Out-of-step protection enhancements

Power systems are subjected to a wide range of small or larger disturbances during operating conditions and they are designed to survive disturbances caused by faults, loss of a large generator, or line switching. The power system typically adjusts to these disturbances and continues to operate satisfactorily and within the desired bounds of voltage and frequency. Multiple system disturbances, however, could cause loss of synchronism between interconnected power systems that lead to loss of generation and load, and sometimes to wide-area blackouts. To mitigate the effect of these disturbances, it is common practice to provide controls called special protection systems that aid in maintaining system stability. In addition, properly designed power systems include out-of-step (OOS) protection systems that detect loss of angular instability and perform controlled network islanding to preserve stability within smaller networks. In this paper, we describe the application philosophy of OOS protection systems in transmission systems and discuss recent enhancements in the design of out-of-step tripping (OST) and blocking protection functions that improve the security and reliability of the power system.