0 200 400 600 800 1000 1200-505101520253035Current in kA, trip command to CB, rotor angle in rad →Time in milliseconds →pos. seq. current in kAtrip command to CBrotor angle in radianfaultoccurs← normal load current← min. currentvery high currents dueto out-of-step condition← after 1stpole slip← 2nd← rotor angleangle towards 0°current decreasesfault cleared →current increases underfault conditions→ ← tBreaker = 60 mstrip command →issued hereIEC10000115-1-en.vsdIEC10000115 V1 ENFigure 82: Trip initiation when the break-time of the circuit breaker is known7.4.7.5 DesignWhen the complex impedance Z(R, X) enters the limit-of-reach region, the algorithmdetermines the direction impedance Z moves, that is, the direction the lens is traversed andmeasures the time taken to traverse the lens from one side to the other. If the traverse timeis more than the limit 40 or 50 ms, a pole-slip is declared. If the complex impedance Z(R,X) exits the lens on the same side it entered, then it is a stable case and the protectedmachine is still in synchronism. If a pole-slip has been detected, then it is determined inwhich zone the centre of oscillation is located. If the number of actual pole-slips exceedsthe maximum number of allowed pole-slips in either of the zones, a trip command isissued taking care of the circuit breaker safety.Section 7 1MRK 502 048-UUS AImpedance protection186Technical manual
