Overcurrent Protection 50, 51, 50N, 51N; General - Siemens siprotec SJ62 User Manual

2 Functions
2.2

Overcurrent Protection 50, 51, 50N, 51N

Applications
2.2.1

General

59
General time overcurrent protection is the main protective function of the 7SJ62/63/64
relay. Each phase current and the ground current is provided with three elements. All
elements are independent of each other and can be combined in any way.
If it is desired in isolated or resonant-grounded systems that three-phase devices
should work together with two-phase protection equipment, the time-overcurrent pro-
tection can be configured such that it allows two-phase operation besides three-phase
mode (see Section 2.1.3.2).
High-current element 50-2 and overcurrent element 50-1 always operate with definite
tripping time, the third element 51, operates always with inverse tripping time.
• The non-directional time overcurrent protection is suited for networks that are radial
and supplied from a single source or open looped networks or for backup protection
of differential protective schemes of all types of lines, transformers, generators,
motors, and busbars.
Depending on parameter 613 Gnd O/Cprot. w. the overcurrent protection for the
ground current can either operate with measured values I
calculated from the three phase currents. Devices featuring a sensitive ground current
input, however, generally use the calculated quantity 3I0.
All overcurrent element enabled in the device may be blocked via the automatic reclo-
sure function (depending on the cycle) or via an external signal to the binary inputs of
the device. Removal of blocking during pickup will restart time delays. The Manual
Close signal is an exception. If a circuit breaker is manually closed onto a fault current,
it can be re-opened immediately. For overcurrent or high-set element the delay may
be bypassed via a Manual Close pulse, thus resulting in high speed tripping. This
pulse is extended up to at least 300 ms.
The automatic reclosure function 79 may also initiate immediate tripping for the over-
current and high-set elements depending on the cycle.
Pickup of the 50Ns elements can be stabilized by setting the dropout times. This pro-
tection comes into use in systems where intermittent faults occur. Combined with elec-
tromechanical relays, it allows different dropout responses 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 swapping (see Section 2.4).
Tripping by the 50-1, 51 elements (in phases), 50N-1 and 51N elements (in ground
path) may be blocked for inrush conditions by utilizing the inrush restraint feature.
or with the quantities 3I0
N
SIPROTEC 4, 7SJ62/63/64 Handbuch
C53000-G1140-C147-A, Edition 07.2015