Abstract
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For power electronic converters-based variable speed doubly-fed-induction-generator (DFIG), the operation near the synchronous speed (usually a very low slip) can cause the rotor side converter to become significantly stressed due to the high temperature fluctuations which can lead to a reduced reliability over time. In this paper, the semiconductor junction temperatures and power losses have been quantified. The thermal stress can be reduced by decreasing the switching frequency. But a low switching frequency can cause harmonics and torsional vibration. A switching control technique is proposed in this paper to deal with the tradeoff between the pulse width modulation (PWM) harmonics and semiconductor junction temperature. A reference point is determined so that the parameters e.g. the junction temperature and the total harmonic distortion (THD) follows this reference point. In this paper, limited reactive power support is considered to reduce the rotor current amplitude and improve the temperature fluctuations is by limiting reactive power support from the DFIG. This paper also presents a coordinated control scheme with switching control and rotor current derating to effectively reduce the conductor thermal stress without compromising its performance reliability.