Note:The R6max reach must be greater than the maximum resistive reach of any distance zone to ensure correct initiation of the25 ms and Delta T timers. However, the R5min reach could be set below the distance maximum resistive reach (inside thedistance characteristic) if an extensive resistive coverage is required, meaning that Out-of-Step protection does not pose arestriction to the quadrilateral applications.For each zone, we receommend setting the positive and negative limits to be the same so, OST R5’ = OST R5, OSTZ5’ = OST Z5, OST R6’ = OST R6, and OST Z6’ = OST Z6.4.4.1.2 SETTING OST Z5 AND OST Z6Setting of the reactance lines OST Z5 and OST Z6 depends on how far from the protection location the poweroscillations are to be detected. Normally, there is only one point for initial splitting of the system; and that point willbe determined by system studies. For that reason, the Out-of-Step protection must be enabled at that location anddisabled on all others. To detect the Out-of-Step conditions, the OST Z5’, OST Z5, OST Z6’, and OST Z6 settings mustbe set to comfortably encompass the total system impedance ZT. A typical setting could be:OST Z5 = OST Z5’ = ZTThe OST Z6 and OST Z6’ settings are not of great importance and could be set to 1.1 x OST Z5.4.4.1.3 SETTING OST R5, OST R6 AND DELTA TThe R5min and R6max settings determined above represent limit values. The actual OST R5 and OST R6 valuesneed to be determined in relation to the Delta T timer.Predictive OST settingFor the Pred. OST Trip setting, it is important to:● Set OST R6 equal to R6max● Set OST R5 as close as practical to R6maxThe aim of pushing the OST R5 setting to the right is to detect fast oscillations as soon as possible, in order to gainsufficient time to operate the breaker before the two source voltages are in opposite directions. The only restrictionis the limitation of the Delta T minimum time delay of 30ms and the speed of oscillation.You should set Delta T such that it does not expire after the positive sequence impedance has passed the OST R6– OST R5 region.For this setting, you need to know the rate-of-change of swing impedance when crossing the OST R6 – OST R5region. This must therefore be based on system studies.Note:You cannot assume that the rate-of-change of positive sequence impedance while crossing the OST R6 – OST R5 region is thesame as the average rate-of-change of positive sequence impedance for the whole swing cycle. A false assumumption couldlead to incorrect predictive OST operation.Note:For a fault, the OST R6 – OST R5 region will be crossed faster than 25ms, therefore even very fast oscillations up to 7Hz willnot be mistaken as a fault condition and predictive OST will not operate.OST settingFor the OST Trip setting option, such a precise setting of the blinders and Delta T is not necessary. This isbecause for a wide ∆R region and a short Delta T setting, any oscillation will be successfully detected. However,the fault impedance must pass through the ∆R region faster than the Delta T setting.Chapter 10 - Power Swing Functions P446SV246 P446SV-TM-EN-1
