Inverse Time Overcurrent Elements 51, 51N - Siemens siprotec SJ62 User Manual

Figure 2-8 Logic of the dropout delay for 50N-1 ground current element
2.2.4

Inverse Time Overcurrent Elements 51, 51N

Pickup Behaviour
SIPROTEC 4, 7SJ62/63/64 Handbuch
C53000-G1140-C147-A, Edition 07.2015
Inverse time elements are dependent on the variant ordered. They operate with an
inverse time characteristic either according to the IEC- or the ANSI-standard or with a
user-defined characteristic. The characteristics and associated formulas are given in
the Technical Data. If inverse time characteristics have been configured, definite time
elements 50-2 and 50-1 are also enabled (see Sections "Definite Time High-Set Ele-
ments 50-2, 50N-2" and "Definite Time Overcurrent Elements 50-1, 50N-1").
Each phase and ground current is separately compared with the pickup values of the
inverse time overcurrent protection element 51 and 51N. If a current exceeds 1.1
times the setting value, the corresponding element picks up and is signalled individu-
ally. If the inrush restraint feature is applied, either the normal pickup signals or the cor-
responding inrush signals are output as long as inrush current is detected. Pickup of
a relay element is based on the rms value of the fundamental harmonic. When the 51
element picks up, the time delay of the trip signal is calculated using an integrated
measurement process. The calculated time delay is dependent on the actual fault
current flowing and the selected tripping characteristics. Once the time delay elapses,
a trip signal is issued assuming 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 indicating when the
overcurrent element time delay elapses.
These elements can be blocked by the automatic reclosure feature (79).
For ground current element 51N the characteristic may be selected independently of
the characteristic used for phase currents.
Pickup values of elements 51 (phases) and 51N (ground current) and the associated
time multipliers may be individually set.
2.2 Overcurrent Protection 50, 51, 50N, 51N
66