Signature POWIs for DFIG under LVRT conditions: Analysis, experiments and recommendations

As one of the important characteristics of voltage sags, the point on wave of initiation (POWI) has been widely studied in its influence on sag sensitive devices. However, it is less mentioned in the research and test standards related to the low voltage ride through (LVRT) of doubly fed inductor generators (DFIGs). To fill this gap, a comprehensive study for the influence of POWIs together with the phase angle jump (PAJ) is carried out in this paper. Taking rotor current as the evaluation index, mechanism analysis and experimental verifications are conducted under different asymmetrical sags. The results demonstrate that POWIs affect the LVRT response through the transient dc stator flux and rotor current. The two POWIs with the greatest and lightest impact within half a cycle are defined as the signature POWIs, which depends on the sag type, sag magnitude and PAJ. Analysis under the signature POWIs, especially the most severe POWI, can more accurately evaluate the LVRT response of DFIGs. Besides, the PAJ will have a significant detrimental impact together with the POWI, manifested as the increase of the negative ac and transient dc rotor current. Suggestions for improving the LVRT testing standard are further put forward, that performing tests within the range of ± 15° around the most severe POWI.