The ImpedanceZB is the reverse impedance as show in figure 234. ZB should be equal tothe generator transient reactance X'd. The impedance is given in % of the base impedance,see equation 425.The ImpedanceZC is the forward impedance giving the borderline between zone 1 andzone 2. ZC should be equal to the transformer reactance ZT. The impedance is given in %of the base impedance, see equation 425.The angle of the impedance line ZB – ZA is given as AnglePhi in degrees. This angle isnormally close to 90°.StartAngle: An alarm is given when movement of the rotor is detected and the rotor angleexceeds the angle set for StartAngle. The default value 110° is recommended. It should bechecked so that the points in the impedance plane, corresponding to the chosen StartAngledoes not interfere with apparent impedance at maximum generator load.TripAngle: If a pole slip has been detected: change of rotor angle corresponding to slipfrequency 0.2 – 8 Hz, the slip line ZA – ZB is crossed and the direction of rotation is thesame as at start, a trip is given when the rotor angle gets below the set TripAngle. Thedefault value 90° is recommended.N1Limit: The setting N1Limit gives the number of pole slips that should occur before trip,if the crossing of the slip line ZA – ZB is within zone 1, that is, the node of the pole slipis within the generator transformer block. The default value 1 is recommended tominimize the stress on the generator and turbine at out of step conditions.N2Limit: The setting N2Limit gives the number of pole slips that should occur before trip,if the crossing of the slip line ZA – ZB is within zone 2, that is, the node of the pole slipis in the external network. The default value 3 is recommended give external protectionspossibility to split the network and thus limit the system consequencies.ResetTime: The setting ResetTime gives the time for (PSPPPAM ,78) function to resetafter start when no pole slip been detected. The default value 5s is recommended.8.16.3.1 Setting example for line applicationIn case of out of step conditions this shall be detected and the line between substation 1 and2 shall be tripped.1MRK 506 369-UUS - Section 8Impedance protectionLine distance protection REL670 2.2 ANSI 457Application manual
