Non-directional / Directional function: In most applications the non-directionalfunctionality is used. This is mostly the case when no fault current can be fed from theprotected object itself. In order to achieve both selectivity and fast fault clearance, thedirectional function can be necessary.Choice of delay time characteristics: There are several types of delay time characteristicsavailable such as definite time delay and different types of inverse time delaycharacteristics. The selectivity between different overcurrent protections is normallyenabled by co-ordination between the function time delays of the different protections. Toenable optimal co-ordination between all overcurrent protections, they should have thesame time delay characteristic. Therefore a wide range of standardized inverse timecharacteristics are available: IEC and ANSI. It is also possible to tailor make the inversetime characteristic.Normally it is required that the phase overcurrent protection shall reset as fast as possiblewhen the current level gets lower than the operation level. In some cases some sort ofdelayed 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 levelfor some time. A typical case is when the protection will measure the current to a largemotor. At the start up sequence of a motor the start current can be significantly larger thanthe rated current of the motor. Therefore there is a possibility to give a setting of amultiplication factor to the current pick-up level. This multiplication factor is activatedfrom a binary input signal to the function.Power transformers can have a large inrush current, when being energized. Thisphenomenon is due to saturation of the transformer magnetic core during parts of theperiod. There is a risk that inrush current will reach levels above the pick-up current of thephase overcurrent protection. The inrush current has a large 2nd harmonic content. Thiscan 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 avalue above a set percentage of the fundamental current.The phase overcurrent protection is often used as protection for two and three phase shortcircuits. In some cases it is not wanted to detect single-phase ground faults by the phaseovercurrent protection. This fault type is detected and cleared after operation of groundfault 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 issufficient to have high current in one phase only. If set 2 of 3 or 3 of 3 single-phase groundfaults are not detected.7.1.3 Setting guidelinesWhen inverse time overcurrent characteristic is selected, the operate timeof the stage will be the sum of the inverse time delay and the set definiteSection 7 1MRK 505 337-UUS -Current protection162Application manual
