REAL-TIME POWER SWING DETECTION USING EQUAL AREA CRITERION

Abstract

Power systems are interconnected for enhancing the availability of power supply to the customers and for reducing the prices charged to them. A natural consequence of the interconnections is that the interconnected power systems are likely to swing following faults and other disturbing events. The systems should be allowed to settle down to a stable state if one exists but should be disconnected from each other if the swing is likely to result in dynamic instability. For maintaining generation-load balance, it becomes necessary to shed load in some cases and shed generation in other cases. During power-system swings, currents and voltages swing in amplitude and phase. The impedances seen by distance relays, which calculate the impedances from the measured voltages and currents, also swing in magnitude and phase. At certain locations in the power system, the impedances calculated from the voltages and currents become so small that they enter the protection zones of distance relays. Power swing relays are traditionally used to block distance relays from tripping line circuit breakers if the swing is not likely to cause dynamic instability but open the interconnecting lines if the systems are likely to become unstable. This is achieved by using delay timers but deciding on the delays is a very difficult and involved issue. Also the needed delay times are different for different operating states of the systems. This thesis presents a new approach for use in out-of-step relays. The technique is based on the well known equal area criterion in the time domain. The basis of the protection technique is described in the thesis. The results from computer simulations show that the technique is able to correctly identify swings that are likely to result in dynamic instability. Based on the information provided by the technique, appropriate decisions for shedding loads or generation can be made with reasonable certainty.