2 Functions
90
7ST6 Manual
E50417-G1176-C251-A3
2.6
Overvoltage and Undervoltage Protection
This function is optional, i.e. not included in all device variants.
Voltage protection has the function to protect electrical equipment against undervolt-
age and overvoltage. Both operational states are unfavourable as overvoltage might
e.g. cause insulation problems or undervoltage might cause stability problems.
Both the overvoltage and the undervoltage protection in the 7ST6 use the overhead
contact line voltage U. The voltage protection functions can be combined as desired,
and can be separately switched on and off. Each voltage protection function is two-
stage, i.e. it is provided with two threshold settings each with the appropriate times
delay.
Abnormally high voltages often occur e.g. on long overhead contact lines under low
load conditions, especially in auto-transformer systems with several auto-transform-
ers along the line, in islanded systems when generator voltage regulation fails, or after
full load shutdown of a generator and with the generator disconnected from the
system. The undervoltage protection can be applied, for example, for disconnection or
load shedding tasks in a system. Furthermore, this protection scheme can detect men-
acing stability problems. With induction machines undervoltages have an effect on the
stability and permissible torque thresholds.
2.6.1
Overvoltage Protection
The figure below shows the logic diagram of the overvoltage protection.
Figure 2-20
Control and pick-up/tripping logic of the overvoltage protection
With the parameter
3701
OVERVOLTAGE
the overvoltage protection can be switched
ON
or
OFF
. The indication
„OVER. OFF“
is output when the function is switched
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