signal is common for all three phases and all steps. It shall be noted that the selectionof measured value (DFT or RMS) do not influence the operation of directional part ofOC4PTOC.Service value for individually measured phase currents are also available on the localHMI for OC4PTOC function, which simplifies testing, commissioning and in serviceoperational checking of the function.A harmonic restrain of the function can be chosen. A set 2nd harmonic current inrelation to the fundamental current is used. The 2nd harmonic current is taken from thepre-processing of the phase currents and the relation is compared to a set restraincurrent level.The function can be directional. The direction of the fault current is given as currentangle in relation to the voltage angle. The fault current and fault voltage for thedirectional function is dependent of the fault type. To enable directional measurementat close in faults, causing low measured voltage, the polarization voltage is acombination of the apparent voltage (85%) and a memory voltage (15%). Thefollowing combinations are used.Phase-phase short circuit:1 2 1 2 1 2 1 2= - = -refL L L L dirL L L LU U U I I IEQUATION1449 V1 EN (Equation 35)2 3 2 3 2 3 2 3= - = -refL L L L dirL L L LU U U I I IEQUATION1450 V1 EN (Equation 36)3 1 3 1 3 1 3 1= - = -refL L L L dirL L L LU U U I I IEQUATION1451 V1 EN (Equation 37)Phase-earth short circuit:1 1 1 1= =refL L dirL LU U I IEQUATION1452 V1 EN (Equation 38)2 2 2 2= =refL L dirL LU U I IEQUATION1453 V1 EN (Equation 39)3 3 3 3= =refL L dirL LU U I IEQUATION1454 V1 EN (Equation 40)1MRK 504 135-UEN A Section 8Current protection149Technical manual
