Doctorandus/a PhD student

Naam: Kristof Engelen

Partners

KU Leuven

Promotor / Supervisor

Prof. dr. ir. Johan Driesen

Prof. dr. ir. Geert Deconinck

Samenvatting van het onderzoek / Summary of Research

Over the years, a steady increase in the use of grid coupled power electronic converters is observed. Many converters are connected to one phase of the grid, and many contain some kind of DC/DC converter stage. Some applications where these converters are used are small-scale renewable energy conversion, electric vehicle charging and discharging, and small-scale energy storage. A common building block of this type of converter is the half-bridge. This is a standard switching cell consisting of two switches and two anti-parallel diodes. In order to ensure correct operation of a half-bridge, dead-time has to be used in the switching signals.

In this work, the effects of dead-time on the operation of a half-bridge DC/DC converter are studied extensively. It has been observed that dead-time causes discontinuous conduction in certain operating regions. In these discontinuous regions, the behaviour of the converter differs significantly from the ideal theoretical behaviour. The converter current is a function of the duty cycle whereas outside of the discontinuous regions, the duty cycle affects the change of current. A formula is given for calculating the average current in discontinuous conduction mode as function of the duty cycle. The presence of dead-time disturbs the normal operation of a standard PI-controller. Therefore, a gain scheduling controller with duty cycle presetting is proposed. This control approach vastly improves the response of the current controller to step inputs.

In order to improve the attenuation of the switching harmonics, LCL filters are increasingly used instead of L filters. The control challenges arising from this transition are investigated. Specifically, a simple but effective method for active damping is studied. The issue of dead-time is revisited for this situation and controller presetting is used to improve the response of the controller to step inputs.

The inductance variation of the filter inductors is taken into account during the design of the current controller of a DC/DC converter with LCL filter.

Normally, when interfacing with a single phase of the utility grid, a large DC-bus capacitor is used to absorb the pulsating power flow at twice the grid frequency, thus minimizing the resulting DC-bus voltage ripple. The required DC-bus capacitance can be reduced by adding a ripple port that diverts the ripple power to an auxiliary capacitor. The voltage swing of this capacitor is much larger than that of the DC-bus capacitor, and is therefore much better utilized.

The auxiliary circuit contains an inductance and it is therefore important that the switching frequency of the ripple port is sufficiently high to limit the added volume, so that the overall volume of the auxiliary circuit together with the DC-bus capacitor is still smaller than that of the original DC-bus capacitor. It then becomes possible to replace the standard electrolytic capacitors with film capacitors that are less energy dense but have a longer life expectancy.

A resonant controller is designed that acts on the measured DC-bus voltage ripple and controls the current flowing in the auxiliary circuit. The effectiveness of the controller is verified both in simulation and in practice. Attention is given to the dynamical aspects of the system. It is found that when the power transients on the DC-bus are more gradual, the DC-bus capacitor can be reduced further.

Volledige tekst van het doctoraat / full text

Examencommissie / Board of examiners

Prof. dr. ir. Johan Driesen (promotor) 
Prof. dr. ir. Geert Deconinck (promotor) 
Prof. dr. Bart Demoen (voorzitter/chairman) 
Prof. dr. ir. Ruth Vazquez Sabariego (secretaris/secretary) 
Prof. dr. ir. Ronnie Belmans 
Prof. dr. ir. Joseph Vandewalle 
Dr. Johan Gyselinck , Universié Libre de Bruxelles (ULB)