Thesis (PhD (Electrical Engineering))--Cape Peninsula University of Technology, 2019 / Energy systems based on fossil fuel have demonstrated their abilities to permit economic development. However, with the fast exhaustion of this energy source, the expansion of the world energy demand and concerns over global warming, new energy systems dependent on renewable and other sustainable energy are gaining more interests. It is a fact that future development in the energy sector is founded on the utilisation of renewable and sustainable energy sources. These energy sources can enable the world to meet the double targets of diminishing greenhouse gas emissions and ensuring reliable and cost-effective energy supply. Fuel cells are one of the advanced clean energy technologies to substitute power generation systems based on fossil fuel. They are viewed as reliable and efficient technologies to operate either tied or non-tied to the grid to power applications ranging from domestic, commercial to industrial. Multiple fuel cell stacks can be associated in series and parallel to obtain a fuel cell system with high power up to megawatts. The connection of megawatts fuel cell systems to a utility grid requires that the power condition unit serving as the interface between the fuel cell plant and the grid operates accordingly. Different power conditioning unit topologies can be adopted, this study considers a multilevel inverter. Multilevel inverters are getting more popularity and attractiveness as compared to conventional inverters in high voltage and high-power applications. These inverters are suitable for harmonic mitigation in high-power applications whereby switching devices are unable to function at high switching frequencies. For a given application, the choice of appropriate multilevel topology and its control scheme are not defined and depend on various engineering compromises, however, the most developed multilevel inverter topologies include the Diode Clamped, the Flying Capacitor and the Cascade Full Bridge inverters. On the other hand, a multilevel inverter can be either a three or a five, or a nine level, however, this research focuses on the three-level diode clamped inverters. The aim of this thesis is to model and control a three-level diode clamped inverter for the grid connection of a megawatt fuel cell stack. Besides the grid, the system consists of a 1.54 MW operating at 1400 V DC proton exchange membrane fuel cell stack, a 1.26 MW three-level diode clamped inverter with a nominal voltage of 600 V and an LCL filter which is designed to reduce harmonics and meet the standards such as IEEE 519 and IEC 61000-3-6. The inverter control scheme comprises voltage and current regulators to provide a good power factor and satisfy synchronisation requirements with the grid. The frequency and phase are synchronised with those of the grid through a phase locked loop. The modelling and simulation are performed using Matlab/Simulink. The results show good performance of the developed system with a low total harmonic distortion of about 0.35% for the voltage and 0.19% for the current.

This paper presents renewable energy, power electronics, and distributed generators. The focus is on wind farm generator, photovoltaic cell, and battery bank system. Power Conditioning system improves the performance of a power system. Apart from the benefits of converting between DC/AC, there is adequate control of real power and additional control of economic reactive power. This is possible because of multiple sources in the system. This project throws light on the basic principle of power system conditioning, its operation and control, and the economic studies.

One of the most pressing problems nowadays is climate change and global warming. As it name indicates, it is a problem that concerns the whole earth. There is no doubt that the main cause for this to happen is human, and very related to non-renewable carbon-based energy resources. However, technology has evolved, and some alternatives have appeared in the energy conversion sector. Nevertheless, they are relatively young yet. Since the growth in renewable energies technologies wind power and PV are the ones that have taken the lead. Wind power is a relatively mature technology and even if it still has challenges to overcome the horizon is clear. However, in the PV case the technology is more recent. Even if it is true that PV modules have been used in space applications for more than 60 years, large scale production has not begun until last 10 years. This leaves the uncertainty of how will PV plants and modules age. The author will try to analyse the aging of a specific 63 kWp PV plant located in the roof of a building in Gävle, monitoring production and ambient condition data, to estimate the degradation and the new nominal power of the plant. It has been found out that the degradation of the system is not considerable. PV modules and solar inverters were studied, and even if there are more elements in the system, those are the principal ones. PV modules suffered a degradation of less than 5%, while solar inverters’ efficiency dropped from 95,4% to around 93%.

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Nowadays, a high penetration level of distributed energy resources (DERs), such as renewables, energy storage, and electric vehicles, are integrated into modern electric power grids, especially power distribution sections, through inverter-based interfaces. Depending on the interfacing technologies and capacities of different DERs, the power distribution networks with inverter-based DERs feature different characteristics, which motivates this dissertation to investigate the modeling, design, and control of heterogeneous inverter-based power distribution networks. First, an example of a DER power distribution network, a PV system, is studied and an optimal design framework for PV systems is proposed considering two objectives, levelized cost of energy (LCOE) and power density (PD). Second, to further improve the performance of the inverter-based distribution networks, the harmonic characteristics of a generic grid-interactive inverter is investigated. A holistic mathematical harmonic state space (M-HSS) model of a grid-interactive inverter is derived to calculate each order of harmonics of grid-connected current. Moreover, to further reduce the computation burden caused by repetitive usage of the mathematical HSS model during the optimal design process, a data-driven HSS (D-HSS) modeling method is proposed by incorporating the data-driven techniques into the aforementioned M-HSS modeling. Based on the M- and D-HSS models, an effective optimal design framework is proposed to determine the closed-loop inverter system parameters. Furthermore, due to the increasing deployment of power electronic devices and nonlinear loads, power grids in the distribution network typically present certain degrees of low and/or high order harmonics. Thus, a harmonic compensation control (HCC) scheme is proposed to ensure that the inverter-based distribution network could provide high-quality grid current injection under distorted grid voltage conditions. Additionally, an energy-stored quasi-Z source converter (qZSC) based interlink converter is proposed for hybrid AC/DC microgrids in the distribution networks. The proposed system not only interlinks both AC and DC sub-microgrids but also incorporates energy storage. The operating principle, operating states as well as control schemes are presented in detail. Finally, another DER power distribution network, a medium voltage DC (MVDC) distribution network, is investigated in the study. First, the dissertation proposes an effective fault management scheme for MVDC networks, which includes a virtual-impedance-based fault current limiter (VI-FCL) on the DC side and a positive-negative-sequence (PNS) control scheme on the AC side. Finally, another DER power distribution network, a medium voltage DC (MVDC) distribution network, is investigated in the study. First, the dissertation proposes an effective fault management scheme for MVDC networks, which includes a virtualimpedance-based fault current limiter (VI-FCL) on the DC side and a positive-negativesequence (PNS) control scheme on the AC side. Then, a detailed 2ω mathematical model of the MVDC network under unbalanced AC voltage conditions is derived to investigate how the 2ω ripple propagates across the network and the corresponding control scheme is investigated to mitigate the 2ω ripple. / Electrical and Computer Engineering

Semiconductor devices are widely used in the automotive industry, and they are key components of the inverter and the converter in an electric vehicle. Thus, the concern of protecting the inverter and the converter from damaging operation has been raised. The junction temperature is one of the main considerations. It is directly related to the inverter power loss and overheat which can lead to fatigue or even failures. Therefore, to protect the semiconductor device from thermal runaway and apply active thermal control, the junction temperature must be obtained.To derive the junction temperature of an Insulated-Gate Bipolar Transistor (IGBT) and a diode, an instant model is proposed.   The instant model updates the junction temperature at the PWM switching frequency (around 2-5 kHz in this thesis). Compared with the traditional algorithm which calculates the average temperature over one fundamental cycle, the instant algorithm can show the instant temperature swing.  A high power IGBT module, FF1200R12IE5 Infineon, is selected to verify the algorithm with temperature results from IPOSIM and Power HIL tests. The temperature deviations for different cases are below 6 ◦C and the relative errors are below 10 %.With an accurate estimation of the junction temperature, the current limit is set to avoid that average temperature, maximum temperature and temperature swing exceed their limits. The currentlimits are derived from the curve/ surface fitting method. / Halvledare används mycket i fordonsindustrin, och är viktiga komponenter i växelriktare i elektriska fordon. Vikten av att skydda växelriktarna från skadliga driftsförhållanden ökat. Övergångstempe- raturen hos halvledarna är en av de viktigaste parametern att beakta. Den är direkt relaterad till väx- elriktarens förluster, vilket kan leda till termisk utmattning, och i värsta fall haveri. För att skydda halvledarna från termisk rusning och tillämpa aktiv termisk styrning måste övergångstemperaturen kunna uppskattas.För att härleda övergångstemperaturen hos en Insulated-Gate-Bipolar Transistor (IGBT) och en diod föreslås en momentan modell. Den momentana modellen uppdaterar övergångstemperaturen vid PWM-frekvensen (cirka 2-5 kHz i denna avhandling). Jämfört med den traditionella algoritmen som beräknar medeltemperaturen under en grundtonscykel kan den momentana algoritmen visa den omedelbara temperatursvängningen. En IGBT-modul, Infineon FF1200R12IE5, valdes för att verifiera algoritmen mot resultat från simuleringar och mätningar. Temperaturavvikelserna för olika fall är under 6 ◦C och de relativa felen är under 10 %.Med en noggrann uppskattning av övergångstemperaturen kan strömgränsen ställas in för att undvika att medel- och maxtemperaturen och temperatursvängningenen överskrider sina gränser.Strömgränserna härleds genom kurv- och ytanpassning.

The thesis deals with the design of a permanent magnet synchronous machine controller that isimplemented on an embedded platform to replace the off-the-shelf controller currently being used in theelectric race car of the KTH Formula Student team. Software implementation of the control algorithmwas tested in laboratory environment on the hardware prototype of a 2-level three-phase voltage sourceinverter.Field oriented control and finite control set model predictive control algorithms were implemented insimulation environment. The latter performed better in terms of reducing switching activity and torqueripple, but needs vastly more computational resources due to its nature of being an online optimizationproblem. Trade-off curve of phase current harmonic distortion and switching activity showed that themodel prediction control algorithm performs better in the low frequency range (1-20 kHz). Obtainedsimulation results were used for power electronics component selection.Field oriented control was implemented on a TMS320F28335 DSP. SPI communication was employedto configure gate driver circuits and perform error handling. The DSP program follows interrupt basedorganization and the main control loop runs on the variable frequency of the pulse width modulation.Low voltage test results on three-phase inductive-resistive load showed that the controller outputssinusoidal current. Efficiency measurement, high voltage and motor testing were hindered by interferencefrom the Silicon-Carbide MOSFETs that prohibited correct operation of hardware. / Den här uppsatsen handlar om designen och implementeringen av en motorstyrning för en permanen- magnetiserad synkronmotor, med syfte att ersätta standardmotorstyrningsenheten i KTH Formula Students tävlingsbil. Implementationen av styralgoritmen testades experimentellt tillsammans med en prototyptillverkad frekvensomriktare i labbmiljö. Regleralgoritmer för field oriented control och finite control set model predictive control implementerades och testades i simuleringsmiljö. Den senare algoritmen visade sig prestera bättre i form av lägre vridmomentsoscillationer trots lägre switch-frekvens men den kräver samtidigt mer beräkningskraft. Övertonsinnehållet (THD) i fasströmmarna som funktion av switchfrekvensen undersöktes för de båda regleralgoritmerna, algoritmen för model predictive control gav lägre THD vid lägre frekvenser (1-20 kHz). Simuleringsresultaten användes för att motivera valet av komponenter till frekvensomriktaren. Regleralgoritmen för field oriented control implementerades och testades experimentellt med hjälp av ett utvecklingskort (TMS320F28335) från Texas Instruments. SPI-kommunikation användes för att konfigurera drivkretsana samt för att utläsa felkoder. Experimentalla tester som utfördes på låg spänningsnivå visade att strömmen till lasten var sinusformad. Mätning av verkningsgrad och provning tillsammans med motorn på en högre spänningsnivå gick inte att geno av att de snabba switchförloppen i kiselkarbidtransistorerna störde ut motorstyrningen.