Abstract: As the interconnection of our country's power system becomes more and more stronger, it is an important problem to make assessment of the reasonable grid construction scale. The purpose of this study is evaluating the construction scale of power grid by calculating the blackout risk under different scales of the grid. From the view of blackout risk, the risk assessment index system and risk assessment models about various wire connection were established in this study to evaluate the rationality of grid construction scale. And the IEEE-14 bus system was used to verify the validity of the model. In the last, the assessment theory and models were applied to a particular ultra high voltage （UHV） synchronous power grid to demonstrate its reasonable construction scale in the blackout risk perspective. Research shows that the number of the large transmission line and the length of the transmission line are in the reasonable range. This method has practical value in engineering.

Key words: reasonable grid construction scale, risk assessment, blackout, various wire connection