Single-Stage Power Electronic Converters with Combined Voltage Step-Up/Step-Down Capability

Power electronic converters are typically either step-down converters that take an input voltage and produce an output voltage of low amplitude or step-up converters that take an input voltage and produce an output voltage of higher amplitude. There are, however, applications where a converter that can step-up voltage or step-down voltage can be very useful, such as in applications where a converter needs to operate under a wide range of input and output voltage conditions such as a grid-connected solar inverter. Such converters, however, are not as common as converters that can only step down or step up voltage because most applications require converters that need to only step down voltage or only step up voltage and such converters have better performance within a limited voltage range than do converters that are designed for very wide voltage ranges. Nonetheless, there are applications where converters with step-down and step-up capability can be used advantageously. The main objectives of this thesis are to propose new power electronic converters that can step up voltage and step down voltage and to investigate their characteristics. This will be done for two specific converter types: AC/DC single-stage converters and DC-AC inverters. In this thesis, two new AC/DC single-stage converters and a new three-phase converter are proposed and their operation and steady-state characteristics are examined in detail. The feasibility of each new converter is confirmed with results obtained from an experimental prototype and the feasibility of a control method for the inverter is confirmed with simulation work using commercially available software such as MATLAB and PSIM

Optimization And Design Of Photovoltaic Micro-inverter

To relieve energy shortage and environmental pollution issues, renewable energy, especially PV energy has developed rapidly in the last decade. The micro-inverter systems, with advantages in dedicated PV power harvest, flexible system size, simple installation, and enhanced safety characteristics are the future development trend of the PV power generation systems. The double-stage structure which can realize high efficiency with nice regulated sinusoidal waveforms is the mainstream for the micro-inverter. This thesis studied a double stage micro-inverter system. Considering the intermittent nature of PV power, a PFC was analyzed to provide additional electrical power to the system. When the solar power is less than the load required, PFC can drag power from the utility grid. In the double stage micro-inverter, the DC/DC stage was realized by a LLC converter, which could realize soft switching automatically under frequency modulation. However it has a complicated relationship between voltage gain and load. Thus conventional variable step P&O MPPT techniques for PWM converter were no longer suitable for the LLC converter. To solve this problem, a novel MPPT was proposed to track MPP efficiently. Simulation and experimental results verified the effectiveness of the proposed MPPT. The DC/AC stage of the micro-inverter was realized by a BCM inverter. With duty cycle and frequency modulation, ZVS was achieved through controlling the inductor current bi-directional in every switching cycle. This technique required no additional resonant components and could be employed for low power applications on conventional full-bridge and half-bridge inverter topologies. Three different current mode control schemes were derived from the basic theory of the proposed technique. They were referred to as Boundary Current Mode (BCM), Variable Hysteresis Current Mode (VHCM), and Constant Hysteresis Current Mode (CHCM) individually in this paper with their advantages and disadvantages analyzed in detail. Simulation and experimental iv results demonstrated the feasibilities of the proposed soft-switching technique with the digital control schemes. The PFC converter was applied by a single stage Biflyback topology, which combined the advantages of single stage PFC and flyback topology together, with further advantages in low intermediate bus voltage and current stresses. A digital controller without current sampling requirement was proposed based on the specific topology. To reduce the voltage spike caused by the leakage inductor, a novel snubber cell combining soft switching technique with snubber technique together was proposed. Simulation and experimental waveforms illustrated the same as characteristics as the theoretical analysis. In summary, the dissertation analyzed each power stage of photovoltaic micro-inverter system from efficiency and effectiveness optimization perspectives. Moreover their advantages were compared carefully with existed topologies and control techniques. Simulation and experiment results were provided to support the theoretical analysis

High Speed flywheel and test rig design for rural energy storage

There is considerable growth in the renewable energy sector to contribute to sustainable development, environmental conservation and most importantly to provide affordable energy to isolated rural communities of sub-Saharan Africa. Renewable energy sources such as solar and wind require energy storage since the source of energy is intermittent. Electrochemical batteries especially from lead acid are commonly used to store energy in Solar Home Systems (SHS) for rural electrification in sub-Saharan Africa. Disadvantages such as low efficiencies, low life cycle costs, high maintenance, comparatively short life and serious environmental and human toxicity effects exist. Since recycling is not widespread, replacement costs are high, as are the resultant environmental damage and health hazards from lead and sulphuric acid. In this thesis, an electromechanical flywheel energy storage device is proposed as an alternative to a lead acid battery in order to increase efficiency, life expectancy, increased high depth of discharge, low life cycle cost and elimination of adverse environmental effects. Due to income and service skill constraints in rural areas, the proposed, high speed flywheel systems (for long time energy storage) will require the use of low cost configurations and topologies, special considerations on the flywheel rotor profile design, robust electrical machines, simple power electronics and a low cost bearing set. Low loss magnetic bearings are also possible but were limited by time while also making their maintenance complex especially in rural areas. Conventional high strength composite materials used in flywheel rotor manufacture for high speed operation are expensive. Therefore there is a need to develop techniques to profile the rotor shape so as to improve on material usage and exhibit lower mechanical stresses. A robust electrical machine topology for high speed operation and a simple drive system are investigated to ensure simple assembly, low cost and low maintenance. vii The various flywheel components were designed using analytical and numerical methods. Two techniques were used to develop two optimal profiles for the flywheel rotor structure. Partial differential equations and analytical solutions were employed to develop the profiles. Analytical equations were used to design the electrical machine, drive, bearing system and other accessories. The final electromechanical battery prototype consisted of a composite flywheel rotor made from E-glass fibre materials, double rotor Axial Flux Permanent Magnet (AFPM) machine and a drive system using Brushless DC (BLDC) mode of operation. The system was designed for 300Wh of energy storage for the delivery of 100W and 500W of power and an operating speed range of 8,000 rpm-25,000 rpm. The design and development of the flywheel energy storage system and test rig using locally available materials was investigated. Experiments were conducted for speeds up to 6,000 rpm. The electromechanical battery was able to store a maximum of 77Wh of energy. The shortfall of the system to meet its design specifications was investigated and found to have been caused by vibrations resulting from prototyping issues. A thermal model was developed to predict the temperature rise in the system which showed a good correlation with the experimental results

A GaN-Based Four-Switch Buck-Boost Converter Using Ripple Correlation Control for Maximum Power Point Tracking in Dynamic Deep Space Environments

As the demand for high-performance power conversion in spacecraft continues to grow and spacecraft mass and volume budgets become increasingly tight, it is essential to design DC-DC converters with higher efficiency and power density. Although photovoltaic (PV) efficiency has increased over time, solar irradiance and temperatures can fluctuate dramatically in deep space. This causes significant variations in the maximum power point (MPP) of the PV array, which can decrease the overall system efficiency unless accounted for. Thus, it is imperative to track the MPP of the PV panels to maintain optimal efficiency. This paper presents the experimental development of a four-switch, GaN-based buck-boost converter with an implementation of the Ripple Correlation Control (RCC) MPPT algorithm for dynamic deep space environments. Due to the use of GaN HEMTs, the experimental system achieves better efficiency and power density compared to the previous state of the art implementations. A simulation of the prototype buck-boost converter was implemented in SaberRD (Synopsis), and a digital design of the RCC-based MPPT controller utilizing the StateAMS tool is presented. The simulation results show that this controller swiftly and precisely converged to the MPP of the source PV panels in a dynamic solar irradiance condition

A Survey, Classification and Critical Review of Light-Emitting Diode Drivers

Based on a survey on over 1400 commercial LED drivers and a literature review, a range of LED driver topologies are classified according to their applications, power ratings, performance and their energy storage and regulatory requirements. Both passive and active LED drivers are included in the review and their advantages and disadvantages are discussed. This paper also presents an overall view on the technical and cost aspects of the LED technology, which is useful to both researchers and engineers in the lighting industry. Some general guidelines for selecting driver topologies are included to aid design engineers to make appropriate choices.published_or_final_versio

Single-Stage Grid-Connected Forward Microinverter with Boundary Mode Control

This paper presents a microinverter to be integrated into a solar module. The proposed solution combines a forward converter and a constant off-time boundary mode control, providing MPPT capability and unity power factor in a single-stage converter. The transformer structure of the power stage remains as in the classical DC-DC forward converter. Transformer primary windings are utilized for power transfer or demagnetization depending on the grid semi-cycle. Furthermore, bidirectional switches are used on the secondary side allowing direct connection of the inverter to the grid. Design considerations for the proposed solution are provided, regarding the inductance value, transformer turns ratio and frequency variation during a line semi-cycle. The decoupling of the twice the line frequency power pulsation is also discussed, as well as the maximum power point tracking (MPPT) capability. Simulation and experimental results for a 100W prototype are enclose

Analysis of Dual Phase Dual Stage Boost Converter for Photovoltaic Applications

In the previous two decades, the depletion of fossil fuels has led to applications with renewable energy sources. The approach for renewable energy application is to achieve eminent static boost ratio from with miniaturized ripple content in both current and voltage. The novel converter discussed in this paper is derived by combining the concept of interleaving and cascading of boost converters. The presented design has a dual phase boost converter followed by a stage of boost converter which as a whole acts like dual phase dual stage boost converter. The interleaving concept is utilized in dual phase boost converter to reduce the voltage current stress created in boosting process and its output is boosted by the dual stage boost converter which improves overall efficiency with respect to the existing systems. The converter is designed for a power rating of 200W with output voltage of 192V for an input voltage of 12V obtained from photovoltaic source at a switching frequency of 50KHz this illustrate the advantages over other existing converters . The different parameters of the proposed boost converter are contrasted with that of a conventional boost converter. Furthermore, the simulations results of the proposed converter are presented to validate the system design. The results exhibit that this converter achieves a predominant performance over other dc-dc boost converters by offering improved efficiency and voltage gain, while having lower input current ripple

Interleaved multi-cell isolated three-phase PWM rectifier system for aircraft applications

Recently there has been an important increase in electric equipment, as well as, electric power demand in aircrafts applications. This prompts to the necessity of efficient, reliable, and low-weight converters, especially rectifiers from 115VAC to 270VDC because these voltages are used in power distribution. In order to obtain a high efficiency, in aircraft application where the derating in semiconductors is high, normally several semiconductors are used in parallel to decrease the conduction losses. However, this is in conflict with high reliability. To match both goals of high efficiency and reliability, this work proposes an interleaved multi-cell rectifier system, employing several converter cells in parallel instead of parallel-connected semiconductors. In this work a 10kW multi-cell isolated rectifier system has been designed where each cell is composed of a buck type rectifier and a full bridge DC-DC converter. The implemented system exhibits 91% of efficiency, high power density (10kW/10kg), low THD (2.5%), and n−1 fault tolerance which complies, with military aircraft standards

Stirling Convertor Control for a Lunar Concept Rover

NASA Glenn Research Center is developing various circuits for a lunar concept rover powered by both a stirling convertor and lithium ion batteries. To begin, a survey of six analog, non-power factor correcting controllers was done for an Advanced Stirling Convertor (ASC) design one was selected to control the stirling convertor. Next, a constant power circuit and lithium ion battery charger was designed, built and tested based on simulation in PSpice. The constant power circuit enables the stirling convertor to maintain a constant power when additional power is required from the batterie

A multi-MHz wireless power transfer system with mains power factor correction circuitry on the receiver

This paper proposes the implementation of a new system topology for multi-MHz inductive power transfer (IPT) systems, which achieves unity power factor when fed from a mains power supply without traditional active circuitry in the front-end as a mains interface. Experiments were performed using an IPT-link which consists of two 20 cm two-turn air-core printed-circuit-board (pcb) coils separated by an air-gap of 13 cm. At the transmit side, a push-pull load-independent Class EF inverter fed from a rectified 60 Hz power supply with no bulk capacitor was designed to drive the transmit coil at 13.56 MHz. This inverter, which has two choke inductors between the voltage source and the two switches, similar to that of an interleaved boost converter, is suitable to be fed directly from a rectified mains source because it tolerates large changes on the input voltage. The IPT rectifier in the experiments was built using a dual current-driven Class D-based topology which allows for higher output voltage when the induced electromotive force (emf) on the receive coil is low. The final power conversion stage on the receive side is a power factor correction (PFC) boost converter that regulates the output voltage and shapes the current waveform at the input of the system. This stage is the only part of the system with closed-loop control. The end-to-end efficiency was measured at 73.3% with 99.2% power factor, when powering a load of 150 W