The global electrical energy consumption is rising and there is a steady increase of the demand on the power capacity, efficient production, distribution and utilization of energy. The traditional power systems are changing globally, a large number of dispersed generation (DG) units, including both renewable and nonrenewable energy sources such as wind turbines, photovoltaic (PV) generators, fuel cells, small hydro, wave generators, and gas/steam powered combined heat and power stations, are being integrated into power systems at the distribution level. Power electronics, the technology of efficiently processing electric power, play an essential part in the integration of the dispersed generation units for good efficiency and high performance of the power systems. This paper reviews the applications of power electronics in the integration of DG units, in particular, wind power, fuel cells and PV generators.

Power electronics as efficient interface in dispersed power generation systems

Power Electronics as Efficient Interface in Dispersed Power Generation Systems

Multiport DC-DC converters are of potential interest in applications such as generation systems utilising multiple sustainable energy sources. A family of multiport bidirectional DC-DC converters derived from a general topology is presented. The topology shows a combination of DC-link and magnetic coupling. This structure makes use of both methods to interconnect multiple sources without the penalty of extra conversion or additional switches. The resulting converters have the advantage of being simple in topology and have a minimum number of power devices. The proposed general topology and basic cells show several possibilities to construct a multiport converter for particular applications and provide a solution to integrate diverse sources owing to their flexibility in structure. The system features a minimal number of conversion steps, low cost and compact packaging. In addition, the control and power management of the converter by a single digital processor is possible. The centralised control eliminates complicated communication structures that would be necessary in the conventional structure based on separate conversion stages. A control strategy based on classical control theory is proposed, showing a multiple PID-loop structure. The general topology and a set of three-port embodiments are detailed.

/pdf/family-of-multiport-bidirectional-dc-dc-converters-403bgg29nl.pdf

Family of multiport bidirectional DC¿DC converters

Four-leg voltage-source converters can effectively provide the neutral connection in three-phase four-wire systems. They can be used in inverter, rectifier, and active filter applications to handle the neutral current caused by the unbalanced and/or nonlinear load or unbalanced source. In this paper, three-dimensional (3-D) space vector modulation (SVM) schemes are proposed for controlling the four-leg voltage-source converters. Important issues for 3-D SVM, such as definition of 3-D vectors, identification of adjacent switching vectors in the 3-D space, and switching vector sequencing schemes and comparisons are addressed. The proposed 3-D SVM is a superset of the traditional two-dimensional (2-D) SVM, and thus it inherits all the merits of the traditional 2-D SVM. A 100 kW 5 kHz four-leg inverter and a 20 kHz four-leg rectifier prototypes are built and controlled by the proposed 3-D SVM. Experimental results are presented to validate the effectiveness of the 3-D SVM.

Three-dimensional space vector modulation for four-leg voltage-source converters

As fuel cells are likely to be used in many future applications, dedicated power converters must be developed and optimized. A thorough knowledge of the fuel cell operation is thus required for power electronics engineers. This paper proposes a theoretical and experimental study of the behavior of a fuel cell stack subject to current harmonics. The fundamental role of the internal double layer capacitor is demonstrated

Interactions Between Fuel Cells and Power Converters: Influence of Current Harmonics on a Fuel Cell Stack

An efficient dc/dc converter is needed as the interface between a low-voltage fuel cell source and a high-voltage bus for inverter operation. In this paper, a three-phase transformer-isolated dc/dc converter utilizing phase-shift (PS) modulation is proposed. The converter must be able to boost the voltage significantly and operate at current levels above 240 A on the source side. Key features of the proposed converter include reduced transformer turns ratio by a factor of two while maintaining the same output voltage, reduced size of passive components including output filter and input dc bus capacitor using three-phase interleaving, eliminated inductor current ripple at PS angles above 120/spl deg/, and achieved soft switching over a wide load range without auxiliary circuitry. The proposed converter has been analyzed, simulated, and implemented in hardware. An efficiency of above 96% was achieved using the prototype unit. Experimental results were used to verify all designs and analyses.

A novel three-phase high-power soft-switched DC/DC converter for low-voltage fuel cell applications

An efficient DC/DC converter is needed as the interface between a low-voltage fuel cell source and a high-voltage bus for inverter operation. In this paper, a three-phase transformer-isolated DC/DC converter utilizing phase-shift modulation is proposed. The converter must be able to boost the voltage significantly and operate at current levels above 240 amps on the source side. Key features of the proposed converter include: reduced transformer turns-ratio by a factor of two while maintaining the same output voltage, reduced the size of passive components including output filter and input DC bus capacitor using three-phase interleaving, eliminated inductor current ripple at phase-shift angles above 120/spl deg/, and achieved soft switching over a wide load range without auxiliary circuitry. The proposed converter has been analyzed, simulated and implemented in hardware. An efficiency of above 96% was achieved using the prototype unit. Experimental results were used to verify all design and analysis.

/pdf/a-novel-three-phase-high-power-soft-switched-dc-dc-converter-2k5a1qjrhn.pdf

A novel three-phase high-power soft switched DC/DC converter for low voltage fuel cell applications

In this paper, an averaged model for a recently proposed novel three-phase transformer-isolated phase-shift DC/DC converter is derived. Fuel cell static and dynamic characteristics are used for the converter power stage design consideration. The controller is then designed based upon the power stage parameters and system dynamic requirements. The entire system has been designed, simulated, and verified with a 3 kW converter prototype and a commercial 1.2 kW fuel cell. The experimental results match the simulation results very well on both fuel cell source dynamic and step load transient responses.

Modeling and control of a novel six-leg three-phase high-power converter for low voltage fuel cell applications

Due to the long time-constant of the fuel cell, the generated power and load become unbalanced under dynamic conditions, resulting in control difficulty for a front-end DC-DC converter. To maximize the fuel cell output power, a current control is employed; however, the DC bus voltage becomes unregulated. Thus, a voltage conditioner utilizing a battery source is proposed to balance the power and stabilize the DC bus voltage. Control design for the front-end DC-DC converter and voltage conditioner are presented. Based on the design, simulation is performed and the results are shown to verify the control strategy.

Power balance control and voltage conditioning for fuel cell converter with multiple sources

A novel three-dimensional space vector modulation (3-D SVM) along with two different overmodulation correction schemes is proposed for three-phase four-legged converters. Although the overmodulation is desired in motor drive applications for better bus voltage utilization, it can cause voltage distortion, and thus is undesirable in utility interface applications. The overmodulation case may occur during transient conditions. When it happens, the duty cycle will saturate, and the output voltage waveform quality deteriorates. In this paper, the overmodulation surface for a 3D SVM scheme is defined, and the correction methods that keep the system from entering into overmodulation are evaluated with computer simulation.

Changrong Liu

Papers

A novel three-phase high-power soft-switched DC/DC converter for low-voltage fuel cell applications

A novel three-phase high-power soft switched DC/DC converter for low voltage fuel cell applications

Modeling and control of a novel six-leg three-phase high-power converter for low voltage fuel cell applications

Power balance control and voltage conditioning for fuel cell converter with multiple sources

Four-legged converter 3-D SVM scheme over-modulation study