Reliable operation of the power transformer is affected by the health the transformer is in

environment

state of operation

meterological environment

etc. These factors make the operation condition of power transformers more stochastic and complicated. However

current models cannot effectively quantify the impact of different factors on the forced outage process. A novel time-varying forced outage rate(FOR)model is proposed based on the proportional hazard model and semi-Markov process. The proposed model could integrate different factors and consider the stochastic process of the influencing factors themselves. To overcome the dilemma of evaluating reliability

Monte Carlo technique is introduced to evaluate reliability indices with the proposed FOR model. Numerical examples show the rationality of the forced outage model

and the effectiveness of the reliability evaluation method.