Flexible high voltage direct current(HVDC)technology is an effective solution in building DC grid

transmiting large-scale power via long distance and grid connection of renewable energy. Aiming at large-scale renewable energy source grid connection

a flexible DC grid topology for integrating the renewable energy source(such as wind energy and solar energy)is proposed based on multi-terminal HVDC technology. Firstly

a general start-up control strategy for the DC grid is designed. Based on the PSS/E calculation results

a steady-state operating condition for the DC grid is designed. Specifically

DC pole to ground fault

AC fault and large-scale wind turbine tripping off are simulated and analyzed. Finally

the DC power grid model is simulated on the PSCAD/EMTDC platform. The start-up control strategy and the steady-state power flow distribution are verified by the simulation results. Combined with the simulation waveforms

the power grid transient characteristics are discussed. This work is supported by “Thousands of People Plan” Special Support Project of State Grid Corporation of China(No. SGRIzlsdgcjsyjsJS〔2014〕264).