characteristics. 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.The time characteristic for step 1 and 4 can be chosen as definite time delay or inverse timecharacteristic. Step 2 and 3 are always definite time delayed and are used in system whereIDMT is not needed.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.7.3.3 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 definitetime delay. Thus, if only the inverse time delay is required, it is importantto set the definite time delay for that stage to zero.The parameters for Four step phase overcurrent protection 3-phase output OC4PTOC(51/67) are set via the local HMI or PCM600.The following settings can be done for OC4PTOC (51/67).GlobalBaseSel: Selects the global base value group used by the function to define (IBase),(VBase) and (SBase).MeasType: Selection of discrete Fourier filtered (DFT) or true RMS filtered (RMS)signals. RMS is used when the harmonic contents are to be considered, for example inapplications with shunt capacitors.Operation: The protection can be set to Disabled or Enabled2ndHarmStab: Operate level of 2nd harmonic current restrain set in % of the fundamentalcurrent. The setting range is 5 - 100% in steps of 1%. Default setting is 20%.Section 7 1MRK 506 334-UUS ACurrent protection180Application manual
