The first prerequisite is therefore the symmetry of the
measured currents. A condition for power swing detection is that the positive sequence component of the
current exceeds an adjustable limit I
2
while the negative sequence current remains below an adjustable value
I
1
.