UPTEC F 15038 Examensarbete 30 hp Juni 2015 Active rectification and control of magnetization currents in synchronous generators with rotating exciters Implementation of the SVPWM algorithm using MOSFET technology Tomas Johansson Abstract Active rectification and control of magnetization currents in synchronous generators with rotating exciters Tomas Johansson Teknisk- naturvetenskaplig fakultet UTH-enheten This thesis aims to design and build a power electronics system for the rectification and control of magnetization currents in synchronous generators with rotating Besöksadress: exciters. Ångströmlaboratoriet Lägerhyddsvägen 1 Hus 4, Plan 0 The rotating exciter provides three-phase AC while the generator rotor needs DC with a high degree of control. The system needs to be able to rectify the three-phase Postadress: AC to a stable DC without unwanted harmonic content, neither on the DC or the Box 536 751 21 Uppsala AC side. For control purposes it is also important that the current in the rotor can be changed very swiftly, preferably by several amperes during a single revolution of Telefon: the machine. 018 – 471 30 03 Telefax: The system of choice is a synchronous rectifier bridge consisting of six MOSFET 018 – 471 30 00 switches operated using the Space vector pulse width modulation (SVPWM) algorithm. This method gives a stable and controllable DC voltage while it keeps the Hemsida: harmonic content of the input currents at a minimum. However the DC voltage will http://www.teknat.uu.se/student always be higher than the peak line-to-line voltage from the exciter. To be able to lower the voltage below this value a Buck-converter is placed after the rectifier bridge. To gain a higher degree of control of the current density in the rotor windings the windings have been subdivided into three parts. To provide individual control of the current in the three rotor parts each part have been outfitted with a Push and Pull H-bridge. The proposed system has been both simulated using MATLAB Simulink and built and tested in the laboratory with satisfactory results. Handledare: Johan Abrahamsson Ämnesgranskare: Urban Lundin Examinator: Tomas Nyberg ISSN: 1401-5757, UPTEC F15 038 Sammanfattning I detta examensarbete presenteras ett kraftelektroniksystem f¨orf¨orb¨attradkontroll av magnetiseringsst¨ommari vattenkraftsgeneratorer som ¨arutrustade med roterande matare. Generatorer anv¨andsf¨oratt konvertera energi fr˚anr¨orelseenergitill elektrisk energi. Detta g¨orsgenom att man uts¨atterspolar f¨orvarierande magnetf¨alt; d˚ainduceras sp¨anningi spolarna. I vattenkraftsgeneratorer anv¨andsoftast stora elektromagneter placerade i en rotor f¨oratt skapa dessa magnetf¨alt. F¨oratt magnetisera elektromag- neterna beh¨ovsstr¨omsom p˚an˚agots¨attm˚aste¨overf¨orasmellan den statiska och den roterande sidan i generatorn. Traditionellt g¨orsdetta med hj¨alpav sl¨apringar och kol- borstar som genom mekanisk kontakt ¨overf¨orelektriciteten. En roterande matare kan beskrivas som en liten generator som har sina elektriska utg˚angarp˚aden roterande sidan ist¨alletf¨orp˚aden statiska sidan. Genom att placera en roterande matare p˚asamma axel som den stora generatorn kan man ist¨alletalstra den elektricitet som beh¨ovsf¨oratt magnetisera generatorn direkt p˚aden roterande sidan. D¨aregenomundviks m˚angaproblem som ¨arassocierade med l¨osningenmed sl¨apringar. Den roterande mataren ger dock v¨axelstr¨ommedan magnetiseringsstr¨ommenm˚astevara likstr¨om. Det ¨arh¨arkraftelektroniken kommer in i bilden. Det finns flera s¨attatt ˚astadkomma likriktning av str¨om.I det h¨arprojektet har ett fullst¨andigtaktivt system byggts. Systemet ¨aruppbyggt av transistorer av MOSFET typ och kan kontrolleras tr˚adl¨ostmed hj¨alpav Bluetoothteknik. Systemet ger full kontroll ¨over str¨ommaroch sp¨anningarb˚adep˚av¨axelstr¨omssidanoch p˚alikstr¨omssidanoch ska anv¨andastill en testgenerator p˚aavdelningen f¨orell¨aravid Uppsala Universitet. D¨arska den ut¨okade kontroll som systemet ger f¨oruts¨attningartill anv¨andasf¨oratt unders¨oka hur den h¨artypen av system kan optimera de magnetiska krafterna inuti generatorn. En s˚adanoptimering kan minska vibrationerna i generatorn och d¨arigenomminska slitaget p˚alager och andra delar i maskinen. Contents 1 Introduction 1 1.1 Project background . .1 1.2 Project description . .2 1.3 Limitations . .2 2 Theory 4 2.1 Rotating exciter . .4 2.2 Power Electronics . .4 2.2.1 Devices . .4 2.2.2 Pulse Width Modulation . .7 2.2.3 Buck converter . .7 2.2.4 Boost converter . .9 2.2.5 Three-phase Boost Rectifier . .9 2.2.6 Push-Pull H-Bridge . 10 2.3 Control system . 11 2.3.1 Space Vector Pulse Width Modulation . 11 2.3.2 PID - control . 13 2.3.3 Hysteresis control . 14 3 Method 15 3.1 Power electronics . 15 3.1.1 System topology . 15 3.1.2 Devices . 16 3.2 Simulations . 17 3.3 Control system . 18 3.3.1 Instrumentation . 18 3.3.2 Driver circuits . 19 3.3.3 FPGA-programming . 19 3.4 Mechanical assembly . 20 3.5 Measurements . 21 3.5.1 Three-phase rectifier . 22 3.5.2 Buck-converter . 22 4 Results 23 4.1 Simulations . 23 4.2 Measurements . 25 4.3 Mechanical assembly . 25 5 Discussion and Conclusions 28 6 Bibliography 29 A Component information 31 A.1 List of devices . 31 A.2 Driver auxiliaries . 31 A.2.1 Input side . 31 A.2.2 Output side . 33 B Labview program 34 B.1 Main program structure . 34 B.2 Angular frequency . 34 B.3 Switch sequence calculator . 35 B.4 Switch sequence control . 36 B.5 Buck converter control . 38 B.6 Measurement readings . 39 1 Introduction Hydropower has been an important source for energy throughout history. In today's society it is mainly utilized to produce electricity and in this capacity it still remains a crucial component of the energy systems in many modern countries. The benefits of hydropower are many: it is a renewable energy source, has a high efficiency and, probably most important, it has a high regulation capability. The last factor makes hydropower excellent to use as a stabilizer for the electrical grid. One of the main problems for hydropower is maintenance, it is very expensive, therefore any system that reduces the amount of necessary maintenance can significantly reduce the cost of power production. One of the driving factors for maintenance of a hydropower generator is the use of slip-rings and carbon-brushes to transfer the magnetization current for the rotor, from the stationary side to the rotating side. The brushes get worn down and needs to be replaced and as they wear down they generate carbon dust. This dust can get mixed up with the lubricant oil, and in the worst case scenario the electrically conductive dust can even cause short circuits in the electric system. A dusty environment is also a health issue for people working at the power plant. One way to avoid the use of slip-rings is to use a rotating exciter, it can be described as a small generator placed on the same shaft as the main generator, the small generator has its electrical output on the rotating side and this electricity is used to magnetize the main rotor. 1.1 Project background An experimental salient pole synchronous generator, called Svante, located at the divi- sion for Electricity at Uppsala Universitet, was built to be able to test different aspects of electro-mechanical properties in a controlled environment. One of the main research aims was to investigate how rotor eccentricity, with regard to the stator, give rise to unbalanced magnetic pull (UMP) leading to varying mechanical loads throughout the generator structure. [22] The generator has now been outfitted with a permanent magnet rotating exciter provid- ing six-phase alternating current, AC, with a line-to-line voltage of 120 V (rms). It is also possible to connect it as a three-phase machine by connecting the poles in series. Since the exciter generates AC current while the main rotor needs direct current, DC, to operate, a rectifying process is needed. Prior to this project such a rectification has been achieved with a twelve-pulse thyristor bridge. While being a relatively efficient and simple solution it introduces harmonics in the current flow from the exciter which can lead to torque ripple in the exciter. 1 CHAPTER 1. INTRODUCTION 2 1.2 Project description This project aims to create a system that rectifies the current from the exciter while keeping the input harmonics at a minimum. The rotor needs to be supplied with DC, at current levels up to 30 A. The current needs to have a low ripple and have a fast rise and fall regulation. In addition it is desirable to be able to control the rotor windings separately to achieve a higher degree of control during operation. To make this possible the twelve poles of the rotor have been divided into three parts, each containing four poles with individual current control for each part. The rotor windings have a total inductance of 0.5 H and a resistance of 3.3 Ω, and it is therefore to be considered as a highly inductive load. The control of the system needs to be managed at the stationary side, in order to enable this a parallel project has been conducted by Fredrik Evestedt [5]. His project deals with wireless communication with Bluetooth technology and with the sensor systems mounted on the rotating side. The requirements on the system can be summarized as: • Rectification from three phase AC to DC • No, or low, harmonic content on the input side • Current levels up to 30 A with low ripple (DC-side) • Regulating capabilities fast enough to overlay a sinus wave with amplitude 1 A and frequency 100 Hz • Individual control of three rotor sections 1.3 Limitations The system needs to be fitted inside a hexagonal metal box to be mounted on top of the shaft of the generator.