Abstract: This paper studies the unstable problem of capacitor voltage in the DC-link of STATCOM, and proposes a new control strategy with additional circuits, which are used to control charge and voltage stability. The use of additional circuit multi-winding transformer effectively isolates the direct electrical contact between the additional circuits and the main circuits of devices, and avoids the occurrence of instantaneous zero capacitance voltage when the additional circuits and the main circuits of devices are put into operation concurrently. This method makes the control strategy of additional circuit independent of the control strategy of the main circuit. The additional circuit can either connects to the large power grids, or use independent stand-alone power supply separated from the grid, or the large capacity UPS, which can preferably achieve devices starting, fast power switching and so on. In addition, the dynamic response of the devices can also be enhanced, and the STATCOM capacitor voltage stability and normal reactive output can be ensured at the same time, when the main circuits have larger voltage fluctuations.

Key words: cascade structure, STATCOM, DC voltage, unbalance, control strategy