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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 suited for radial systems with single-side infeed or open ring
systems, for backup protection of all kinds of comparison protection equipment for lines, transformers, gen-
erators, motors, and busbars.
2.2.1
General
The overcurrent protection for the ground current can either operate with measured values I
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 is used in
systems where intermittent faults occur. Combined with electromechanical relays, it allows different dropout re-
sponses to be adjusted and a time grading of digital and electromechanical relays to be implemented.
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.
The following table gives an overview of the interconnections to other functions of the devices 7SK80.
Table 2-2
Overcurrent protection
elements
50-1
50-2
50-3
51
50N-1
50N-2
50N-3
51N
SIPROTEC, 7SK80, Manual
E50417-G1140-C344-A5, Release date 11.2012
Interconnection to other functions
Manual
CLOSE
Dynamic cold load pickup
function
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2.2 Overcurrent Protection 50, 51, 50N, 51N
or with the quan-
N
Inrush restraint
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Functions
59
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