where the IN is the fault current level in the faulty phase. A high sensitivity need not to beachieved as the two-phase fault level normally is well above base current.tIN: The time delay for detecting that the fault is cross-country. Normal time setting is 0.1- 0.15 s.tVN: The time delay for a secure VN detecting that the fault is an ground-fault or doubleground-fault with residual voltage. Normal time setting is 0.1 - 0.15 s.tOffVN: The VN voltage has a reset drop-off to ensure correct function without timingproblems. Normal time setting is 0.1 s6.6 Power swing detection ZMRPSB (68)6.6.1 IdentificationFunction description IEC 61850identificationIEC 60617identificationANSI/IEEE C37.2device numberPower swing detection ZMRPSBZpsbSYMBOL-EE V1 EN686.6.2 Application6.6.2.1 GeneralVarious changes in power system may cause oscillations of rotating units. The mosttypical reasons for these oscillations are big changes in load or changes in power systemconfiguration caused by different faults and their clearance. As the rotating masses striveto find a stable operate condition, they oscillate with damped oscillations until they reachthe final stability.The extent of the oscillations depends on the extent of the disturbances and on the naturalstability of the system.The oscillation rate depends also on the inertia of the system and on the total systemimpedance between different generating units. These oscillations cause changes in phaseand amplitude of the voltage difference between the oscillating generating units in thepower system, which reflects further on in oscillating power flow between two parts of thesystem - the power swings from one part to another - and vice versa.1MRK 506 334-UUS A Section 6Impedance protection157Application manual
