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This paper presents an active front-end converter for medium-voltage current-source drives. It comprises two series-connected pulsewidth-modulation (PWM) rectifiers fed from a delta-wye isolation transformer. These employ sequential-sampling synchronous space-vector modulation (SVM), and operate at a switching frequency of 100 Hz, thus reducing the switching losses by 66% compared to previous PWM approaches. Digital processing requirements of this technique are minimized by using decision-making SVM. This online modulation technique ensures full harmonic current cancellation through the magnetic interface, and minimizes the sampling/control-action delay. Therefore it enables the use of advanced control strategies notwithstanding the low sampling frequency of the control system. Exploiting this fact, the proposed converter is controlled using a nonlinear strategy in the synchronous frame. The strategy employs input-output and feedback linearization, and is derived from the complex-signal flow diagram of the converter. The proposed strategy linearizes and partially decouples the converter d-q-axes dynamics, ensuring a dynamic response totally independent from the operating point, even under regeneration. This is a highly desirable feature for high-performance drives presenting continuous transitions from motoring to regeneration. Moreover, the converter presents an independent active and reactive power flow, which enables it to operate as a reactive power compensator if desired. Computer analysis and experimental work with a TMS320C32 digital-signal-processor-based 5-kVA laboratory prototype validate the excellent results attained by the proposed converter.
Date of Publication: Dec. 2003