Marcel A. M. Hendrix

Three-Port Bidirectional Converter for Hybrid Fuel Cell Systems

The implementation of a hybrid fuel cell/battery system is proposed to improve the slow transient response of a fuel cell stack. This system can be used for an autonomous device with quick load variations. A suitable three-port, galvanic isolated, bidirectional power converter is proposed to control the power flow. An energy management method for the proposed three-port circuit is analyzed and implemented. Measurements from a 500-W laboratory prototype are presented to demonstrate the validity of the approach

Multi-input bidirectional DC-DC converter combining DC-link and magnetic-coupling for fuel cell systems

This paper presents a new multi-input bidirectional dc-dc converter which connects a fuel cell, storage and load by a combination of a dc-link and magnetic-coupling. A boost-dual-half-bridge and a bidirectional direct-connected switching cell are used. The topology is simple and only needs six power switches. The load and the sources are galvanically isolated. Furthermore, the proposed converter draws/injects smooth current from the fuel cell and the supercapacitor. The system is suitable for medium-power applications where simple topology, autonomous operation, compact packaging, and low cost are required. Different control schemes to manage power flow are proposed and compared. Simulation and experimental results that verify the effectiveness of the topology and its control scheme are presented. Moreover, it is shown that the idea of combining the dc-link and magnetic-coupling can be further developed to a general topology for multi-port bidirectional dc-dc converters.

Predictive DC voltage control of single-phase PV inverters with small DC link capacitance

This paper proposes a novel control method for DC voltage in single-phase voltage-source inverters fed by constant-current or constant-power sources. The technique predicts the inverter power require to correct a DC voltage error within one fundamental AC cycle. This is based on the power balance between DC input and AC output, and the energy stored in the DC link capacitor. The fast response means a smaller capacitor can be used, and operation with a large double-line-frequency ripple on the DC bus is possible without causing distortion of the AC output current. This scheme is suitable for applications where substantial DC link buffer energy is unnecessary, e.g.: grid-connected photovoltaic generators. The reliability and lifetime of the inverter can be significantly improved if a non-electrolytic type capacitor is used. Experimental results are presented that verify the inverter operation.

A Soft-Switched Three-Port Bidirectional Converter for Fuel Cell and Supercapacitor Applications

This paper presents a zero-voltage-switching (ZVS) three-port bidirectional converter for fuel cell and supercapacitor applications. A simple and effective method to extend the soft-switching range is proposed, especially for the three-port active-bridge topology. By continuously adjusting the duty cycle on the supercapacitor side H-bridge according to the supercapacitor voltage level, soft-switching conditions are achieved over the full operating range due to the equivalent volt-second products applied to the transformer over half the switching period. Detailed analysis is provided for both the two-port and the three-port converters. Furthermore, a dual PI-loop based control strategy is proposed to achieve constant output voltage and precise power flow management, as well as soft-switching. The converter is implemented and tested

A predictive control scheme for DC voltage and AC current in grid-connected photovoltaic inverters with minimum DC link capacitance

This paper describes a discrete time control algorithm for regulation of both DC voltage and AC currents in three-phase inverters. It is suitable for application in grid-connected photovoltaic (PV) generation plants due to the intrinsic ability to perform maximum power point tracking (MPPT) of the PV array. Furthermore, the transient response time is of the order of a few PWM sampling periods, therefore the energy storage requirements of the DC capacitor are significantly reduced, and a small nonelectrolytic type can be employed. Considering that the DC buffer capacitor of an inverter is often the limiting factor in terms of reliability, the proposed approach can lead to a substantial improvement in the way the entire system is perceived. The technique has similar harmonic performance to regularly sampled PWM, with a fixed switching frequency and low current distortion, and operates at unity power factor. This predictive method avoids problems of stability and loop interactions present in controllers employing cascaded DC voltage and AC current feedback loops. The theory of the algorithm is presented in this paper, together with simulation results.

Triple-half-bridge bidirectional converter controlled by phase shift and PWM

This paper presents a transformer-coupled three-port triple-half-bridge bidirectional dc-dc converter controlled by phase shift in combination with pulse-width-modulation (PWM). A typical application of the proposed converter is a fuel cell system with a supercapacitor/battery as the storage. The boost-half-bridge that connects to the supercapacitor plays a key role in accommodating the wide operating voltage of the supercapacitor. With this technique, the zero-voltage-switching (ZVS) operating range is extended to the entire phase shift region. Furthermore, both the current stress and the conduction losses of the power switches are reduced compared to other methods. In addition, a control scheme employing multiple PI regulators for this converter is presented. A prototype has been built. Simulation and experimental results show the effectiveness of the proposed converter and the control scheme.

Dynamic Characteristics of PEM Fuel Cells

Fuel cell applications have become increasingly attractive. Therefore, comprehensive models, simulation and analysis tools are required to characterize fuel cell behavior. For years, the focus has been on describing the steady state characteristics of fuel cells, which is an important application but certainly not the only one. This paper presents methods to measure steady state and transient behavior of polymer electrolyte membrane fuel cells (PEMFC). An active load that enables all required measurements is described. Furthermore, a simple large-signal dynamic PEM fuel cell model is introduced of which the parameters can be found with an interrupted current method. This method estimates the parameters of the dynamic model. However, there was doubt whether the influence of small-signal disturbances in the fuel cell operation could be analyzed with the interrupted current method. Therefore, impedance spectroscopy measurements were performed on the fuel cell stack to determine if the large-scale model was applicable to small-signal excitations. Practical measurements show that an additional small-signal model should be introduced to characterize a fuel cell stack when switching ripple is present in the output current

Optimal design of a pot core rotating transformer

This paper discusses the optimal design of a pot core rotating transformer to replace wires and slip rings in mechatronic systems by means of contactless energy transfer. Analytic models of the transformer are derived in the electromagnetic and thermal discipline. The models are compared with both 2D/3D FEM simulations and measurements. The analytical models are combined and used in a multi-objective sequential quadratic programming algorithm to find the minimal Pareto front in terms of volume and power loss for comparison of the adjacent and coaxial winding topologies. Finally, the optimization algorithm is used for the design of two prototype rotating transformers for a power transfer of 1kW peak, rotating at 4000 rpm. The prototypes are manufactured and tested in an experimental setup.

Islanding behaviour of grid-connected PV inverters operating under different control schemes

This paper compares the islanding behaviour of different inverter implementations, which have current source or voltage source characteristics. Measurements were made on a number of commercially available inverters, and simulation models were developed to enable further investigation of the influence of the type of control system on islanding operation. Additionally, a modified active-frequency-drift detection method is presented which reduces the amount of distortion introduced to the grid compared with the conventional approach often described in the literature.

A Distributed Fuel Cell Based Generation and Compensation System to Improve Power Quality

A small single-phase fuel cell based energy generation and compensation system is proposed in this paper. The power conditioning unit of the system comprises a grid-interfacing inverter and a three-port bidirectional converter which connects a fuel cell and supercapacitor to the inverter. The system can operate in both island and grid-connected modes. By taking advantage of the transient storage capability offered by the supercapacitor, the function of reactive power compensation is integrated into the system. Simultaneously, the inverter is operated as a shunt active power filter and, by means of a proposed control strategy, compensates for reactive and harmonic current demanded by local loads. The system is suitable for residential applications and can improve the quality of a weak power grid. Simulation and experimental results show the validity and feasibility of the proposed system