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Functions
2.20 Out-of-Step Protection (ANSI 78)
Figure 2-69
Detection of an out-of-step condition requires, additionally, that the impedance vector enters a power swing
characteristic at one side, passes through the imaginary axis or characteristic dividing line, and exits the
polygon at the opposite side (loss of synchronism, cases (1) and (2)). This is characterized in that the real com-
ponents of the complex impedances (referred to the coordinate system possibly rotated about ϕ
changed sign crossing the characteristic.
On the other hand it is also possible for a power swing vector to enter and exit the power swing polygon on the
same side. In this case, the power swing tends to stabilize (cases (3) and (4)).
When an out-of-step condition is recognized, i.e. when the impedance vector has passed through a power
swing characteristic, an annunciation is issued which also identifies the crossed characteristic. Additionally, a
counter n1 (for characteristic 1) or n2 (for characteristic 2) is incremented.
Out-of-step protection pickup is activated when a counter reaches the value 1. A further out-of-step indication
is set for an adjustable indication time period, each time a counter is incremented. After a likewise adjustable
holding time pick-up resets to zero. The holding time is started anew each time a counter is incremented.
A trip command is issued when the number of power swing polygon crossings has reached a selectable
number. This command is maintained for at least the set time T-HOLDING. The minimum trip command dura-
tion T TRIPCOM MIN. does not start until the pickup has reset.
Next comes the logic diagram of the out-of-step protection. The feature has two stages and can be blocked by
a binary input.
168
Polygonal Out-of-Step Characteristic with Typical Power Swings
SIPROTEC, 7UM62, Manual
C53000-G1176-C149-7, Release date 03.2010
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