In order to solve the bipolar short-circuit fault problem on the DC transformer medium-voltage DC（MVDC） side of the photovoltaic MVDC collection system

this paper discusses the bipolar short circuit problem at the sending terminal and receiving end of DC cable

respectively. The contributions of the MVDC short circuit current at the fault point are quantitatively analyzed by different fault circuits

which provides a reference for parameter setting of protection devices. Furthermore

the mechanism of DC low-voltage bus overvoltage is analyzed

and an overvoltage active suppression method is proposed to avoid frequent photovoltaic arrays from cutting off from the system. The simulation results show that the current limiting reactor at the MVDC side of the converters can reduce the rise speed of short-circuit current greatly. By changing the operation point of the PV module

the voltage rise of the low-voltage DC bus can be quickly suppressed. The research can provide theoretical and technical supports for the renewable power generation construction of medium-voltage and low-voltage DC distribution network demonstration project.