Undisturbed grid connection control strategy for two-terminal flexible low-frequency transmission system

Low-frequency transmission system (LFTS) based on modular multilevel matrix converter (M3C) has great application potential in offshore wind power transmission,urban power grid and long-distance power transmission. In order to study the startup process and the control strategies of LFTS and connect the converter station to the low-frequency transmission line without voltage or current impulse of M3C,a startup method of undisturbed grid connection is proposed. Firstly,the structure and operating principle of the LFTS are analyzed. Then,the selection principle of sub modules and the charging process of M3C are analyzed with two-terminal LFTS as the target. Taking the advantage that the blocked M3C does not exchange power with the low-frequency power grid when the capacitor is at rated voltage,the problems of over-voltage and over-current that tend to occur during LFTS startup can be solved by flexible control of M3C and reasonable design of startup strategy,without additional control devices and detecting the synchronization of the breaker. Finally,a real time digital simulation system (RTDS) is built to verify the proposed startup method. The simulation results show that converter stations connect to the low-frequency transmission line without voltage or current impulse. Also,the proposed strategy is correct and has a great prospect of the engineering application.