中文摘要：

提出阶段式电流保护的运行风险评估模型，揭示了继电保护对电力系统可靠性的影响机理。首先阐述了继电保护在某一电网运行方式下拒动和误动的原理，指出保护拒动和误动产生于设备硬件失效和运行时保护设置不当。然后根据电网中保护的实际配置情况、动作值和动作时限，推导出基于实际保护范围或灵敏系数的运行拒动概率和运行误动概率两组风险指标。此外，还提出受保护错误动作影响的失负荷风险指标。利用一个典型实例，阐述了阶段式电流保护运行风险评估模型的应用，以说明该方案的实用性和有效性。

英文摘要：

To reveal the mechanism of how much impact the protection system can have on the power system reliability, the paper proposes an operational risk evaluation model of stepped current protection. Firstly, the principles of the fail-operation and false-operation of protection relay are described considering operational conditions of power grids, and the truth that fail-operation and false-operatlon are due to the invalidation of hardware and incorrect protection scheme in the operation is pointed out. According to the scheme of relay protection systems, the tripping setting and time of stepped current protection in grids, two sets of risk indices based on actual protective range and sensitivity coefficient, such as operational fail-operation probability and operational falseoperation probability, are deduced. Furthermore, the loss of load risk index incorporating protection misoperation effect is put forward. The application of the presented operational risk evaluation model of stepped current protection is clarified using a typical example to show the practicality and validity of the proposed approach.