Functions
2.2 Overcurrent Protection 50, 51, 50N, 51N
SIPROTEC, 7SJ62/64, Manual
C53000-G1140-C207-2, Release date 01.2008
70
2.2.4
Inverse Time Overcurrent Elements 51, 51N
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.
During configuration of the inverse time characteristics, the definite time relay elements 50-3, 50-2 and 50-1
are also enabled (see Section "Definite Time High-set Elements 50-3, 50-2, 50N-3, 50N-2" and "Definite Time
Overcurrent Elements 50-1, 50N-1").
A voltage restraint can optionally be set (see Section „Inverse Time Overcurrent Protection (Voltage-controlled
/ Voltage-restraint“).
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.
These elements can be blocked by the automatic reclosing feature (79 AR).
For ground current element 51N the characteristic may be selected independently of the characteristic used for
phase currents.
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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