Abstract: Traditional static instability detecting for line protection depends on artificial setting the specific value.And when the power swing center is outside the line,the swing may not be detected.If faults occur again under such circumstances,the distance protection will wrongly act and needs swing block.A novel static instability detecting criterion based on the characteristic of Ucos φ and its change rate is presented.The criterion is independent of setting and immune from the swing center location.Simulation results demonstrate that the protection at both sides of the line can detect static instability fast and reliably with the criterion proposed,so that mal-operation of the line protection may be prevented by adopting effective measures.