Recent developments in high power rated Voltage Source Converters (VSCs) have resulted in their successful application in Multi-Terminal HVDC (MTDC) transmission systems and also have potential in the Medium Voltage DC (MVDC) distribution systems. This work presents the findings of stability studies carried out on a zonal MVDC architecture for the shipboard power distribution system. The stability study is confined to rotor angle stability of the power system, i.e. the transient and small signal stability analysis. The MTDC ring structure similar to MVDC shipboard power system was implemented in MATLAB/Simulink to look at the transient behavior of the MVDC system. Small signal stability analysis has been carried out with the help of Power System Toolbox (PST) for both MVAC as well as MVDC architectures. Later, Participation Analysis has been carried out to address the small signal instability in the case of MVAC architecture and methods for enhancement were also presented.

This thesis deals with metrology specialized to DC voltage. In the begining of thesis is explanation of technical terms from metrology, summarization of units from International System and explanation of the standards and its kinds in the context of metrology. Primary standards of DC voltage based on Zener diodes and Josephson effect are described as well. In cooperation with Czech metrology institute in Brno this work is focused on automatization of measuring Zener reference standards (ZRS). Goal of this project is to automatize the measuring of Zener reference standards and its relationship with Josephson voltage system using scanner. To do this, there is a need to find out offsets, that can develop on scanner's channels. There is a theoretical analysis of measuring made on the scanner and their uncertainties. Aim of the solution is computation of scanner channels offsets using everyday automatic measuring the system of Zener reference standards (ZRS). For measuring the LabView was used and for computing the data from measured values the Matlab was used. This thesis should short the time needed when measuring DC voltage references, measure values of DC voltage standards and set the scanner channels offsets. In the end there is a summarization of offsets measured on individual scanner channels.

AC-DC converters find application in every day life as a front-end to DC-DC and DC-AC converters. Active three-phase converters shape the three-phase input current to be sinusoidal and to be in-phase with the input voltage, as well as to provide a steady DC output voltage. This thesis investigates various active three-phase rectifier and control topologies and identifies a rectifier and control topology most suitable for use in converting a variable voltage variable frequency generator output to a DC voltage. In this dissertation, design relations are derived for determining the plant transfer response (for the suitable topology/controller), design equations are derived for designing/choosing the filter components, and guidelines are derived that will assist in choosing the right semi-conductor components and to give an estimation of expected system efficiency. The dissertation investigates the implementation of both analogue and digital control and provides implementation methodologies for both controllers. Expected results are verified by simulation and a build-up prototype. It was shown that the VIENNA rectifier is able to convert a generator type input, with variable input voltage amplitude and variable frequency, to a constant DC-bus voltage whilst controlling the input current to be sinusoidal and in phase with the input voltage. The rectifier was able to maintain a constant DC voltage at the output for input voltages as low as half the rated input voltage and for an equivalent output power of half the rated output power. This suggests that the VIENNA rectifier, controlled as a dual-boost rectifier, is suitable for applications that require power factor corrections and simultaneously operate from a wide input voltage range. / Dissertation (MEng (Electrical Engineering))--University of Pretoria, 2007. / Electrical, Electronic and Computer Engineering / MEng / unrestricted

This diploma thesis deals with elektrical power quality in DC grids. Electrical power is commodity and therefore it must be represented not just with quantitative, but also with qualitative parameters. Nowadays, there is an increasing demand for bigger implementation of renewable energy sources and more efficient power systems, which motivates installation of DC grids. However, development of DC grids is decelerated by lack of standardization alson in power quality. The work includes definition of power quality parameters and their measurement methodology.

This thesis focuses on the design of an electrical drive system for the purpose of high precision motion control. A modern electrical drive is usually equipped with a current regulated voltage source along with powerful motion controller system utilizing one or more micro-controllers and/or digital signal processors (DSPs). That is, the motor drive control is mostly performed by a dedicated digital-motion controller system. Such a motor drive mostly interfaces with its host processor via various serial communication protocols such as Profibus, CAN+, RS-485 etc. for the purpose of receiving commands and sending out important status/control signals. Considering that the motor drives lie at the heart of every (multi-axis) motion control system, the aim of this thesis is to explore the design and implementation of a conventional DC motor drive system suitable for most industrial applications that require precision and accuracy. To achieve this goal, various underlying control concepts and important implementation details are rigorously investigated in this study. A low power DC motor drive system with a power module, a current regulator and a motion controller is built and tested. Several design revisions on these subsystems are made so as to improve the overall performance of the drive system itself. Consequently, important &ldquo / know-how&rdquo / required for building high performance (and high power) DC motor drives is gained in this research.

This Master of Science thesis was written based on the shipbuilder Kockums AB feasibility study regarding the development of an All- Electric Ship for the Swedish Navy. The thesis was aiming at addressing voltage stability issues in a dc system fed by PWM rectifiers operating in parallel when supplying constant power loads. A basic computer model was developed for investigating the influence from various parameters on the system. It was shown that the voltage stability is dependent upon the ability to store energy in large capacitors. It was also shown that a voltage droop must be implemented maintaining load sharing within acceptable limits. Different cases of operation were modelled, faults were discussed, and the principal behaviour of the system during a short-circuit was investigated. It was shown that the short-circuit current is much more limited in this type of system in comparison to an ac system. It was concluded that more research and development regarding the components of the system must be performed.

The trend in power electronics converters is to be highly efficient, compact, reliable, and cost-effective. Conventionally, power converters supply or consume power from the low-voltage utility, which impacts the size and efficiency of the system. For example, the recent proliferation of electric vehicles has demanded the rapid installation of dc fast chargers (DCFC) across the country. However, most of the commercial DCFCs operate at 480 V transferring hundreds of kilowatts, resulting in large line currents which could hinder the size, cost, and efficiency of conventional DCFCs. Conversely, modular multilevel power topologies can be directly tied to the medium-voltage (MV) grid, eliminating the line-frequency transformer and the bulky line cable requirements. Among these topologies, the cascaded H-bridge (CHB) has been extensively used in the industry for MV-high-power applications because of its cost and efficiency in this operation range. Thus, it is one of the prevalent topologies for MV solid-state transformers (SSTs) The asymmetrical hybrid binary CHB (HBCHB) allows increased output voltage levels at the expense of modularity. Based on the HBCHB, a converter new modular topology regarded as the pseudo-binary CHB (PBCHB) is proposed for the ac-dc front-end stage SSTs. To operate the PBCHB, a new hybrid modulator is developed to operate the three modular structures of the PBCHB with step-like sinusoidal waveforms at near-line-frequency commutations while an asymmetrical floating capacitor (FC) module operates at high-frequency PWM commutation. The FC module does not transfer active power but serves only as a power quality enhancer of the PBCHB. However, the modular structures symmetrically transfer all the power from the MV grid to the load. With the SST structure of the PBCHB, the dc-link voltages of the H-bridges are naturally balanced; yet the proposed hybrid modulator enables equal power transfer in the three modular structures. In addition, a controller for the FC voltage is designed, analyzed, and implemented in the proposed hybrid modulator The effectiveness of the proposed front-end SST with the proposed modulation and control technique is verified in a 1.2 kV/3 kW single-phase prototype, where each module was able to transfer 1kW each. Electromagnetic interference (EMI) also impacts the cost, size, and reliability of three-phase systems because they may require bulky EMI filters to avoid self-pollution and polluting the grid. The common-mode voltage (CMV) is one important factor of EMI emissions. Thus, reducing or eliminating it could improve the cost and size of the system. Space vector pulsewidth modulation (SVWPM) can directly design the CMV output and the switching sequence of three-phase converters. However, its implementation can become complex in converters with many levels such as MV grid-tied SST converters. This dissertation uses the digital gh coordinate and proposes a set of computations to easily retrieve the converter states with a reduced CMV and generate a symmetrical switching sequence with reduced number of commutations. To do this, a single vector among the nearest three vectors (NTVs) is sufficient to implement the switching sequence for the reduced commutation and reduced CMV SVPWM. Additionally, the dc bus can be fully utilized. Unlike conventional approaches, the developed technique is easily scalable because its computational complexity does not depend on the number of levels of the converter. The proposed reduced CMV technique was verified in a three-phase 15-level 311 V/600 W unit. Moving forward in the objective of CMV reduction, a new jk-coordinate system for multilevel converters is proposed for SVPWM with eliminated CMV. With the jk coordinates, the converter states that yield zero CMV (ZCMV) can be directly computed. In addition, a single jk vector is sufficient to generate the switching sequences of NTVs. Moreover, the switching sequences feature reduced losses for high-power-factor applications in the phase that naturally commutes twice during a sampling period. Similarly, the computation burden of the ZCMV SVPWM technique presented in this dissertation is not affected by the number of levels of the converter, thus, it is scalable. The three-phase 15-level 311 V/600 W prototype was utilized to verify this technique. / Doctor of Philosophy / The recent demands for fast chargers for electric vehicles (EV), photovoltaic (PV) energy integration, and data centers for artificial intelligence (AI) have driven the research and development of efficient, compact, and cost-effective power electronic solutions. Under these motivations, the solid-state transformer (SST) is a power electronics configuration that can benefit the EV, PV, AI, and several other applications. By eliminating the requirement of a line-frequency transformer, SSTs can be directly connected to the medium-voltage (MV) grid, reducing the weight and volume, and improving efficiency. The main reason for these advantageous attributes is the utilization of multilevel ac/dc or dc/ac converters. Among these, the cascaded H-bridge (CHB) converter has been extensively used in the industry for MV-high-power applications because of its cost, fault tolerance, and efficiency, making it a favorable converter for MV SSTs. Symmetrical modules in the CHB must commutate at the same pulsewidth modulation (PWM) when operating in an SST. An asymmetrical configuration such as the hybrid binary CHB (HBCHB) allows increased output voltage levels and low-frequency commutation at the expense of modularity. This dissertation proposes a pseudo-binary CHB (PBCHB) inspired by the HBCHB to obtain low-frequency commutations, thus, negligible switching losses in the SST. The PBCHB has symmetrical modules that transfer balanced active power with negligible switching losses while an asymmetrically smaller module enhances the power quality with PWM operation. To do this, a new hybrid modulator and controller were designed, analyzed, and verified in this dissertation. The effectiveness of the proposed front-end PBCHB-based SST with the developed modulation and control techniques is verified in an MV 1.2 kV/3 kW single-phase prototype. Electromagnetic interference (EMI) filters can impact the cost, size, and reliability of SSTs. The common-mode voltage (CMV) that power converters generate is one type of EMI emissions that could impact the cost and size of the system. The modulation technique called space vector pulsewidth modulation (SVWPM) has the freedom to design a switching sequence able to reduce or eliminate the CMV. However, implementing the SVPWM can become complex in MV grid-tied SST converters (PBCHB, CHB, HBCHB) with many voltage levels. This dissertation uses the digital gh coordinate system and a new jk coordinate system to reduce and eliminate the CMV, respectively. These coordinates systems have the advantage of reduced computational complexity in multilevel converters with large number of output voltage levels increases. The proposed techniques can retrieve back the abc signals for the PWM drivers without repetitive iterations. Moreover, the proposed techniques can generate symmetrical switching sequences with reduced number of commutations and switching losses in the converter. To do this, the computation of a single vector among the nearest three vectors is sufficient to implement the switching sequences of SVPWM. As a result, the computational complexity of the SVPWM techniques in this dissertation is constant and does not vary with the number of output voltage levels, making them easily scalable solutions compared to previous solutions in the literature. The proposed reduced and eliminated CMV SVPWM techniques were verified in a three-phase 15-level 311 V/600 W HBCHB unit in inverting mode.

This thesis proposes a new bidirectional three-phase AC-DC power converter using matrix converter and Z-source inverter topologies. Advantages of the AC-DC matrix converter are the inherently controllable power factor, the tight DC voltage regulation, the wide bandwidth with quick response to load variation, the single-stage buck-voltage AC-to-DC power conversion; advantages of the z-source inverter are the increased reliability by allowing the shoot-through between upper and lower power switches of one inverter leg, insensitivity to DC bus voltage due to the extra freedom of controlling DC-link voltage. The proposed Matrix-Z-source converter (MZC) marries up both advantages of AC-DC matrix converter and Z-source inverter. It can achieve voltage-boost DC-AC inversion capable of variable voltage variable frequency (VVVF) AC output; it can achieve voltage-buck AC-DC rectification capable of inherent control over AC current phase angle and DC output regulation with a (VVVF) AC source supply. Both foresaid performance in DC-AC inversion and AC-DC rectification can be implemented in a simple open-loop control manner. Three constraints of VSI, in the bidirectional AC-DC power conversion, are the peak AC voltages are always less than DC-link voltage, closed-loop control has to be employed when DC regulation and/or AC current phase angle control are required, and AC voltage is sensitive to the variation of the DC-link voltage in DC-AC inversion. The voltage-boost inversion and/or voltage-buck rectification of MZC overcomes the first constraint; thus MZC enables the AC machine voltage increased higher than DC-link voltage hence advantages of running AC machine at relatively high voltages are enabled. The direct DC voltage regulation and inherent AC-current-phase-angle control of MZC overcomes the second constraint in an open-loop manner; hence a simplified system design is obtained with sufficient room for the further improvement by closed-loop control schemes. The extra freedom in controlling DC-link voltage of MZC overcomes the third constraint hence a DC source voltage adaptable inverter is obtained. This thesis focuses on the study of the feasibility of the proposed MZC through theoretical analysis and experimental verification. At first, the proposed MZC is conceptually constructed by examining the quadrant operation of AC-DC matrix converter and Z-source inverter. After the examination of the operating principles of both AC-DC matrix converter and Z-source inverter, the configuration of MZC is then proposed. The MZC has two operating modes: DC-AC inversion and AC-DC rectification. Circuit analysis for both operating modes shows that the new topology does not impose critical conflict in circuit design or extra restriction in parameterization. On the contrary, one version of the proposed MZC can make full advantage of Z-source network components in both operating modes, i.e. a pair of Z-source inductor and capacitor can be used as low-pass filter in AC-DC rectification. The modulation strategy, average modeling of system, and features of critical variables for circuit design of the proposed MZC were examined for each operating mode. Simulations of the proposed MZC and its experimental verification have been presented. Analytical models of conduction and switching losses of the power-switch network in different operating mode have shown that the losses in the MZC compare favorably with conventional VSI for a range of power factor and modulation indices.

No description available.

The installation of wind power plant has significantly increased since several years due to the recent necessity of creating renewable and clean energy sources. Before the accomplishment of a wind power project many pre-studies are required in order to verify the possibility of integrating a wind power plant in the electrical network. The creation of models in different software and their simulation can bring the insurance of a secure operation that meets the numerous requirements imposed by the electrical system. Hence, this Master thesis work consists in the creation of a wind turbine model. This model represents the turbines installed at Lillgrund wind farm, the biggest wind power plant in Sweden. The objectives of this project are to first develop an accurate model of the wind turbines installed at Lillgrund wind farm and further to use it in different kinds of simulations. Those simulations test the wind turbine operating according to different control modes. Also, a power quality analysis is carried out studying in particular two power quality phenomena, namely, the response to voltage sags and the harmonic distortion. The model is created in the software PSCAD that enables the dynamic and static simulations of electromagnetic and electromechanical systems. The model of the wind turbine contains the electrical machine, the power electronics (converters), and the controls of the wind turbine. Especially, three different control modes, e.g., voltage control, reactive power control and power factor control, are implemented, tested and compared. The model is tested according to different cases of voltage sag and the study verifies the fault-ride through capability of the turbine. Moreover, a harmonics analysis is done. Eventually the work concludes about two power quality parameters.