Functions
2.2 Overcurrent Protection 50, 51, 50N, 51N
SIPROTEC, 7SK80, Manual
E50417-G1140-C344-A4, Release date 08.2010
54
2.2 Overcurrent Protection 50, 51, 50N, 51N
The overcurrent protection is provided with a total of four elements each for the phase currents and the ground
current. All elements are independent from each other and can be combined as desired.
If it is desired in isolated or resonant-grounded systems that three-phase devices should work together with
two-phase protection equipment, the overcurrent protection can be configured in such a way that it allows two-
phase operation besides the three-phase mode (see Chapter 2.1.3.2).
The high-set elements 50-2, 50-3, 50N-2, 50N-3 as well as the overcurrent elements 50-1 and 50N-1 always
operate with a definite tripping time (51), the elements 51 and 51N always with an inverse tripping time (50).
Applications
• The non-directional overcurrent protection is applicable for networks that are radial and supplied from a
single source or open looped networks, for backup protection of differential protective schemes of all types
of lines, transformers, generators, motors, and busbars.
2.2.1 General
The overcurrent protection for the ground current can either operate with measured values I
N
or with the quan-
tities 3I0 calculated from the three phase currents. Which values are used depends on the setting of parameter
613 Gnd O/Cprot. w. and the selected type of connection of the current transformers. Information on this
can be found in Chapter 2.1.3.2, connection examples in the Appendix A.3. Devices featuring a sensitive
ground current input, however, generally use the calculated quantity 3I0.
The timer for each Element can be blocked via binary input thus suppressing the trip command. If the blocking
is cancelled during pickup, the timer will be restarted. The Manual Close signal is an exception in this case. If
a circuit breaker is manually closed onto a fault, it can be re-opened immediately. For the overcurrent or high
current Element the delay can be bypassed via a Manual Close pulse which results in instantaneous tripping.
This pulse is extended up to at least 300 ms.
Pickup of the definite-time elements can be stabilized by setting the dropout times. This protection comes into
use in systems where intermittent faults occur. Combined with electromechanical relays, it allows different
dropout responses to be adjusted and a time grading of numerical and electromechanical relays to be imple-
mented.
Pickup and delay settings may be quickly adapted to system requirements via dynamic setting changeover (see
Section 2.4).
Tripping by the 50-1 and 51 elements (in phases), 50N-1 and 51N elements (in ground path) may be blocked
for inrush conditions by utilizing the inrush restraint feature. 4
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