Realistic testing of power swing blocking and out-of-step tripping functions

Sudden changes in the power system, such as faults, network changes due to line trip outs, and disconnection of large loads or a generating plant, causing oscillations until remaining generators adjust and settle to new stable conditions. In some cases these power oscillations or swings are so large that some generators run out of step and lose synchronism. With generators having different frequencies these power swings result in voltages and currents in different parts of the networks to swing in amplitude and phase angle. The impedance measurements based on these varying voltages and currents will also oscillate. If the measured impedance becomes very small and enters the distance relay zones it can lead to an undesired trip of the distance relay. Distance relays have power swing blocking features to detect these conditions and prevent a misoperation for stable power swings. However, during the blocking period a fault can occur on the line and the distance relay must detect this condition and unblock the distance zones.