EXAMENSARBETE INOM ELEKTROTEKNIK, AVANCERAD NIVÅ, 30 HP STOCKHOLM, SVERIGE 2017 Construction of an Active Rectifier for a Transverse-Flux Wave Power Generator OLOF BRANDT LUNDQVIST KTH SKOLAN FÖR ELEKTRO- OCH SYSTEMTEKNIK 1 Sammanfattning Vågkraft är en energikälla som skulle kunna göra en avgörande skillnad i om- ställningen mot en hållbar energisektor. Tillväxten för vågkraft har dock inte varit lika snabb som tillväxten för andra förnybara energislag, såsom vindkraft och solkraft. Vissa tekniska hinder kvarstår innan ett stort genombrott för våg- kraft kan bli möjligt. Ett hinder fram tills nu har varit de låga spänningarna och de resulterande höga effektförlusterna i många vågkraftverk. En ny typ av våg- kraftsgenerator, som har tagits fram av Anders Hagnestål vid KTH i Stockholm, avser att lösa dessa problem. I det här examensarbetet behandlas det effekte- lektroniska omvandlingssystemet för Anders Hagneståls generator. Det beskriver planerings- och konstruktionsprocessen för en enfasig AC/DC-omvandlare, som så småningom skall bli en del av det större omvandlingssystemet för generatorn. Ett kontrollsystem för omvandlaren, baserat på hystereskontroll för strömmen, planeras och sätts ihop. Den färdiga enfasomvandlaren visar goda resultat under drift som växelriktare. Dock kvarstår visst konstruktionsarbete och viss kalibre- ring av det digitala kontrollsystemet innan omvandlaren kan användas för sin uppgift i effektomvandlingen hos vågkraftverket. 2 2 Abstract Wave power is an energy source which could make a decisive difference in the transition towards a more sustainable energy sector. The growth of wave power production has however not been as rapid as the growth in other renewable energy fields, such as wind power and solar power. Some technical obstacles remain before a major breakthrough for wave power can be expected. One obstacle so far has been the low voltages and the resulting high power losses in many wave power plants. A new type of wave power generator, which has been invented by Anders Hagnestål at KTH in Stockholm, aims to solve these problems. This master’s thesis deals with the power electronic converter system for Anders Hagnestål’s generator. It describes the planning and construction process for a single-phase AC/DC converter, which will eventually be a part of the larger converter system for the generator. A control system based on hysteresis current control is planned and assembled. The finished single-phase converter shows agreeable results working as an inverter, generating a distinctly sinusoidal AC voltage. However, some additional construction and calibration in the digital control system remain, before the converter can be used in the power conversion for a wave power plant. 3 3 Acknowledgements To my parents and to my brother I want to express my appreciation for their love and support throughout my life. To Anders Hagnestål for letting me be part of the development in his inno- vative research project, which is contributing to the technical development and future prospects of wave power. To Aliro Cofre Osses for his good contribution to the project work and for being a good friend. To Nicholas, Matthijs, Rudi, Keijo, Panos, Dieter, Stefanie and the other friendly people in the electrical laboratory for the good company and the help- ful assistance during the practical work with the converter construction. To captain Gregor, first mate Willy Wonka and the other sailors on the At- lantic Cartier cargo ship who meet the power in the waves everyday. To all the people working towards an expansion of renewable energy. It is certainly an exciting time to enter the work life within electric power engineering, considering the important difference that clean electrical energy can make in building a sustainable future. It is my sincere wish to be a part in the work towards this goal. 4 4 Table of contents 1 Sammanfattning 2 2 Abstract 3 3 Acknowledgements 4 4 Table of contents 5 5 Nomenclature 10 I Introduction 11 6 Background 11 7 Summary of the technical work 12 8 Goals and scope limitations 12 9 Method 13 II Literature review 14 10 Technical theory review 14 10.1 Electrical machines . 14 10.1.1 Electric generators . 14 10.1.2 Rotating generators and linear generators . 14 10.1.2.1 Rotating generators . 15 10.1.2.2 Linear generators . 15 10.1.3 Electrical machine types by magnetic flux direction . 15 10.1.3.1 Radial-flux machines . 15 10.1.3.2 Axial-flux machines . 15 10.1.3.3 Transverse-flux machines . 15 10.2 Power electronics . 15 10.3 Power semiconductors . 16 10.3.1 Power diodes . 16 10.3.2 Power transistors . 16 10.3.2.1 Power MOSFETs . 16 10.3.2.2 Insulated-gate bipolar transistors . 17 10.3.2.3 Silicon carbide power MOSFETs . 17 10.3.2.4 Comparison between SiC MOSFETs and Si IGBTs 17 10.4 Switch-mode converters . 17 10.4.1 Pulse-width modulation . 17 10.4.2 DC-DC converters . 18 10.4.3 DC/AC converters and AC/DC converters . 18 10.4.4 Single-phase voltage-source converters . 18 10.4.5 Active rectifiers . 18 10.4.6 Three-phase voltage-source converters . 19 5 10.4.7 Total harmonic distortion . 19 10.5 PWM control algorithms for voltage-source converters . 20 10.5.1 Control of single-phase voltage-source converters . 20 10.5.1.1 Sinusoidal pulse-width modulation . 20 10.5.1.2 Hysteresis current control . 21 10.5.2 Bipolar and unipolar PWM . 21 10.5.2.1 Bipolar voltage switching mode . 22 10.5.2.2 Unipolar voltage switching mode . 23 10.5.3 Frequency modulation index . 23 10.5.4 Amplitude modulation index . 24 10.6 Microcontroller applications for control of voltage-source converters 24 10.7 MOSFET gate driver circuits . 24 10.8 Snubber circuits . 24 10.9 The DC-link and its function . 25 10.9.1 Polarity of electrolytic capacitors . 25 10.9.2 Bleeder resistors . 25 10.10Back-to-back coupling of voltage-source converters . 25 10.11Level shifters . 26 11 Electric power generation from sea waves 26 11.1 The power in the waves . 26 11.2 Challenges in the design of wave power generators . 27 11.3 Current status of wave power generation in the world . 27 11.4 Future potential for the field of wave power . 28 12 Characteristics of the wave power generator of Anders Hagnestål 28 12.1 Generator characteristics . 28 12.2 Reducing the resistive losses . 29 12.3 Active power factor correction . 30 12.4 Power level in the generator . 30 12.5 Cogging in the generator . 30 III Planning 31 13 Dimensioning the generator’s power electronic converter sys- tem 31 13.1 Overview of the power electronic converter system . 31 13.2 AC/DC-converter characteristics . 31 13.2.1 Active power factor correction . 32 13.3 DC/AC-converter characteristics . 32 13.4 BeagleBone Black microcontroller . 32 13.5 Sizing of the converter’s electrical components . 33 13.5.1 Selection of power transistors . 33 13.5.2 Selection of the converter’s voltage levels . 33 13.5.2.1 DC-link voltage level . 34 13.5.2.2 Generator side voltage level . 34 13.5.3 Selection of the converter’s current levels . 34 13.5.4 Maximum power flow through the power converter . 34 13.5.5 Selection of MOSFET drivers . 34 6 13.5.6 PWM switching frequency . 35 13.5.7 Sizing of a filter circuit on the generator side . 35 13.5.8 Sizing of the snubber circuits . 35 13.5.9 Sizing of the DC-link filter capacitor . 36 13.6 Electrical components for the initial laboratory test setup . 36 13.6.1 DC-link capacitor for the initial lab testing . 36 13.6.2 Bleeder resistor for the initial lab testing . 37 13.6.3 Snubber circuits for the initial lab testing . 37 13.6.4 Level shifters . 38 13.7 Electrical isolation paper . 39 13.8 Heat sinks . 39 14 Planning for the control system of the power electronic con- verter 39 14.1 Beaglebone Black and the choice of the Python programming language . 39 14.2 Development of a SPWM control Python code . 40 14.3 Development of a hysteresis control Python code . 40 14.3.1 Flow chart for the bipolar hysteresis control code . 41 14.3.2 Flow chart for the unipolar hysteresis control code . 41 14.4 Hysteresis control simulations for different sampling frequencies . 43 14.4.1 Switching frequencies for different sampling frequencies . 43 14.4.2 Current deviation from the reference current for different sampling frequencies . 43 14.4.3 Conclusions about the necessary sampling frequency for unipolar PWM hysteresis control . 44 15 Planning for the construction of the active rectifier 44 15.1 Laboratory setup with machines and two converters . 45 15.2 Two modules instead of four during the initial testing phase . 45 15.3 Circuit diagrams . 45 15.3.1 Simplified block diagram for the final laboratory setup with two machines . 46 15.3.2 Simplified circuit diagram for one single-phase converter with four phase-legs . 46 15.3.3 Simplified circuit diagram for one single-phase converters with two phase-legs . 47 15.3.4 Detailed circuit diagram for one single-phase converter with two phase-legs . 48 15.3.5 Circuit diagram for the connection of the current sensor . 48 15.4 Practical design aspects to take into account . 50 15.4.1 Copper plate dimensions . 50 15.4.2 Elevation of the copper plates above the DC-link capacitor 50 15.4.3 Mechanical and electrical connection of the power modules 50 15.4.4 Placement of electrical cables . 50 15.4.5 Attachment of the heat sinks .