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Power system operation risk assessment can comprehensively take into account the occurrence possibility and severity of disturbances, and it is an effective complement to traditional deterministic security analysis. Calculating transient state probability of components in future short time duration is one of the key issues in power system operation risk assessment. In this paper, a novel probability distribution named 'log-normal distribution' was presented for repair time. The shape of probability density of log-normal distribution can describe the distribution characteristics of component repair times well. The numerical test result using real-life data showed that the log-normal distribution matched the real-life data well, and calculation of the transient state probability was very easy. Then, two different time-varying component outage models for overhead transmission line described by Markov model and transformer described by non-Markov mode1 are developed respectively. Subsequently, the utility function is used to measure the degree of dissatisfaction induced by fault and the utility theory based risk indices of overload, low voltage, voltage collapse and angle instability are given. The proposed risk indices can sensitively reflect the trends of risk indices and accord with the actual power system operation. Based on the above research, the practical result shows that this algorithm is easy to be realized and the accuracy and practicality have been are verified.