A Single-Phase Four-Switch Rectifier With Significantly Reduced Capacitance

A single-phase four-switch rectifier with considerably reduced capacitance is investigated in this paper. The rectifier consists of one conventional rectification leg and one neutral leg linked with two capacitors that split the dc bus. The ripple energy in the rectifier is diverted into the lower split capacitor so that the voltage across the upper split capacitor, designed to be the dc output voltage, has very small ripples. The voltage across the lower capacitor is designed to have large ripples on purpose so that the total capacitance needed is significantly reduced and highly reliable film capacitors, instead of electrolytic capacitors, can be used. At the same time, the rectification leg is controlled independently from the neutral leg to regulate the input current to achieve unity power factor and also to maintain the dc-bus voltage. Experimental results are presented to validate the performance of the proposed strategy

Control of Ripple Eliminators to Improve the Power Quality of DC Systems and Reduce the Usage of Electrolytic Capacitors

The problem of voltage/current ripples has become a primary power quality issue for DC systems, which could seriously degrade the performance on both the source side and the load side and lead to reliability concerns. In this paper, a single-phase PWM-controlled rectifier is taken as an example to investigate how active control strategies can improve the power quality of DC systems, reduce voltage ripples and, at the same time, reduce the usage of electrolytic capacitors. The concept of ripple eliminators recently proposed in the literature is further developed and the ratio of capacitance reduction is quantified. With such ripple eliminators, this power quality problem is formulated as a control problem to actively divert the ripple current on the DC bus. The main focus of this paper is to investigate how advanced control strategies could improve the performance of ripple eliminators. An advanced controller on the basis of the repetitive control is proposed for one possible implementation of ripple eliminators in the continuous current mode (CCM). Experimental results are presented to verify the effectiveness of the strategy with comparison to another ripple eliminator operated in the discontinuous current mode (DCM). It has been shown that the proposed instantaneous ripple-current diversion in CCM leads to a nearly fourfold improvement of performance

DC-Link Current Harmonic Mitigation via Phase-Shifting of Carrier Waves in Paralleled Inverter Systems

DC-connected parallel inverter systems are gaining popularity in industrial applications. However, such parallel systems generate excess current ripple (harmonics) at the DC-link due to harmonic interactions between the inverters in addition to the harmonics from the PWM switching. These DC-link harmonics cause the failure of fragile components such as DC-link capacitors. This paper proposes an interleaving scheme to minimize the current harmonics induced in the DC-link of such a system. First, the optimal phase-shift angle for the carrier signal is investigated using the analytical equations, which provides maximum capacitor current ripple cancellation (i.e., at the main switching frequency harmonic component). These optimally phase-shifted switching cycles lead to variations of the output current ripples, which, when summed together at the DC-link, result in the cancellations of the DC-link current ripples. The results show that when the carrier waves of the two inverters are phase-shifted by a 90° angle, the maximum high-frequency harmonic ripple cancellation occurs, which reduces the overall root-mean-square (RMS) value of the DC-capacitor current by almost 50%. The outcome of this proposed solution is a cost-effective DC-harmonics mitigating strategy for the industrial designers to practically configure multi-inverter systems, even when most of the drives are not operating at rated power levels. The experimental and simulation results presented in this paper verify the effectiveness of the proposed carrier-based phase-shifting scheme for two different configurations of common DC connected multi-converter systems

An approach to harmonic current-free AC/DC power conversion for large industrial loads: the integration of a series active filter with a double-series diode rectifier

This paper proposes a new harmonic current-free AC/DC power conversion system characterized by the integration of a small-rated series active filter, with a large-rated double-series diode rectifier. The DC terminals of the active filter are directly connected in parallel with those of the diode rectifier, thereby forming a common DC bus. The active filter enables the diode rectifier to draw three-phase sinusoidal currents from the utility. In addition, it can provide the supplementary value-added function of regulating the common DC-bus voltage to a limited extent of ±5%, slightly increasing the RMS voltage rating, but not increasing the peak voltage rating. Experimental results obtained from a 5 kW laboratory system verify the practical viability and cost effectiveness of the proposed system </p

Active Control of Voltage Ripples in Power Electronic Converters

Two major challenges, i.e., bulky electrolytic capacitors and isolation transformers, remain as critical obstacles for further improvement on reliability, power density and efficiency of power electronic converters, which are mainly used to reduce low-frequency voltage ripples and high-frequency common-mode voltage ripples, respectively. In order to overcome the two challenges, the most straightforward way is to simply combine existing solutions developed for each of them. However, this would considerably increase system complexity and cost, which should be avoided if possible. In this thesis, these two challenges are innovatively addressed in a holistic way by using active control techniques. This thesis first focuses on the reduction of low-frequency voltage ripples in conventional half-bridge converters, after adding an actively-controlled neutral leg. As a direct application of this strategy, a single-phase to three-phase conversion is then proposed. After that, a ρ-converter with only four switches is proposed to significantly reduce both low-frequency ripples and high-frequency common-mode ripples in a holistic way. It is found that the total capacitance can be reduced by more than 70 times compared to that in conventional full-bridge converters. As a result, there is no longer a need to use bulky electrolytic capacitors and isolation transformers. Then, the ρ-converter equipped with the synchronverter technology is operated as an inverter for PV applications. Another converter is also proposed for the same purpose but with reduced voltage stress. In order to further reduce the total capacitance and to reduce the neutral inductor in the ρ-converter, a new type of converter, called the θ-converter, is proposed. Finally, two actively-controlled ripple eliminators are proposed to reduce low-frequency ripples in general DC systems while the aforementioned research is focused on some specific topologies. Extensive experimental results are presented to validate most of the developed systems while the rest are validated with simulation results

Reducing the inductors of rectifiers having two outputs to improve power density

In this paper, a recently reported single-phase rectifier with two outputs (RECTO) is improved to reduce the neutral inductor and the grid inductor, by moving the neutral inductor away from the path of the grid current. The neutral inductor does not carry the grid current any more so the current stress of the neutral inductor can be significantly reduced, and the size of the inductor becomes much smaller. In theory, the current stress can be reduced by at least three times and the size of the inductor can be reduced by nine times. At the same time, the grid inductor can be reduced to achieve the same level of grid-current switching ripples because of the changed operation modes and modulation strategy. Together, the reduced neutral and grid inductors help improve the system power density. It is worth noting that the voltage and current stresses of the switches and the other features of the RECTO, e.g., two dc outputs and unity power factor, are not affected. Comparative experimental results are presented to demonstrate the reduction. If the two load currents are the same then the neutral inductor is only required to handle the switching ripples. This improved RECTO is particularly suitable for applications with two balanced loads without increasing the cost much

Integration of an Active Filter and a Single-Phase AC/DC Converter with Reduced Capacitance Requirement and Component Count

Existing methods of incorporating an active filter into an AC/DC converter for eliminating electrolytic capacitors usually require extra power switches. This inevitably leads to an increased system cost and degraded energy efficiency. In this paper, a concept of active-filter integration for single-phase AC/DC converters is reported. The resultant converters can provide simultaneous functions of power factor correction, DC voltage regulation, and active power decoupling for mitigating the low-frequency DC voltage ripple, without an electrolytic capacitor and extra power switch. To complement the operation, two closed-loop voltage-ripple-based reference generation methods are developed for controlling the energy storage components to achieve active power decoupling. Both simulation and experiment have confirmed the eligibility of the proposed concept and control methods in a 210-W rectification system comprising an H-bridge converter with a half-bridge active filter. Interestingly, the end converters (Type I and Type II) can be readily available using a conventional H-bridge converter with minor hardware modification. A stable DC output with merely 1.1% ripple is realized with two 50-μF film capacitors. For the same ripple performance, a 900-μF capacitor is required in conventional converters without an active filter. Moreover, it is found out that the active-filter integration concept might even improve the efficiency performance of the end converters as compared with the original AC/DC converter without integration

A plug-and-play ripple mitigation approach for DC-links in hybrid systems

© 2016 IEEE.In this paper, a plug-and-play ripple mitigation technique is proposed. It requires only the sensing of the DC-link voltage and can operate fully independently to remove the low-frequency voltage ripple. The proposed technique is nonintrusive to the existing hardware and enables hot-swap operation without disrupting the normal functionality of the existing power system. It is user-friendly, modular and suitable for plug-and-play operation. The experimental results demonstrate the effectiveness of the ripple-mitigation capability of the proposed device. The DC-link voltage ripple in a 110 W miniature hybrid system comprising an AC/DC converter and two resistive loads is shown to be significantly reduced from 61 V to only 3.3 V. Moreover, it is shown that with the proposed device, the system reliability has been improved by alleviating the components' thermal stresses