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A method of automatic frequency ratio control (AFC) by a dc system is suggested. This control also provides a tie line bias control action (TBC) in acÂ¿dc systems, and can cooperate with the AFC and TBC in ac systems. The operating principle is that the frequency deviations of two ac systems are detected and its controller adjusts the dc power for constant frequency control. A logic circuit is provided in the controller to give the signals to regulate the governors of the generators. By the proper adjustment of the sensitivity of dc power, the dead band of frequency deviations, and the regulation sensitivity of the governors, the frequency control in the acÂ¿dc system can be acheived quite satisfactorily.