This paper focuses on the cascaded static synchronous compensator(STATCOM)with energy gained from the DC side

and does researches on the stability of DC voltage when STATCOM is in pre-charge state. The reason that the DC voltage would diffuse if no precaution is taken is analyzed. Then

three methods of shunt impedance are summed up

viz. shunt resistors on DC side

shunt resistors on AC side and shunt capacitors on AC side. The mechanism of balancing the DC voltage is analyzed in detail

and parameter design procedures are given for every method by formula derivation. Simplified formulas are also given to make calculation easier. At last

experiment is done to validate the theoretical analysis through low voltage experiments of four modules. This work is supported by National Natural Science Foundation of China(No. 51577088)and Joint Research of Industry and Academia of Jiangsu Province(No. BY2015003-008).