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2.2.5
Inverse Time Overcurrent Protection 51V (Voltage-controlled / Voltage-
restraint)
Undervoltage Consideration
The inverse time overcurrent protection is provided with an undervoltage detection that can be disabled (ad-
dress 1223 VOLT. INFLUENCE). This function can influence overcurrent detection by means of two different
methods:
• Voltage-controlled: If a set voltage threshold is undershot, the overcurrent element is released.
• Voltage-restraint: The pickup threshold of the overcurrent element depends on the voltage magnitude. A
lower voltage decreases the current pickup value (see Figure 2-14). In the range between V/V
0.25 a linear, directly proportional dependence is realized, and therefore the following applies:
Figure 2-14
The 51 PICKUP value is decreased proportional to the voltage decrease. Consequently, for constant current I
the I/ 51 PICKUP ratio is increased and the tripping time is reduced. Compared with the standard curves rep-
resented in Section „Technical Data" the tripping curve shifts to the left side as the voltage decreases.
Switching to the lower pickup value or decreasing the pickup threshold is carried out phase-selectively. The
assignment of voltages to current-carrying phases is shown in the following table.
Table 2-3
Current
I
A
I
B
I
C
In order to avoid an unwanted operation in case of a voltage transformer fault, a function blocking is implement-
ed via a binary input controlled by the voltage transformer protection breaker as well as via the device-internal
measuring voltage failure detection ("Fuse Failure Monitor").
The following two figures show the logic diagrams for the inverse time overcurrent protection with undervoltage
consideration.
SIPROTEC, 7SK80, Manual
E50417-G1140-C344-A5, Release date 11.2012
Voltage influence of the pickup value
Controlling voltages in relation to the fault currents
Voltage
V
– V
A
B
V
– V
B
C
V
– V
C
A
2.2 Overcurrent Protection 50, 51, 50N, 51N
Functions
= 1.00 to
Nom
69
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