Manual 7SJ46
Settings
C53000-K1174-C002-4
51
high-impedance equipment (transformers, paralleling reactors).
The high-current stage is set such that it only picks up in case of
a short-circuit upstream or inside of such equipment. It should be
kept in mind that transformers, reactors and motors may absorb
on power-up an increased magnetizing current (inrush current).
A short delay time may be required in the case of machines to
bridge the inrush current. Therefore a delay can be set for the I>>
stage.
The earth current stage IE> is a useful function in networks with
effective or resistance or reactance earthing. Set this stage to
somewhat less than the lowest earth fault current to be expected.
For the time setting, a separate selective tripping schedule for
earth faults can be used.
The setting of the overcurrent stage I> is mainly determined by
the maximum load current. It must be excluded that the device
picks up due to an overload while it is operated as a short-circuit
protection. In that case, the overcurrent stage should be set for
line protection to approx. 20% above the highest (over-)load
current to be expected, and to approx. 50% for transformer and
motor protection. When using the device downstream of
controlled or parallelly operated transformers, make sure that the
setting value is lower than the lowest short-circuit current to be
expected for the planned protection zone. Refer to the selective
tripping schedule for setting times.
Where the device is used for inverse-time overcurrent protection,
an interval of approx. 0.3 s (typical interval for selective tripping)
must be observed between the set characteristics of two adjacent
relays. These characteristics are set on the basis of the scale
factor TIp or D (see Figure 2-11 to Figure 2-20). Since device
7SJ46 operates with disk emulation, please observe the reset
characteristics.
The definite time overcurrent protection of 7SJ46 operates with
disk emulation. It emulates the behavior of disks of
electromechanical relays. In case several faults occur
successively it is ensured that the "prehistory" is taken into
consideration due to the inertia of the disk and that the time
behavior is matched. The 7SJ46 divides the operation range of
the disk emulation into an incrementing zone (1.1 to 20
⋅
I/I
P
) and
a decrementing zone (< 0,9
⋅
I/I
P
). The zone in between is called
idle state zone since this is the zone where the virtual disk is in
idle state.
If you press the "ENTER" key or disconnect the device from the
auxiliary voltage, the disk emulation will be reset.
The disk emulation offers its advantages when the grading
coordination chart of the time overcurrent protection is combined
(on electromechanical base) with other devices connected to the
system.
The current thresholds relating to the rated device current are set
on the device. These thresholds include:
Current transformer I
NP
/ I
NS
100 A / 5 A
Rated device current I
N
5 A
Maximum load current
120 A
Safety factor (assumed)
1.25
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