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Active rectifiers interfacing the grid have gained much attention in recent years due to their current control capability that enables the application to exchange a certain range of power at the benefit of low harmonic distortion compared to line commutated converters. Moreover, the amount of active and reactive powers can be arbitrarily chosen within certain limits. These limits as well as the degree of harmonic attenuation is determined by the line-side filter. This paper basically focuses on the current control of active rectifiers incorporating a line-side filter of first-up to third-order degree. Thus, it treats both the current-source converter (CSC) as well as the voltage-source converter (VSC). The basic models of the plant are derived for each case, and important dualities are pointed out. As a conclusion, a recipe for the controller design of particularly CSCs with LC filters and VSCs with LCL filters is proposed. Index Terms--AC-DC power conversion, current control, pulsewidth modulation (PWM), voltage control.

Voltage--oriented PI control of three-phase grid-connected pulsewidth-modulation rectifiers with LCL filters is addressed. LCL filters require resonance damping. Active resonance damping is state of the art to face the problem, but it is still under investigation because of the manifold solutions. It is often realized using many sensors and/or complex control algorithms. In contrast, pure PI control requires only one set of current sensors, and its implementation and design are rather simple and well known from the L filter control. PI control has already been shown to be a suitable solution also for LCL filters, but there are limitations. These are investigated in this paper. System stability is analyzed with respect to different ratios of LCL filter resonance and control frequencies. The latter are important parameters for system design and control. Both line and converter current control are analyzed. For a certain range of frequency ratios, the voltage-oriented PI control gives stable performance without additional feedback, but for ratios outside this range, stable operation is impossible. Experimental tests validate the theoretical results. In addition, an experimentally determined LCL filter transfer function is shown in this paper, which shows a lower resonance peak as expected from commonly used filter models. Index Terms--Active damping (AD), grid-connected pulsewidth-modulation (PWM) rectifier, LCL filter, PI current control.

Novel switched link pulse-width-modulated (PWM) current source converters are presented. These converters supply a controlled dc current to the load with a concurrent elimination of selected harmonics in the ac mains. The new topologies permit minimal constraints to be formulated resulting in significantly more efficient PWM patterns than hitherto reported in the literature. A higher number of harmonics are thus eliminated at the mains for a given switching frequency while maintaining a control capability of the output dc current in a wide range. Simulation results confirm the theoretical findings. Also discussed in the paper is the selection of capacitor filters to mitigate the residual harmonics. Index Terms--Amplitude modulation, current source converter, harmonic elimination, pulsewidth modulation.