Abstract: It is meaningful for the electric power grid company to reduce the electric power purchase risk costs through optimizing purchase quantities of power and electric reserve,so as to reduce the customers' electric usage costs,The purchase price in these two markets is regarded as the risk factors in this paper,and because the Mean Variance may result to unreasonable decision for its enlarging the risks scope,the Mean Semi-Variance is used to describe the risk purchase problem,and the basic optimal purchase model is given out.Based on this,fuzzy membership is applied to represent the experts' uncertain realization of resonable reseve rate,and then the Expansion Principle is used to infer the membership of the optimal purchase strategies,the core method is also used to defuzzify it so as to obtain the final optimal purchase strategies.The model integrated the expert qualitative understanding and quantitative analysis technique,which provides a new solution to the power purchase problem,and these can be properly reflected in the example.

Key words: electricity market, reserve capacity market, risk purchase strategy, expansion principle