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Origins of stator current spectra in DFIGs with winding faults and excitation asymmetries

In this paper the authors derive simple expressions for the frequencies of the harmonic components in the steady state stator line current of a DFIG operating under various conditions of supply and/or winding unbalance. The underlying purpose of the work is to identify signature frequencies which may be used in condition monitoring instrumentation to both identify and discriminate between a range of likely faults that may arise in practice. The analytical expressions are verified first by comparison with the spectra calculated using an advanced time-stepped circuit model, and then by comparison with measurements made on an experimental test rig.