Activating the vast potentials of demand response(DR)in loads is currently an important research topic of the smart grid. However

because of the characteristics of mass

diversity and responsiveness of DR resources

it is a huge challenge to incorporating the DR resources in real-time simulation. A digital real-time co-simulation method of the smart grid for integrating large-scale fast DR resources is proposed. First

the network with a large number of DR resources is decoupled from the rest of the grid

which asks for building the multi-rate digital simulation according to different dynamic responses. Then

an interface algorithm including loop control is designed for the co-simulation. After that

typical application scenarios about microgrid and high-voltage grid are analyzed

and constitutions of the digital real-time co-simulation system are given based on RTDS and GridLAB-D. Finally

by taking the large-scale air-conditioning load as an example

the effectiveness of the proposed co-simulation method is verified. By using personal computers

the method is able to extend the simulation ability of existing digital real-time simulation systems for large-scale fast DR resources

which shows the good scalability and economic benefits of the proposed method.