Flexible DC power grid contains a number of modular multilevel converters and DC breakers

hence fast and accurate models of converter and DC breaker are needed. For the sake of accelerating the simulation of modular multilevel converters

an energy equipartition model is proposed by equal distribution of the power of capacitor to simplify the voltage balancing control and retaining the external characteristic of the bridge arm in blocked and unblocked status. As for the cascaded full-bridge DC breaker

a fast simulation model is developed by simplifying the full-bridge sub-modules in breaker sub-circuits. Electromagnetic transient simulations on PSCAD/EMTDC are carried out to verify the accuracy and acceleration of the proposed model. This work is supported by National Key Research and Development Program of China(No. 2016YFB0900600)and National Natural Science Foundation of China(No. 51577148).