Functions
2.20 Out-of-Step Protection (ANSI 78)
SIPROTEC, 7UM62, Manual
C53000-G1176-C149-7, Release date 03.2010
165
2.20 Out-of-Step Protection (ANSI 78)
Depending on power network conditions and feeding generators, dynamic occurrences such as load jumps,
short-circuits not disconnected quickly enough, auto-reclosure or switching actions, may cause system swings.
Such power swings endanger power network stability. Stability problems often result from active power swings
which can lead to pole-slipping and generator overloading.
2.20.1 Measuring Principle
General
The out-of-step protection is based on the well-proven impedance measurement and evaluation of the complex
impedance vector trajectory. The impedance is calculated from the positive sequence fundamental frequency
components of the three voltages and currents. The decision wether or not to separate the generator from the
network is made dependent on the course of the impedance vector and the location of the electrical centre of
the power swing.
The out-of-step case is illustrated using a simple model. The following figure shows the generator voltage U
G
and the network equivalent voltage U
N
. The generator, transformer and network impedances lie between these
two voltages and constitute a total impedance Z
tot
.
Figure 2-67 Equivalent Model of a Power Swing
The measurement location divides the total impedance into impedances m Z
tot
and (1-m) · Z
tot
. The following
applies for the impedance at measurement location m:
The current I is independent of the location of the measurement:
The voltage U
at measurement location m is:
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