Functions
2.2 Overcurrent Protection 50, 51, 50N, 51N
SIPROTEC, 7SK80, Manual
E50417-G1140-C344-A4, Release date 08.2010
61
2.2.4 Inverse Time Overcurrent Elements 51, 51N
Inverse time overcurrent elements are dependent on the ordering version. They always operate with an inverse
time Curve in accordance with IEC or ANSI standards. The characteristics and associated formulas are given
in the Technical Data. During configuration of the inverse time characteristics, the definite time relay elements
50-1, 50-2 and 50-3 are also enabled (see Sections "Definite Time High-set Elements 50-2, 50-3, 50N-2, 50N-
3" and "Definite Time Overcurrent Elements 50-1, 50N-1").
Pickup Behavior
For each Element an individual pickup value 51 PICKUP or 51N PICKUP is set. Apart from Fundamental,
the True RMS can also be measured. Each phase and ground current is separately compared with the setting
value 51 or 51N per Element. If a current exceeds 1.1 times the setting value, the corresponding Element picks
up and is signaled individually. If the inrush restraint feature is used, either the normal pickup signals or the
corresponding inrush signals are issued as long as inrush current is detected. If the 51 Element picks up, the
tripping time is calculated from the actual fault current flowing, using an integrating method of measurement.
The calculated tripping time depends on the selected tripping curve. Once this time has elapsed, a trip signal
is issued provided that no inrush current is detected or inrush restraint is disabled. If the inrush restraint feature
is enabled and an inrush condition exists, no tripping takes place but a message is recorded and displayed in-
dicating when the overcurrent element time delay elapses.
For ground current element 51N the Curve may be selected independently of the Curve used for phase cur-
rents.
Pickup values of elements 51 (phase currents) and 51N (ground current) and the relevant time multiplicators
may be set individually.
The following two figures show the logic diagrams for the inverse time overcurrent protection.
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