In order to meet the protection requirements of high-capacity medium-voltage（MV） direct-current（DC） power system

an MV DC air circuit breaker has been developed. To overcome the difficulties in large current breaking of MV DC air circuit breaker（ACB）

we performed simulation and experiment researches. Temperature field and flow distribution of the arc chamber during breaking process were obtained by establishing and simulating the three-dimensional magnetic hydro-dynamics（MHD） model of the air arc. The motion of arc was also observed by a high-speed camera in large current breaking

by which the validity of the MHD model was verified. At last

a 5 kV/110 kA DC ACB prototype was developed according to the research results

and a high current breaking test at DC 5 kV/110 kA was carried out. The test results show that the high speed MV DC ACB could break the short circuit current successfully

in which the breaking current is 91.8 kA and the total breaking time is 17 ms.