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The guidelines to set an out of step blocking relay is that with maximum slip between systems, it will take an impedance trajectory more than 4 cycles to traverse the distance from out of step characteristic to mho tripping curve. The 4 cycle time is the required pick up time of auxiliary relay which establishes blocking function. As in case if setting out of step tripping relays, precaution has to be taken that out of step blocking characteristic does not encroach the load impedance point and establish increment blocking of line tripping relays.
Usually, blocking is applied only to zone1 and zone2 and not to zone3. |
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This enables clearing of fault which may arise during power swing block condition. The blocking function setting for reclose blocking condition is also similar to that described for trip blocking setting. Blocking automatic reclosing is a must for out of step condition. In case the OSB offset mho characteristic encroaches into the load region, then it has to be appropriately modified. This is achieved by using additional blinders, typically set at  . Separation angle is as shown in fig 26.6. |
