A real-time control pulse generating method for cycloconverter in fractional frequency transmission system

Fractional frequency transmission system (FFTS) is a new approach for large capacity wind power integration into a power system. The cycloconverter is the key equipment for FFTS. Because of the large capacity of a power system, the harmonics generated by cycloconverter must be suppressed to reduce the costs of filters. Meanwhile, in order to capture more wind power, the frequency of the lower frequency side (LFS) of the system should be changed according to the wind velocity. To satisfy these requirements, a new method, namely multi-pointer method, is proposed. With this method, the time-varying sequences of pulses could be generated in real-time to trip the thyristors of the cycloconverter. This method does not need solving of the triangular equations and can produce output voltage with less total harmonic distortion (THD) than other methods, such as the linear approximation method. Meanwhile, the frequency of LFS could be changed online. The principle, hardware design layout and flow chart of this method are illustrated. The validity of this new method is proved by the experimental results. Furthermore, the hardware circuit and the program of this method are easy to realize, thus make it suitable for industrial applications, especially in FFTS.