A new generator control unit (GCU) must be designed to work in a wide frequency range in order to obtain better control performance in variable frequency AC power system. Control algorithms make improvements in terms of determining the control parameters and analyze the influence of the controller parameters on system stability, and compensate for the loss of signals generated in the conditioning process. This paper compares the pros and cons of different RMS algorithms and determine the final RMS calculation scheme. Test results show a considerable performance improvement for the proposed design in dynamic performance and adjustment precision.