In this paper, an inner loop for the control and frequency regulation of the doubly fed induction generator connected to a dc link through a diode bridge on the stator is presented. In this system, the stator is directly connected to the dc link using a diode bridge, and the rotor is fed by only a pulse width-modulated (PWM) converter. If compared to the DFIG connected to an ac grid, this system uses one PWM inverter less and a much less expensive diode bridge. Thus, the cost of power electronics is reduced. The application in mind is for dc networks such as dispersed generation grids and microgrids. These networks use several elements that should work together. Usually, these elements are connected with each other by power electronic devices in a common dc link. This paper presents a control system for the inner control loop in order to regulate the torque and the stator frequency. Simulation and experimental results show that the system works properly and is able to keep the stator frequency near the rated value of the machine.
- Dc link
- Dc microgrids
- Doubly fed induction generator
Fig. 1. Structure of the DFIG-DC. Diode bridge on the stator, PWM converter on the rotor.
EXPECTED SIMULATION RESULTS:
Fig. 2. Torque, stator flux, frequency error, and sinδ.
Fig. 3. Stator and rotor currents in closed loop.
Fig. 4. Torque, stator flux, frequency error, and sinδ.
Fig. 5. Response to a voltage dip down to 0.5 p.u.
Fig. 6. Twelve-pulse rectification curves. Six-pulse stator currents, torque,
and equivalent three-phase current using 12 pulse and torque.
This paper presents a control method for the DFIG connected to a dc link through a diode rectifier on the stator windings. Simulation and experimental results show that it is possible to drive the stator flux at the rated frequency of the machine by using a simple controller that adjusts the rotor d-axis current reference in order to annihilate the orientation error. The method converges to the field orientation and the steady-state frequency error is zero.Agood dynamics is achieved in the electromagnetic torque. The waveforms of the stator current are not sinusoidal, due to the presence of the diode bridge, but have an acceptable harmonic content. The industrial application of this system could be implemented using a 12-pulse rectifier, which reduces not only the torque ripple but also the harmonic content in the rotor currents.
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