Abstract: The core requirement of transmission network enterprise lies in the safe operation of transmission network. Failure in satisfying this requirement will result in disastrous loss on economic and social benefits. Hence, transmission network is expected to possess the capability of enduring possible disturbances in order to ensure the normal operation of grid and equipment, as well as avoid the power transmission interruptions. Through the analysis on the requirement satisfaction of transmission system stakeholders, the overall status of transmission network operation can be evaluated. Based on the stochastic characteristics of transmission network operation, this paper used a risk assessment method to construct evaluation indices set for the security requirement, and proposed the calculation model and method of security risk assessment indicators. Compared with the traditional researches on grid security, a scenario analysis approach was introduced. In detail, breakdown or normal operation of certain components under certain specific operating modes was modeled as a scenario, and a probabilistic model for the scenario was constructed with taking into consideration a variety of random factors. All kinds of security problems were studied in the specific scenarios. The security of the situations could be evaluated comprehensively according to the safety criterion indices； therefore a minimum expense of control could be adopted to measure the consequence of the disturbance on the transmission network operation. Through sampling method, the probability and consequence of each situation occurrence could be acquired； and with the assistance of calculating value at risk and conditional value-at-risk, the security risk of transmission network could be evaluated properly.

Key words: transmission network operation, grid security, index system, needs assessment