Sub-module redundant fault-tolerant scheme of delta-connected cascaded static synchronous compensator(STATCOM)is mainly used to improve the DC voltage loop and modulation algorithm. However it lacks the research of the transient modulation ratio and transfer function. A new kind of dynamic virtual impedance technology is proposed to improve the transient response. Firstly

it is pointed out that the DC voltage instruction uplift process is divided into the transient adjustment and the steady state of the modulation ratio

and the system transient transfer function is analyzed. Secondly

the dynamic modulation ratio is obtained by the real-time tracking of the DC voltage's abrupt change

and the feed-forward virtual impedance is used to improve the current loop. The DC voltage loop is improved by the parameters' dynamical adjustment of the proportional-integral controller. The transient transfer function is remained stable by compensating the disturbance of the fault process in real time. The feasibility of virtual impedance technology is demonstrated by the bode diagram. Finally

the 35 kV STATCOM model is established by PSCAD. The new scheme has improved the transient current and the power response of the device.