Overcurrent Protection 50, 51, 50N, 51N; General - Siemens SIPROTEC 7SJ61 Manual

Functions

2.2 Overcurrent Protection 50, 51, 50N, 51N

2.2 Overcurrent Protection 50, 51, 50N, 51N
Overcurrent protection is the main protection function of the 7SJ61 relay. Each phase current and the ground
current is provided with four elements. All elements are independent from each other and can be combined as
desired.
If it is desired in grounded or compensated 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 twophase
operation besides three-phase mode (see Section 2.1.3.2).
The high-set elements 50-3 and 50-2 as well as the overcurrent element 50-1 always operate with definite trip-
ping time, the fourth element 51 always with inverse tripping time.
Applications
• The non-directional overcurrent protection is suited for networks that are radial and supplied from a single
source or open looped networks, for backup protection of differential protective schemes of all types of lines,
transformers, generators, motors, and busbars.
2.2.1

General

Depending on parameter 613 50N/51N/67N w. the overcurrent protection for the ground current can either
operate with measured values I
featuring a sensitive ground current input, however, generally use the calculated quantity 3I0.
All overcurrent elements enabled in the device may be blocked via the automatic reclosing 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,
it can be re-opened immediately. For overcurrent or high-set elements 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 overcurrent and high-set ele-
ments depending on the cycle.
Pickup of the definite-time elements can be stabilized by setting the dropout times. This protection comes into
use in systems where intermittent faults occur. Combined with electromechanical relays, it allows different
dropout responses to be adjusted and a time grading of digital and electromechanical relays to be implement-
ed.
Pickup and delay settings may be quickly adapted to system requirements via dynamic setting changeover
(see Section 2.3).
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. 4
48
or with the quantities 3I0 calculated from the three phase currents. Devices
N
SIPROTEC, 7SJ61, Manual
C53000-G1140-C210-1, Release date 02.2008