ABB RET670 Applications Manual page 408

Section 3
IED application
402
If VT inputs are not available or not connected, setting parameter
DirModeSelx (x = step 1, 2, 3 or 4) shall be left to default value Non-
directional.
In many applications several steps with different current pick up levels and time delays
are needed. OC4PTOC (51_67) can have up to four different, individual settable, steps.
The flexibility of each step of OC4PTOC (51_67) is great. The following options are
possible:
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
1MRK504116-UUS C
Application manual