A method is proposed to evaluate the security of distance backup protection under power flow transfer conditions

providing support for early interruption of cascading tripping caused by subsequent overload. First

based on the relative position between the measured impedance and the protection operation boundary on the impedance plane

the protection’s ability to avoid overload impedance is quantitatively evaluated. In addition to considering branch transmission power

the method further incorporates factors such as branch power factor angle

protection settings

and operation characteristics of the impedance relay to improve evaluation accuracy. Second

the minimum value of the protection’s ability to avoid overload impedance under multiple simultaneous branch outage events is defined as the evaluation index of protection security under power flow transfer conditions. Consequently

the protection security evaluation problem is transformed into an optimization problem

where multiple branch outage events serve as decision variable. A corresponding mathematical model and a solution approach based on binary particle swarm optimization (BPSO) are developed. Experimental results show that the proposed method effectively captures the combined influence of branch power flow and protection characteristics on overall protection security

allowing for more accurate identification of the initial locations where overload tripping may occur.