Stability analysis of a double fed induction generator with the PLL controller

In the paper, a stability analysis of a double fed induction (DFM) generator with a phase locked loop (PLL) controller is presented. The stator of the machine is connected to the grid and the rotor is fed by a voltage source inverter with a current regulated PWM. The machine is controlled from the rotor side. The PLL controller which makes it possible to overcome variables transformation from the stator to the rotor side is introduced in the control system. The control system can work without a shaft encoder. The PLL controller controls the angle between two chosen vectors e.g. the rotor current vector and a stator flux/voltage vector; at the controller output a frequency of the rotor current exists. The machine and the proposed control system are nonlinear. The transfer functions (after linearization) of the open-loop and closed-loop control systems are presented and a stability analysis of the system is investigated. Experimental results of a DSP controlled 30 kW DFM are presented