The rationality of the power grid planning scheme needs to be judged by comprehensively evaluating its function

economy

reliability

flexibility

adaptability

and so on.On the basis of analysis and summary of existing research achievements about UHV grid-frame performance

we further analyzed the function demand of ultra-high voltage（UHV） grid and differences in function and reliability of different grid-frame.Morever

we discussed the necessity

feasibility

and rationality of development of UHV AC/DC power grid in China as well as the supply capacity adequacy of different grid-frame in the aspect of system reliability.In terms of the complexity of system security

the conditions of occurring cascading failures and the complexity of power system blackout were discussed.Calculation result shows that if a receiving-end power system can only transmit by 20 ultra-high voltage/extra-high voltage（UHV/EHV） DC transmission lines（7 circuits for hydropower & 13 circuits for thermal power transmission） to accept massive power from distant places

the seasonal hydropower reduction and the outages of some dc transmission lines can directly cause the loss of load probability of the system to reach 0.152.It is pointed out that the formation of UHV AC/DC transmission grid with strong frame is more reliable in meeting the performances of transmission demand in China.In terms of system security complexity

cascading failure conditions and the complexity of power system blackouts are expounded.It is pointed out that the defect of power system security defense system is an important reason leading to large-scale power system blackout.While constructing the UHV AC/DC power grid

it is necessary to establish a new generation of power system security monitoring system to improve the security and defense system of large-scale power grid.Finally

a conclusion is drawn that large-scale UHV AC/DC transmission network should be built in China to meet the requirements of power collection

transmission

and distribution in wide range and with large capacity.The research results can provide references for the multi-attribute comprehensive evaluation of UHV power grid planning.