M.Sc. / Please refer to full text to view abstract

M.Ing. / Inverters are daily used in industry, and to a large extent, as induction motor drives. Power levels of inverters vary from a few watt to several megawatt. The most common part of inverters is a phase arm or a pole, consisting of two power electronic switches and two freewheeling diodes. The hard switched phase arm is the standard in industry due to simplicity and cost. Regenerative snubbing is often used at larger power levels to reduce losses in the inverter. Some aspects of the non-linear resonant pole inverter, such as regenerative snubbing and zero voltage switching, are investigated. It is obtained by utilising a non-linear resonant tank. Low current stresses permit the optimal use of the power electronic switches. The non-linear resonant pole inverter can replace the hard switched inverter, with no restriction with respect to power levels. Experimental and analytical modelling was employed to investigate some aspects, such as controllability, losses and feasibility.

M.Ing. / A constant demand exists for ever decreasing size in switch mode supplies. The first step has been the introduction of resonant mode converters. Such converters typically consist of a resonant tank, a transformer and an input or output filter. The soft-switching characteristics of these converters allow an order of magnitude higher frequency, thus reducing the size of the reactive components. The next logical step towards a smaller package is introduced, namely the electromagnetic integration of the resonant tank and, if possible, the transformer into a single component, which; not only saves mass and volume, but can also reduce manufacturing costs. The particular converter investigated is the well known series resonant converter. It is shown that the necessary capacitance can be achieved by using a bifilar primary and the leakage inductance of the transformer replaces the physical inductor. Simulation of a suitable distributed circuit network indicates the same frequency domain characteristics and time domain waveforms for both the integrated component, and the discrete inductor, capacitor an transformer in series. Possible configurations for the integrated LCT-component are proposed, and theoretical analysis predicts an operating frequency in the MHZ region. Notwithstanding the complicated manufacturing, results show An integrated LCT-component, applied in a prototype 1 MHz power supply, with an efficiency of approximately 90 %. If manufacturing of the LCT-component allows an acceptable dimension and an acceptable resonant frequency, this can be a very competitive technology.

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This thesis presents new control approaches for improving the performance, stability, and efficiency of a single-phase grid-connected voltage source inverter (VSI) with an LCL filter that is used in renewable energy power conditioning applications. There are two main controllers that need to be designed: an external DC-bus voltage controller to balance the power flow coming into the VSI, and an internal current controller to control the current injected by the VSI into the utility grid. This thesis aims to find well-tailored control approaches for the aforementioned control loops. First, the stability and behavioral characteristics of the open-loop VSI with an LCL filter are explored using a Poincaré map, and the open-loop system is found to have marginal stability. A current control method is proposed, called composite nonlinear feedback (CNF), which offers significantly improved overall performance compared to the state-of-the-art proportional resonant (PR) controller with state feedback. To reduce the overall number of sensors in the system, two different observers are implemented to estimate the VSI state variables: (1) the Luenberger observer (LO), and (2) the sliding mode observer (SMO). To balance the system power flow, a new DC-bus voltage droop control method is proposed, that provides fast performance during transients. This control approach includes a novel discrete DC-bus voltage sensing technique, which effectively removes the double frequency ripple from the DC-bus voltage signal and prevents it from propagating into the current control loop. A variant on the DC-bus voltage droop control method, called adaptive droop control is proposed, which adaptively changes the droop gains in order to regulate the DC-bus voltage to a constant value. Finally, another variant on the DC-bus voltage droop control method is proposed, called optimized adaptive droop control, which adaptively changes the gains of the controller in order to minimize the overall system power losses. A stability analysis is conducted using the singular perturbation control theory, which allows a nonlinear dynamical system to be broken down into subsystems with different time scales. The results of the stability analysis confirm that the proposed closed-loop grid-connected VSI with an LCL filter is locally stable. / Thesis (Ph.D, Electrical & Computer Engineering) -- Queen's University, 2013-09-30 13:47:56.337

D.Ing. (Electrical and Electronic Engineering) / Conventional power electronic circuits consist of discrete semiconductor devices, tronsformers and reactive components, interconnected according to a circuit diagram. However, as the switching frequencies increase and the weight and volume constraints become more stringent, this traditional approach becomes very limiting. Silicon based IC technology and its high voltage version, HVIC, have made more compact converters possible, but the major reactive elements, such as inductors, capacitors and transformers. must still be constructed and connected discretely ...

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This thesis explores some new applications in power electronics for supercapacitors. This involves the design and development of dc-dc converters to interface the supercapacitor banks with the rest of the power electronic system. Two applications for supercapacitors are proposed and analyzed. The first application is aimed at high power applications such as motor drives. The proposed approach compensates the effect of voltage sags in the dc link of typical adjustable speed drives, thus reducing speed fluctuations in the motor and eliminating the possibility of nuisance tripping on the drive control board. The second approach presented in this thesis explores the use of supercapacitors to extend run-time for mobile devices such as laptop computers and hand held devices. Three possible approaches are explored: a) Supercapacitors connected directly across the battery; b) Battery-inductor-supercapacitor connection; and c) Supercapacitor, and battery connected via a DC-DC converter. Analytical models, simulation and experimental results on a typical laptop computer are presented.

This thesis presents a new family of converters for high power interconnection of dc buses with different voltage levels. Proposed converters achieve high voltage dc-dc conversion without an intermediate ac conversion stage. This function is implemented without series connection of active switches, or the use of isolation transformers. The salient features of proposed converters are (i) design and construction simplicity, (ii) low switching losses through soft turn-on and soft turn-off, (iii) single stage dc-dc conversion without high-current chopping, (iv) modular structure, (v) equal voltage sharing among the converter modules. Three converter circuits are investigated. The first performs unidirectional power transfer from a dc bus with higher voltage to a dc bus with lower voltage. The second performs unidirectional power transfer from a dc bus with lower voltage to a dc bus with higher voltage. Both converters are suitable for interconnecting single pole dc buses with same polarity, or double pole dc buses. A third converter is also presented which performs the function of either the first or the second converter with polarity reversal. The third converter is suitable for interconnecting single pole dc buses with different polarities, or double pole dc buses. By hybrid integration of the proposed three converters, the thesis also investigates other topologies for bidirectional power transfer between two dc buses. Proposed converters operate only in discontinuous conduction mode and exhibit soft switching operation for the active and passive switches. A common feature between the proposed converters is the self current turn-off for the active switches at zero voltage. This allows the use of thyristors as active switches alleviating their reverse recovery losses. For each converter topology, the structure is presented, its operation principle is explained and a complete set of design equations are derived. Comparisons are performed on high-power and high-voltage design examples. The merits and limitations of each converter are concluded. Practical considerations regarding components selection, loss analysis, filter design and the non-idealities of the circuits are studied. Experimental implementation of scaled-down laboratory prototypes is presented to provide a proof of concept and validate the operation principle of the proposed converter topologies.

This thesis presents a new family of converters for high power interconnection of dc buses with different voltage levels. Proposed converters achieve high voltage dc-dc conversion without an intermediate ac conversion stage. This function is implemented without series connection of active switches, or the use of isolation transformers. The salient features of proposed converters are (i) design and construction simplicity, (ii) low switching losses through soft turn-on and soft turn-off, (iii) single stage dc-dc conversion without high-current chopping, (iv) modular structure, (v) equal voltage sharing among the converter modules. Three converter circuits are investigated. The first performs unidirectional power transfer from a dc bus with higher voltage to a dc bus with lower voltage. The second performs unidirectional power transfer from a dc bus with lower voltage to a dc bus with higher voltage. Both converters are suitable for interconnecting single pole dc buses with same polarity, or double pole dc buses. A third converter is also presented which performs the function of either the first or the second converter with polarity reversal. The third converter is suitable for interconnecting single pole dc buses with different polarities, or double pole dc buses. By hybrid integration of the proposed three converters, the thesis also investigates other topologies for bidirectional power transfer between two dc buses. Proposed converters operate only in discontinuous conduction mode and exhibit soft switching operation for the active and passive switches. A common feature between the proposed converters is the self current turn-off for the active switches at zero voltage. This allows the use of thyristors as active switches alleviating their reverse recovery losses. For each converter topology, the structure is presented, its operation principle is explained and a complete set of design equations are derived. Comparisons are performed on high-power and high-voltage design examples. The merits and limitations of each converter are concluded. Practical considerations regarding components selection, loss analysis, filter design and the non-idealities of the circuits are studied. Experimental implementation of scaled-down laboratory prototypes is presented to provide a proof of concept and validate the operation principle of the proposed converter topologies.