1,721,285 research outputs found
Medium Voltage Impedance and Admittance Measurement and System Identification Techniques
No full textThe recent trend of an increasing share of renewable energy sources in modern power systems, as
well as the integration of power electronics equipment, is shaping the requirements for stable grid
infrastructure. These requirements mainly come from the stability-related phenomena arising from
different subsystem interactions. Namely, medium voltage systems require special attention due
to the lack of equipment and solutions intended for their impedance measurement. Hence, this
thesis provides the perception of the problem of medium voltage impedance measurement and system
identification from the point of view of perturbation injection converters. A cascaded H-bridge
the converter supplied from a medium-voltage multi-winding phase-shifting transformer is proposed
in this thesis. Moreover, in combination with wideband injection signal, it imposes itself as a viable
solution to this problem.
The thesis commences with an overview of readily available converter topologies and injection
signals. The converter topologies are discussed, their downsides are pointed out and it is outlined
how these issues can be addressed by using the cascaded H-bridge topology. Additionally, the lack of
flexibility of the state-of-the-art solutions is highlighted.
Furthermore, the modelÄ ing approach of the single power electronics building block of the cascaded
H-bridge converter is presented. Initially, the open-loop control model in the dq-frame is presented.
The dq-frame modelling approach is adopted for both the three-phase active front end and for the
single-phase H-bridge inverter. An estimation of the source-load affected dynamics is provided
on the basis of the closed-loop control modelling. This question needed to be answered out of the
concern for the interaction between the active front end and the H-bridge inverter. For efficient
perturbation injection, the active front end should not limit the output dynamics of the H-bridge
inverter. Real-time simulations in combination with additional single-phase dq-frame measurement
and identification methods revealed that there is in fact very little to no influence between two
sub-converters. Moreover, the terminal characteristics of the active front end, i.e. its input admittance
and output impedance are measured in an experimental setup, providing the first result in the full
experimental verification of the notions proposed.
The hardware and control designs of the active front end are subsequently verified through the power
circulation tests in a setup including two active front ends and the medium voltage multi-winding
transformer. On one side, the verification is made possible due to the transformer primary side
synchronization method, effectively alleviating the need for the filter part of the active front end.
Namely, the transformer leakage inductances are used as the filter inductors. On the other side, the
control verification is performed on the basis of an industrial control system required owing to the
size and complexity of the full-scale converter.
The flexibility issue of the medium voltage perturbation injection converters is addressed through the
hardware and control reconfiguration of the CHB. The ac converter is reconfigured
for dc operation with three different modes of operation possible, depending on the desired voltage
level. The ideas behind the converter flexibility are demonstrated through the simulations covering
the measurement of the terminal characteristics of the MMC.PE
Modeling and Design Optimization of Medium Frequency Transformers for Medium-Voltage High-Power Converters
No full textNovel high-power medium-voltage converter technologies offering galvanic insulation are needed to support the development of the emerging medium-voltage direct-current grids and further improve the performance of various other applications such as traction, renewable energy and e-mobility. With the recent advancements in the power semiconductor industry, resulting in faster and more efficient switches with extended voltage and current capabilities, these converters, often referred to as solid state transformers (SSTs) or power electronic transformers (PETs), have become increasingly attractive.
Besides the power semiconductor modules, the central component of any such converter, having a significant impact on its efficiency and power density, is the medium frequency transformer (MFT) that provides the necessary galvanic insulation and input-output voltage matching. However, the progress of the magnetic components has not been following the same pace as the semiconductor industry. Unlike the traditional line frequency transformers (LFTs), MFTs have not yet reached the technological maturity, thus leaving many areas open for research.
To that end, this thesis focuses on the technical challenges tied to modeling and design optimization of the MFTs for the emerging SSTs. The available technologies and materials, suitable for medium frequency operation are identified and classified in respect to different application requirements. A detailed analysis and modeling of all the relevant phenomena governing the MFT electrical behavior as well as limiting the operation and design range is performed. A synthesis of all these models is done in form of a design optimization algorithm capable of generating the set of all feasible transformer designs. Moreover, design filters are developed allowing to interactively search for the most preferable design alternatives in terms of hot-spot temperatures, weight, volume and efficiency. As a proof of concept, a 100kW, 10kHz MFT prototype has been realized according to the optimal specifications resulting from the proposed design optimization tool. The accuracy of the utilized models was confirmed via thorough testing, including: electric parameter identification, partial-discharge test, full-power loading test and an extensive thermal run within the realized back-to-back resonant test setup. With the established design methodology and reliable models, a technology coordination and an MFT design sensitivity study has been performed on the SST level. A 0.5MW, 10kV input-series output-parallel series resonant converter based topology has been selected for the case study, taking into account different available semiconductor ratings and the resulting converter modularity. This study has shown that, considering the available technologies and materials, the expected MFT power density reaches its apex at around 10 â 20kHz. While modern wide-band-gap semiconductors will for sure increase the efficiency and power density of the converter stage, this result indicates that further size reduction of the magnetic components above these frequencies will only be possible through improvements in the materials - providing core and winding materials with better high frequency loss characteristic.PE
Multi-scale and Multi-physics Modeling of Electrochemical CO2 Reduction
No full textThe electrochemical CO2 reduction is a promising approach to convert this greenhouse gas to valuable products. The membrane electrode assembly (MEA) configuration integrates porous gas diffusion electrodes (GDEs) where the CO2 reduction reaction rapidly occurs thanks to the large surface area and the easy transport of CO2. The MEA electrolyzer also mitigates the ohmic potential loss in the device. Therefore, it becomes a potential device for industrial CO2 electrolysis. Nevertheless, the coupled multi-physical transport-reaction process in porous electrodes and the device is complex and experimentally not directly quantifiable. Additionally, there are various parameters in terms of operation, material, and design which require optimization. A comprehensive multi-scale and multi-physics study is therefore required to aid in the understanding.
In this thesis, the reaction environment was analysed by a pore-level model featuring a liquid-solid interface. Transport of species was resolved around catalyst surface including the electric double layer. The model allows for demonstration of the transport inside the Nernst diffusion layer and electric field distribution around the double layer, making it a key step forward on the path on understanding the local reaction environment in electrochemical CO2 reduction.
In terms of the device level study, a one-dimensional homogenized model for CO2 reduction to CO on an Ag-based GDE was firstly developed. The model was then extended to ethanol and ethylene joint production by a Cu-based catalyst with experimental validation. This model accounts for the gas phase for the delivery of reactant and product, the liquid phase for the transport of dissolved species and support the electrochemical reaction, and the solid phase for the transport of electrons. The dissolution of CO2 into electrolyte, CO2 reduction, H2 evolution and the reactions in aqueous solution are involved. The results demonstrated the influence of operation on CO2 concentration and local current density, which provided guidance on how to obtain fast production rate and high selectivity.
In addition, a two-dimensional model was developed for MEA on full device level for the performance prediction and formulation of design guidance. A combined computational and experimental study on the MEA CO2 electrolyzer with nickel-based single-site catalyst was performed. The continuum model was developed utilizing the kinetic parameters obtained from H-cell experiment, the geometrical characteristics and transport properties obtained by direct pore level simulations on catalyst meso-structure from Nano-tomography. This continuum model was compared with the experimental MEA measurements performed independently. The validated model was applied for extensive parameter studies, which aided in design guidelines, diagnosed the experimental operation, and provided insight on the heterogeneities. Lastly, a catalyst loading study was performed to analyse the mass transport limitation inside the catalyst layer and optimize the layer thickness.
The thesis will benefit the understanding of coupled multi-physics transport-reaction phenomenon in CO2 reduction devices on both pore scale and macroscopic scale, which is difficult to characterize and quantify via experiments. The optimization of operation parameters, materials, and geometry will contribute to the development of practical and scalable electrochemical CO2 reduction devices.PE
Galvanically Isolated Modular Converter
Full text linkThe ever increasing penetration of renewable energy systems in the distribution grids will inevitably require profound changes in the grid infrastructure. One emerging direction, the medium voltage dc (MVdc) grids, is the cornerstone of this work. They are foreseen for offshore wind parks and onshore large scale renewables collection grids, future on-board ship power systems, repurposed ac lines with increased transport capacity, etc. The shift to dc offers energy savings and reduced impact on the landscape. This thesis provides insights on the presumed most suitable conversion topology between a MVdc and a LVac grid, which is not expected to disappear in the near future. The conversion is characterized by a large voltage ratio between the two terminals. A modular multilevel converter (MMC) with an integrated magnetic component, the galvanically isolated modular converter (GIMC), is proposed and preferred for efficiency and cost reasons over a solid-state transformer, whose efficiency is heavily penalized by the inverter stage on the low voltage side.
The thesis opens on a detailed benchmark of the performances of various control, modulation and branch balancing methods, with a focus on medium voltage applications. Extensive simulations are carried out to support the discussion. In case of an application with fast dynamics, the closed-loop control method, comprising energy balancing controllers, offers by far the best performances. For the modulation and branch balancing methods, it was concluded that, as long as both the number of cells per branch and average cell switching frequency are not very low, PWM methods based on the reference branch voltage, rather than the number of inserted cells per branch, feature lower voltage errors.
From there, the two GIMC variants sharing the same three-windings transformer, the interleaved GIMC and stacked GIMC, are analyzed. This solution does not suffer from dc bias in the magnetic device, unlike the open-end windings MMC. Since the obtained model is identical to the one for the conventional dc/3-ac MMC, the same control algorithms can be applied without restriction. The volume and efficiency comparison against the conventional case with discrete air-core inductors, supported by FEM simulations, quantifies the benefits of the proposal. It is concluded that the gains are marginal for the considered modest power ratings (0.5 MVA). However, the magnetic design is considerably simplified. Larger gains are expected for designs with higher branch inductance values, since the stacking of the primary windings gives easy access to high leakage inductances.
A generic and versatile losses estimation method for modular converters is introduced. Compared to detailed switched simulations, the simulation times are improved by two orders of magnitude, if the assumptions to neglect the branch current ripple and branch capacitor voltage spread hold. The estimation error is below 2 % in the considered comparison.
At last, the design of a 0.5 MVA converter prototype connected to 10 kVdc with 96 cells is presented. The cell, with a dedicated Flyback-based auxiliary cell power supply from its dc-link and protection circuits for a cell bypass in case of over-current or -voltage, along with the electric design of the cabinet hosting one converter phase-leg, are verified experimentally. The cell and phase-leg layout provide a platform for further research activities.PE
High frequency IGCT operation for DC transformer
Full text linkIn spite of the dominance of ac technology for the vast majority of power transmission and distribution
since the late 19th century, the past decades have seen an increase in use of dc electrical power. In
particular, this has been the case both in high voltage transmission, and in low voltage islanded dc
grids such as shipboard power distributions systems, or dc buildings. The increase in interest for
dc electrical power is mainly due to its overall decreased losses compared to its ac counterparts,
increased flexibility, and ability to more easily integrate renewable generation and energy storage. In
this context, medium voltage dc grids currently lack in standardisation and are still an active research
topic.
The dc transformer is expected to be a key technology for the operation of future medium voltage
dc systems. Essentially, its function is equivalent to the traditional ac transformer in providing
an isolated interface between dc buses at different voltage levels. Yet, unlike the purely passive
traditional transformer, the dc transformer is also expected to integrate additional functionality,
particularly regarding system protection. Most embodiments of the dc transformer proposed in
academic publications are based on the dual active bridge, with IGBTs being the semiconductor of
choice and with galvanic isolation provided by a medium frequency transformer. Compared to this
solution, alternative technologies have been somewhat overlooked, in terms of topologies and devices.
In particular, resonant conversion for dc transformer applications has not gained much popularity.
The focus of this thesis is on a medium voltage dc transformer employing IGCTs as semiconductor
devices, in a bidirectional series resonant LLC topology. The principle behind the selection of this
topology and device is the synergy between the IGCT, which contributes the lowest conduction losses
of any actively controlled semiconductor switch, and the series resonant LLC converter principle of
operation, which provides low switching loss through soft turn-on and low current turn-off. With this
goal in mind, a significant technical challenge to be overcome is the increase of switching frequency
of the IGCT well beyond the sub-kHz level at which it traditionally finds application, and into the
multi-kHz range, targeted by dc transformer applications.
This thesis contains three main contributions aiming to acquire sufficient knowledge for the design
of dc transformer lab demonstrator. For this purpose, the boundary between zero-voltage and zerocurrent
switching of the IGCT is initially explored to identify the lowest switching loss condition
through the variation of turn-off current value. Then, in these low-loss conditions, thermal steady
state operation of the IGCT is demonstrated at the frequency of 5 kHz for the first time, proving
that the IGCT is a device capable of medium frequency operation. Engineering samples of IGCTs
optimised on the technology curve through varying levels of electron irradiation are also explored
in order to quantify potential benefits in the dc transformer application. Finally, medium frequency
operation of the device is extended to an increased voltage level through series connection of IGCTs,
through custom ultra-low capacitance, purely capacitive snubbers, designed for the challenges of
5 kHz operation.
Ultimately, the thesis demonstrates that while the IGCT has traditionally found use in sub-kHz,
hard-switched applications, its ruggePE
Modeling and optimization of PCB coils for inductive power transfer systems
No full textThe auxiliary power supply for medium voltage converters requires high insulation capability between the source and the load. Inductive power transfer technology, with an air gap between the primary and secondary coil, offers such high insulation capability, making it a potential candidate for auxiliary power supply in medium voltage converters. However, the large air gap between the primary and secondary coil typically results in a loosely coupled inductive power transfer system, necessitating optimization of the inductive power transfer system to achieve high efficiency and power density. This thesis focuses on the coil link optimization. It introduces a novel design of the coil link structure, models the coil pairs, and presents an optimization flow to design optimal winding geometry based on given electric specifications. The research of this thesis revolves around PCB coils, which are favored for their easily controlled manufacturing process. To maintain modularity in medium voltage converters, the inductive power transfer system consists of multiple coil pairs, each supplying one power electronics building block. The primary side of each coil pair is connected to a common power source. To ensure independent operation of the secondary coils without closed-loop control, the LCL-S compensation network is utilized. The advantage of this compensation network is analyzed and its design process is discussed in this thesis. The characteristics of a coil, including self and mutual inductance and coil losses, are defined by its winding geometry. Therefore, by optimizing the winding geometry, high coil link efficiency can be achieved. This thesis develops a model to calculate the magnetic field inside each winding turn when no ferrite is present behind the winding. The brute-force optimization is conducted on the PCB coil pairs with the proposed model, resulting in a Pareto front showcasing the trade-off between high efficiency and high power density. When ferrite is added to the backside of a winding, it alters its inductance and resistance, and therefore has the potential to increase power transfer efficiency. However, the modeling of the coil pair becomes more complex due to the crowding field around ferrite edges. In this thesis, different ferrite shapes are compared, and a magnetic model with 5 mm thick round shape ferrite is proposed, based on a database from finite element simulation together with an artificial neural network, predicting the coil pair characteristics. The validity of the finite element simulation data is pre-verified with impedance analyzer and power tests, and the accuracy of the magnetic model is confirmed with simulation test datasets and characteristic tests on one coil prototype. The design of the coil link requires thermal coordination considering the temperature limit of each component inside coils. This thesis proposes a thermal model to predict the temperature rise in coil pairs. The proposed electric circuit model with LCL-S compensation network, the magnetic model based on artificial neural network, and the thermal model based on thermal network are all independent from external simulations and easily integrated into the optimization flow, ensuring fast optimization time. After exploring all degrees of freedom of winding geometries, one coil pair on the Pareto front is selected and tested under various load conditions.PE
IGCT Based Solid State Resonant Conversion
No full textDriven by the environmental changes, penetration of renewables and the need for more flexible power grid, DC is reemerging as a basis for future power networks. Already established in the area of HVDC for very long distance transmission, more and more attention is drawn to DC for distribution purposes, especially MVDC collection grids for offshore wind farms, solar parks, ship distribution. To realize such a system, safe, reliable and efficient interconnection of different voltage levels within the power network has to be achieved. Considering that the traditional AC transformer cannot be used for this function, power electronics based DC-DC converters must take over, opening a wide area of research focused around the idea of the DC transformer. Given the state of the art of DC-DC topologies, LLC-SRC was chosen for testing the viability of application of the IGCT to meet the high efficiency and reliability demands. The potential of these heavy duty devices was highly overlooked due to the popularity of the IGBT. Sub-resonant operation of the LLC-SRC was analyzed and advantages of operation are outlined. Zero voltage turn-on, low current turn-off and the clamp-less operation with IGCT is theorized promising the high efficiency of the final converter. Simplified design guidelines are summarized as a result of the theoretical analysis providing a quick tool for the future designer to use. Based on these guidelines, a physical test setup was built for multifunctional purposes, capable of testing single and series connected IGCTs under hard-switching and resonant operating conditions. Furthermore, it was designed to operate in the continuous steady state simulating the LLC-SRC operation of the IGCT half-bridge. As the low current hard-switching data is not always available in the datasheet of the IGCT, these tests were performed first. TCAD simulation was initially used to gain understanding into the turn-off behavior followed by the experiment on the test setup. Single- and double-pulse tests were performed and data was summarized for both turn-on and turn-off process. The results served as a baseline for comparison with the resonant based tests and determine if the switching behavior is different for the two switching operation types. The developed tests provided invaluable insight into the switching operation of the IGCT under resonant conditions. Single resonant pulse test was used to observe the turn-off behavior including transient duration, switching losses estimation and over-voltage. The initial results were used to clearly define the required dead-time for the next set of pulse testing - double resonant pulse testing. This test proved the viability of the IGCT operation under the zero-voltage turn-on and switching without the clamp circuit. Finally, the continuous operation of the test setup was demonstrated with IGCTs switching in resonant operation mode. Relatively high switching frequency was achieved for this type of MV voltage switches, mostly used in hard-switching applications with switching frequencies of up to 900Hz. Exclusion of the clamp circuit was justified and practically proved without any damage to the IGCTs under test. Proper selection of the dead-time in the switching bridge ensured zero voltage turn-on of the switching devices with further decrease in switching losses achieved by low current turn-off. High conversion efficiency of the half-bridge was reached for the nominal operating conditions defined byPE
Modeling, Optimization and Design of High Power Medium Frequency Transformers
No full textNowadays, direct current (DC) technology is increasing its presence in alternating current (AC) power systems. This trend is enabled by the progress in energy conversion through semiconductor devices and power electronics. Therefore, it is possible to imagine DC power distribution networks in future energy systems. However, in order to enable this evolution, a highly efficient, reliable and compact conversion principle / device is needed which will be used for DC power transformation. Research of new conversion topologies has been increased recently through concepts of solid-state transformers (SSTs). The core stage of this topology is the medium frequency transformer (MFT), since it provides the galvanic isolation. So far, many designs have been proposed, followed by developed prototypes and using different core materials (silicon steel, nanocrystalline, ferrites) in combination with different types of conductors (Litz wire, foil, hollow) and insulation materials (air, oil, water, solid). One of the tasks of the thesis was to develop tools for MFT design and optimization which are experimentally verified on a laboratory scale prototype of the device. Thereby, the broad design space is explored together with interdependencies between magnetic and insulation materials used for the implementation of high-voltage high-power MFTs. At the same time, characterization tests are carried out in order to determine core, dielectric, conduction losses as well as thermal properties of such devices.PE
MMC-based conversion for MVDC applications
No full textIn recent years, DC grids have gained momentum due to the advantages they display over their AC counterparts. For instance, high voltage DC grids have been established as the most prominent solution in the underwater power transmission or interconnection of asynchronous systems. Consequently, the success of high voltage DC grids is expected to be replicated in the medium voltage domain. However, medium voltage DC grids have not been standardized yet, leaving the space for various research topics.
To realize a flexible and resilient DC grid, dependable connections among its parts operating under different voltage levels must be ensured. To put it differently, the DC-DC converter represents a keystone of any DC grid, operating at an arbitrary voltage level. Owing to the outstanding flexibility, scalability, and availability, the modular multilevel converter established itself in a vast variety of applications. Hence, the possibilities it offers in the high/medium voltage domain deserve to be further inspected.
This thesis is divided into two parts. The first part concerns the modular multilevel converter operating as the DC-AC converter. The presented modeling approach, implying the averaging of the converter equations on the branch level, provided the grounds for the control-related discussions. The control of terminal currents was elaborated in detail and supported by a thorough analysis and comparison of the methods employed to maintain the proper converter energy distribution. Despite the theoretically unlimited voltage scalability, the modular multilevel converter power extension through the current capacity boost represents a significant technical challenge being comprehensively addressed throughout this thesis. It was shown that by paralleling the converter branches, the current, and implicitly the power, capacity of the converter can be increased while requiring no hardware adaptations of the existing parts. Moreover, branch paralleling offers the possibility to improve the spectral content of voltage at either of the converter terminals.
The second part of this thesis refers to the employment of the modular multilevel converter in the domain of the DC-DC conversion. A novel, high power, bidirectional, DC-DC converter, utilizing the Scott transformer connection to provide galvanic separation between two of its ports, was proposed. It features the possibility of interconnecting a bipolar DC grid of high, or medium, voltage level with another DC grid of an arbitrary voltage level. As the Scott transformer connection comprises two separate transformer units, the proposed topology ensures that the operation can be maintained even in case either of the bipolar grid DC feeders is lost. A set of minor modifications to the above-mentioned topology leads to the structure providing the means for unidirectional energy flow. Nevertheless, control principles for both of the proposed solutions were derived and confirmed through the set of detailed simulations. Additionally, the unprecedented shift of the Scott transformer connection operating frequency towards the medium frequency range was proposed.
A comprehensive analysis of the so-called quasi two-level converter leg was provided in the last part of this thesis. Quasi two-level operating principles imply the sequential and smooth transition of the converter leg AC voltage from one polarity to another. However, the transition occurs in the discrete time steps, being referred to asPE
Condition Health Monitoring for Medium-Voltage High-Power Modular Multilevel Converter
No full textAmong the policies to reduce the consequences of GHG emissions, the decarbonization of the electrical, transport, and manufacturing sectors has profoundly changed how the electricity is produced, transmitted, distributed, and consumed, particularly imposing several complex technical challenges to power electronics-based technologies. Future power converter solutions are called to fulfill increasingly demanding requirements of efficiency, power quality, cost, volume, and reliability, foreseeing that the next generation of high-power converters might be based on modern concepts such as the MMC.
Although the MMC exhibits extraordinary characteristics, namely, full modularity, voltage-current scalability, and high efficiency, its development is still in an early stage of maturity, and different topics such as reliability improvement are still research subjects. Around 40000 parts composing the 96 SMs used in the two MV 250kVA MMC to form the PEL MMC research platform illustrates the reasonable concern about technology reliability. CHM arises as an attractive concept to prevent failure events improving reliability and availability of power electronics-based converters. When intended for MMC, CHM development and deployment face several challenges, mainly due to the existing technique's demand for additional hardware, modification of control schemes, particular operating conditions, and complex algorithms. Consequently, MMC CHM state-of-the-art is narrowed to SM power capacitors and power semiconductors.
This thesis aims to provoke and contribute to the CHM body of knowledge by investigating three alternatives applied to the PEL MV MMC. The first is a simple, accurate, and costless online strategy to monitor the SM electrolytic capacitor's condition. The method basis on the relationship between the capacitor's degradation and capacitance level. Thus, the SM capacitance is estimated by employing measurements commonly used for necessary control and protection algorithms and the RWLS technique. RT-HIL simulations and experimental results show the method's performance under different capacitor aging levels and converter operating conditions.
A second alternative responds to the difficulties in gathering information about components other than power capacitors and power semiconductors, as cost, space, and data processing restrictions are common barriers. The strategy uses the single switch flyback-based ASPS consumption monitoring to observe simultaneously different SM sub-circuits instead of a particular component and the fault dictionary concept to detect the SM drifting from expected healthy conditions. It is demonstrated that minor and significant power consumption variations can be noticed and used for health monitoring.
Envisioned in future MMC, the handling, processing and extraction of valuable information from massive amounts of data coming from different CHM techniques is an open issue and certainly a challenge for the health assessment. Inspired by the data fusion framework and MCDM problems, a scheme to integrate various SM health indicators is proposed to compose a comprehensive health index. Systematic approaches considering objective data from the SM and subjective information from expert knowledge are presented and verified through numerical examples based on experimental data. It is demonstrated that entropy, fuzzy-TOPSIS, and game theory-based methods are superior solutions for the SM and converter-level health assessment.PE
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