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2.6.2
Overvoltage Protection 59
Application
Function
SIPROTEC 4, 7SJ62/63/64 Handbuch
C53000-G1140-C147-A, Edition 07.2015
The overvoltage protection has the task of protecting the transmission lines and elec-
trical machines against inadmissible overvoltage conditions that may cause insulation
damage.
Abnormally high voltages often occur, e.g. on low loaded, long distance transmission
lines, in islanded systems when generator voltage regulation fails, or after full load
shutdown of a generator from the system.
With three-phase connection, the fundamental component of the largest of the three
phase-to-phase voltages is supplied to the overvoltage protection elements or, option-
ally, the negative sequence voltage.
If only one voltage transformer is connected, the function is provided with the phase-
to-ground or phase-phase fundamental component voltage in accordance with the
connection type.
The overvoltage protection has two elements. In case of a high overvoltage, tripping
switchoff is performed with a short-time delay, whereas in case of less severe over-
voltages, the switchoff is performed with a longer time delay. When one of the adjust-
able settings is exceeded, the 59 element picks up, and trips after an adjustable time
delay elapses. The time delay is not dependent on the magnitude of the overvoltage.
The dropout ratio for the two overvoltage elements (= V
set.
The following figure shows the logic diagram of the overvoltage protection for phase–
phase voltages.
2.6 Voltage Protection 27, 59
/V
) can be
dropout value
pickup value
136
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