Distance IEDs located in interconnected networks see these power swings as the swingingof the measured impedance in relay points. The measured impedance varies with timealong a locus in an impedance plane, see figure 75. This locus can enter the operatingcharacteristic of a distance protection and cause, if no preventive measures have beenconsidered, its unwanted operation.RjXOperating characteristicImpedance locus atpower swingIEC09000224_1_en.vsdIEC09000224 V1 ENFigure 75: Impedance plane with Power swing detection operating characteristicand impedance locus at power swing6.6.2.2 Basic characteristicsPower swing detection function (ZMRPSB) (68) reliably detects power swings withperiodic time of swinging as low as 200 ms (which means slip frequency as high as 10%of the rated frequency on the 50 Hz basis). It detects the swings under normal systemoperating conditions.ZMRPSB (68) function is able to secure selective operation for internal faults duringpower. The operation of the distance protection function remains stable for external faultsduring the power swing condition, even with the swing (electrical) centre located on theprotected power line.The operate characteristic of the ZMRPSB (68) function is easily adjustable to theselected impedance operate characteristics of the corresponding controlled distanceprotection zones as well as to the maximum possible load conditions of the protectedpower lines.6.6.3 Setting guidelinesSetting guidelines are prepared in the form of a setting example for the protected powerline as part of a two-machine system presented in figure 76.Section 6 1MRK 506 334-UUS AImpedance protection158Application manual
