Non-directional / Directional function: In most applications the non-directional
functionality is used. This is mostly the case when no fault current can be fed from the
protected object itself. In order to achieve both selectivity and fast fault clearance, the
directional function can be necessary.
If VT inputs are not available or not connected, the setting parameter
DirModeSelx (x = step 1, 2, 3 or 4) shall be left to the default value Non-
directional.
Choice of time delay characteristics: There are several types of time delay characteristics
available such as definite time delay and different types of inverse time delay
characteristics. The selectivity between different overcurrent protections is normally
enabled by co-ordination between the function time delays of the different protections. To
enable optimal co-ordination between all overcurrent protections, they should have the
same time delay characteristic. Therefore, a wide range of standardized inverse time
characteristics are available for IEC and ANSI. It is also possible to tailor make the inverse
time characteristic.
Normally, it is required that the phase overcurrent protection shall reset as fast as possible
when the current level gets lower than the operation level. In some cases some sort of
delayed reset is required. Therefore, different kinds of reset characteristics can be used.
For some protection applications, there can be a need to change the current pick-up level
for some time. A typical case is when the protection will measure the current to a large
motor. At the start up sequence of a motor the start current can be significantly larger than
the rated current of the motor. Therefore, there is a possibility to give a setting of a
multiplication factor to the current pick-up level. This multiplication factor is activated
from a binary input signal to the function.
Power transformers can have a large inrush current, when being energized. This
phenomenon is due to saturation of the transformer magnetic core during parts of the
period. There is a risk that inrush current will reach levels above the pick-up current of the
phase overcurrent protection. The inrush current has a large 2
nd
harmonic content. This
can be used to avoid unwanted operation of the protection function. Therefore, OC4PTOC
(51/67) has a possibility of 2
nd
harmonic restrain if the level of 2
nd
harmonic current
reaches a value above a set percent of the fundamental current.
The phase overcurrent protection is often used as a protection for two and three phase short
circuits. In some cases, it is not wanted to detect single-phase ground faults by the phase
overcurrent protection. This fault type is detected and cleared after operation of ground
fault protection. Therefore, it is possible to make a choice how many phases, at minimum,
that have to have current above the pick-up level, to enable operation. If set 1 of 3 it is
sufficient to have high current in one phase only. If set 2 of 3 or 3 of 3 single-phase ground
faults are not detected.
1MRK 506 369-UUS - Section 9
Current protection
Line distance protection REL670 2.2 ANSI 493
Application manual
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