Digital current-control schemes

IEEE Industrial Electronics Magazine (Impact Factor: 3.76). 04/2009; DOI: 10.1109/MIE.2009.931894
Source: IEEE Xplore

ABSTRACT The paper is about comparing the performance of digital signal processor-based current controllers for three-phase active power filters.

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    ABSTRACT: Low-order odd harmonics arise in practical multiphase drives because of the machine and converter nonlinear behavior (e.g., deadtime and saturation). If the windings are distributed, some of the harmonics cause torque ripple, while others cause losses. The latter is aggravated by the small impedance in the no-torque subspaces. The harmonics can be compensated without steady-state error by proportional-integral controllers in multiple synchronous reference frames (SRFs); however, a heavy computational load is required. In three-phase systems, the computational burden of this multiple SRF (MRF) scheme is often avoided by implementing instead resonant controllers (RCs) tuned at the harmonics that are multiples of six in an SRF rotating with the fundamental frequency. A similar structure has been proved to be satisfactory for deadtime compensation in an asymmetrical six-phase machine. Part 1 of this paper extends that harmonic current control strategy to symmetrical machines of any phase number. The optimum frequencies for the SRFs and for the RCs in each plane, so that the total number of RCs is minimized, are assessed. Part 2 studies the computational load and compares it with that of the MRF scheme.
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    ABSTRACT: Using the vector space decomposition approach, the currents in a multiphase machine with distributed winding can be decoupled into the flux and torque producing α-β components, and the loss-producing x-y and zero-sequence components. While the control of α-β currents is crucial for flux and torque regulation, control of x-y currents is important for machine/converter asymmetry and dead-time effect compensation. In this paper, an attempt is made to provide a physically meaningful insight into current control of a six-phase machine, by showing that the fictitious x-y currents can be physically interpreted as the circulating currents between the two three-phase windings. Using this interpretation, the characteristics of x-y currents due to the machine/converter asymmetry can be analyzed. The use of different types of x-y current controllers for asymmetry compensation and suppression of dead-time-induced harmonics is then discussed. Experimental results are provided throughout the paper, to underpin the theoretical considerations, using tests on a prototype asymmetrical six-phase induction machine.
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