Setting Guidelines - ABB REB670 Applications Manual

Section 7
Current protection
7.1.3
162
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.
Choice of delay time characteristics: There are several types of delay time 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: 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 2nd harmonic content. This
can be used to avoid unwanted operation of the protection. Therefore, OC4PTOC (51/67)
have a possibility of 2nd harmonic restrain if the level of this harmonic current reaches a
value above a set percentage of the fundamental current.
The phase overcurrent protection is often used as 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.

Setting guidelines

When inverse time overcurrent characteristic is selected, the operate time
of the stage will be the sum of the inverse time delay and the set definite
1MRK 505 337-UUS -
Application manual