Concerning cost, volume, and efficiency, Adjustable Speed Drive (ASD) systems are commonly employed with diode rectifiers or Silicon Controlled Rectifiers (SCR) as the front-ends (i.e., ac-dc converters). Apart from low cost, small volume, and high reliability, harmonic currents are significantly produced at the grid side by the rectification apparatus (i.e., diode rectifiers or SCR). As a simple strategy, the phase-shifted current control can be applied to the SCR-fed drive systems, where the input currents for the SCR are phase-shifted in such a way to cancel out the harmonics of interest (e.g., the 5th order harmonic). However, this solution is effective only when same amounts of currents are drawn by the rectifiers. Unfortunately, in practice, the loading is different among the parallel drive systems, leading to degradation in the harmonic mitigation. In this paper, unequal loading conditions in multi-drive systems are thus addressed. A load-adaptive scheme by means of varying the dc-link voltage is proposed, where a unified dc-link current modulation scheme is also employed in the dc-link. The proposed load-adaptive scheme can ensure that the rectified currents (i.e., rectifier output currents) are equal, and thus the harmonic reduction enabled by the phase-shifted current control is enhanced. The principle of the proposed method is demonstrated on a two-drive system consisting of a diode rectifier and a SCR. Experimental tests have validated the effectiveness of the proposed scheme in terms of harmonic mitigation for multiple parallel ASD systems.
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