Siemens SIPROTEC 4 - Inverse Time Overcurrent Protection 51 V (Voltage-Controlled; Voltagerestraint)

User-defined Characteristics
When user-defined characteristic are used, the tripping curve may be defined point by point. Up to 20 value
pairs (current, time) may be entered. The device then approximates the characteristic, using linear interpola-
tion.
The dropout curve may be user-defined as well. See dropout behavior for ANSI and IEC curves in the function
description. If no user-defined dropout curve is required, the element drops out as soon as the respective
current falls below approx. 95% of the set pickup value. When a new pickup is evoked, the timer starts at zero
again.
Inverse Time Overcurrent Protection 51V (Voltage-controlled /
Voltagerestraint)
Undervoltage Consideration
The inverse time overcurrent protection is provided with an undervoltage detection that can be disabled
(address 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 2-13). In the range between V/V
Nom
= 1.00
to 0.25 a linear, directly proportional dependence is realized, and therefore the following applies:
[spannungsabhaengigkeit-des-anregewertes, 1, en_US]
Figure 2-13 Voltage influence of the pickup value
The 51 PICKUP value is decreased proportional to the voltage decrease. Consequently, for constant current Ι
the Ι/ 51 PICKUP ratio is increased and the tripping time is reduced. Compared with the standard curves repre-
sented 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 2-2
Controlling voltages in relation to the fault currents
Strom Spannung
Ι
A
V
A
– V
B
Ι
B
V
B
– V
C
Ι
C
V
C
– V
A
2.2.5
Functions
2.2 Overcurrent Protection 50, 51, 50N, 51N
66 SIPROTEC 4, 7SJ62/64, Manual
C53000-G1140-C207-8, Edition 08.2016