Frequency Domain Analysis of Inductor Saturation in Current Controlled Grid Converters

In this paper the effects of non-linear inductance on the performance of current controllers designed to track periodic signals and/or to compensate periodic disturbances are investigated with a frequency-domain model. When the inductance has a non-linear behavior a distorted current waveform is produced; if the saturation is symmetric, the current spectrum contains only odd harmonics. A current-dependent model of the non-linear inductance has been developed using the Volterra series expansion. The model allows proving formally how harmonic compensation provided by resonant and repetitive controllers can also mitigate the effects of the inductance saturation. This result is the main contribution of the paper and it is also substantiated with experimental evidence. It has been proven that the resonant and repetitive controllers are able to reduce harmonic distortion introduced by non-linearities. Moreover the repetitive controller is able to comply with the harmonic limits reported in IEEE 1547 and IEC 61727 even in very hard saturation conditions.