The DC fault characteristics of voltage source converter based high voltage direct current (VSC-HVDC)systems are analyzed in this paper. The phenomenon whereby the capacitor on DC side discharges quickly during a DC fault contributes to a large short-circuit faultcurrent. Neither traditional DC breakers nor DC switches can cut off the fault current under this condition. A fast solid state DC breaker design method is proposed in thispaper. This method is based on the fault current charac-teristics of the inverter in multi-terminal HVDC systems(MTDC), where a fault current appears at the natural zero-crossing point near the inverter. At this point, by coordi-nating the AC breakers near the rectifier, the DC breakercould reliably cut off the DC fault current and protect thesystem. A detailed model for this fast solid state DCbreaker and its operation sequence are studied, based on this design method. Simulations modeling a five-terminalmeshed DC grid and a fast DC breaker were carried outwith PSCAD/EMTDC using this design method. There sults from the simulations confirmed the validity of thedesign method.