Disclosed is a device and method to control the conversion of an arbitrary resource energy into mechanical energy and subsequently into electric energy with a variable-speed generation system utilizing a turbine and a doubly-fed generator, to be connected directly to a power grid of a certain grid frequency. Via electronic control of the rotor speed by controlling the rotor winding excitation frequency, the device has the capability of providing an optimum tradeoff in maximum turbine efficiency and minimal generator losses to secure maximum efficiency of the total conversion process, irrespective of varying resource and electric load conditions. The excitation requirements of the generator are confined to moderate levels and yet a wide rotor-speed variation margin, starting from zero speed, is permitted. An integrated strategy is implemented which effectively coordinates the execution of these tasks and the independent control of the generator terminal voltage as well as the active and reactive electric power output. All signal processing as required correspondingly is carried out without the need for cumbersome measurements of turbine input and output power or generator losses.