DC link faults ride through is one of the most important research objects in modular multilevel converter HVDC system. The HVDC system based on full-bridge sub-module（FBMMC HVDC）can provide counter electromotive force opposite to the current direction after the sub-modules blocked. Therefore it is an ideal topology to fast clearance the DC link faults without opening of the AC circuit breaker. This paper analyzed the sub-module capacitor voltage for FBMMC HVDC at the situation of DC pole-to-pole short circuit fault. The result shows that the module voltage will experience successively drop and uplift. This process is significance for the design of DC fault detection method

the sub module capacitance voltage tolerance

and the sub-module over and under voltage protection value etc. It is also an important factor to the successful implementation of the DC fault ride through.Calculation and simulation show that the results of the analysis result is correct and has a high accuracy.