Variable Stator Frequency Control of Stand-Alone DFIG with Diode Rectified Output

Variable frequency operation of stand-alone doubly fed induction generator (DFIG) is analyzed in this paper, in order to extend the operating speed range of the generator and optimize the power conversion efficiency. In a stand-alone configuration, the stator feeds a three-phase diode rectifier and a stand-alone electrical load. The dc output voltage of the generator is regulated by applying stator flux-oriented control scheme in synchronous reference frame. The losses in the power electronics components depends on the current flowing through the switches and as such also influences the power rating of the converter. In addition, the speed range of the DFIG is also determined by the converter rating. Thus, in the present paper to extend the speed range, for the same converter rating, the slip of the system is limited which can be achieved by setting the stator frequency free. Since the stator voltage amplitude is imposed by the dc-link voltage which is regulated to a constant reference value, a variation in stator flux magnitude results in a frequency variation. Therefore, the stator frequency can be scheduled in such a way that the output power is maximized. After the dc output voltage and inner d-q controller are designed, simulation tests have been run to find the optimal stator frequency for a fixed range of speed and consequently used as a look up table to improve the power conversion efficiency of the generator system for a limited rotor side converter rating (approximately 22%) in a 2.5:1 speed range at constant dc output voltage and power.