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A new control strategy for a ground power unit (GPU) is proposed, which is used for airplanes on the ground. In such a system, the ratio between the switching frequency and the fundamental frequency is low and in order to obtain high output performance, the controller has to be digital. The control is performed by a PWM-modulator and a voltage controller in the synchronously rotating reference frame (d-q). Design rules are given and a simulation tool is used for investigation of the controller. A 90 kVA prototype is used for validation of the principle. It is concluded by the experiments that the new controller gives a high-output performance, both stationary and dynamically, at linear and nonlinear loads. Synchronization with the airplane is also shown to work successfully.