This thesis investigates the design, modeling, and testing requirements of Medium Frequency Transformers (MFTs) for Solid State Transformer (SST) applications, with a focus on insulation coordination, thermal performance, and compliance with international standards. SSTs are gaining traction as a next-generation solution for power distribution, offering size and weight reduction, galvanic isolation, and seamless integration with renewable and bidirectional power systems. Central to their viability is the MFT, which must withstand high-frequency electrical and thermal stresses.

A 25kW, three-phase MFT operating at 1kHz was designed using copper windings and a Metglas core, achieving 98% theoretical efficiency and a target mass of 50kg. Detailed electromagnetic and thermal models were developed and validated through experimental testing on a single-phase air-core prototype, assessing insulation withstand, partial discharge behavior, and thermal rise.
The thesis also evaluates IEC 60076-11 as a baseline for MFT testing. While many procedures remain relevant, modifications are necessary to reflect high-frequency effects and pulsed excitation typical of SST systems. Recommendations are provided for adapting key tests to SST environments. This work contributes to bridging the gap between traditional transformer practices and the emerging demands of solid-state-based power conversion, supporting the future standardization and deployment of SST-compatible MFTs.

Many overhead transmission lines in the Netherlands are multi-circuited; when one circuit is de-energized for maintenance, the other remains energized. The energy transition necessitates more transmission line reinforcements and heavier loading of existing lines to meet the increasing power demand, consequently increasing the need for additional linework. The energized circuit induces a voltage (EMF) and current in the circuit being worked on. As a further safety measure, line workers utilize a portable earthing device (PED) that is attached to the de-energized circuit’s conductors. The induced voltage and current in the circuit can cause arcs up to half a meter long when attaching or detaching the PED, posing a serious threat to workers’ safety. There are no active mechanisms in place to stop the arc, so it must naturally extinguish in air.

This thesis aims to model and characterize naturally extinguishing arcs in air. There is little available literature on this topic, which primarily consists of models for static free-burning arcs in air. There are two key objectives in this research: (1) develop a mathematical expression describing the behaviors of naturally extinguishing arcs in air and (2) use the derived expression to produce an arc model simulation verifying the expression’s validity and accuracy. The provided test data, including arc videos and oscilloscope data, encompassed arcs under various testing conditions and utilized a test setup that mimicked the arcing environment of a PED being detached from a conductor. The arc distances were approximated using MATLAB image processing techniques due to the videos’ poor quality in showcasing the arc distances. These approximated arc distances and oscilloscope data were examined and manipulated to develop an overarching arc expression that represents the resistance-current relationship evident in free-burning arcs in air. Two expressions were derived for comparison: a logarithmic expression and a power law expression. These expressions were implemented in two separate ATP-EMTP software models that each utilized a MODELS-language module to represent the arc for simulation.

The resulting resistance and voltage-current characteristics of the two models were compared against the original test data. The power law expression was found to be the more accurate of the two models. Different testing conditions were then applied to the arc model to identify their natural arc extinction distances, which were also compared against the original test data. The results establish that the power law expression arc model sufficiently predicts the natural arc extinction distance. These predictions can be improved in the future with more precise test data. This thesis’s research fills a gap in the existing literature on modeling natural arc extinction in air. The findings of this research can influence future safety procedures for the line workers who experience these arcs. Furthermore, the results of the arc model for PED removal can potentially be expanded to describe the arcing behavior of disconnectors during bus transfer assessment, leading to a greater understanding of disconnectors and providing a deeper insight into their application and test result interpretations.

This thesis presents the development of a frequency-dependent model for metal oxide surge arresters (MOSAs) to better understand their behaviour under temporary overvoltages with a high harmonic content. This topic has gained importance due to the increasing use of long AC cables in connecting offshore wind farms to the transmission grid, which introduces a high capacitance to the grid and raises the risk of harmonic resonances. These resonances can lead to sustained temporary overvoltages, making accurate arrester modelling essential for selecting appropriate surge arresters to withstand these conditions.

Surge arresters are critical components for protecting high-voltage equipment from overvoltage events. The main objective of this work was to investigate the response of the arrester under both AC and DC voltages at a variety of frequencies, with particular focus on the low-current region of the characteristic voltage-current (V-I) curve.

To achieve this, High-voltage AC and DC tests were performed on a surge arrester block. The measured data were used to extract the resistive and capacitive components of the current, as well as to calculate the power losses. These results formed the basis for developing a four-branch nonlinear model. Curve-fitting techniques and various algorithms in MATLAB simulations were used to determine the model parameters, which were then validated against the experimental data. The model demonstrated good accuracy for peak voltages up to 9 kV and showed clear frequency-dependent behaviour, particularly in the 50 to 500 Hz range.

Overall, the final model aligned well with the RMS values of total current as well as the average power losses, with only minor deviations from the measured results. This work contributes to improving surge arrester modelling and lays the groundwork for further developments in frequency-dependent modelling within simulation tools such as ATP-EMTP.

The world is currently undergoing an unprecedented energy transition from fossil fuel-based energy sources to renewable energy sources. Additionally, advancements in power electronics technology have made High Voltage DC (HVDC) the most cost-effective method for long-distance electrical power transmission over both sea and land. These developments have led to an increased presence of Power Electronic Interfaces (PEIs) in the electricity grid. The insulation of High Voltage (HV) equipment in the electrical power grid is subjected to various electrical stresses due to the solid-state switching of PEIs. This can weaken the dielectric material of the HV equipment and therefore reduce the reliability of the electrical power system. Conventional test sources cannot replicate complex waveforms. Therefore, it is necessary to develop an Arbitrary Waveform Generator (AWG) capable of testing HV equipment with complex waveform shapes. A Modular Multilevel Converter (MMC) is a promising topology for this purpose, where each submodule (SM) requires auxiliary power to supply the Gate Drive Units (GDUs) and Distributed Control Units (DCUs). The objective of this thesis is to design the Auxiliary Power Supply (APS).

This thesis proposes using a flyback-based Input Series Converter as an APS. The proposed converter offers advantages such as integrated active Input Voltage Sharing (IVS), a scalable design, wide-input operation, and a minimal component count. The proposed converter topology is compared with other topologies discussed in the literature through SPICE simulations. Furthermore, various synchronous switch driving methodologies are studied and compared, given the necessity of series-connected switches with the currently available devices on the market.

A critical aspect of the design of a flyback-based Input Series Converter is the multi-winding transformer; hence, this thesis places special emphasis on the electrical and mechanical design of the flyback transformer. The transformer provides HV insulation between the SM capacitor and the output of the APS, as well as active IVS for balancing the voltage across the input stage capacitors and the MOSFETs. A prototype of the multi-winding transformer has been developed and used to assemble an open-loop converter. The converter's performance is tested with an input voltage range of 300V–3600V, and the design is further refined with an output diode snubber circuit.

A scalable, compact, and reliable topology has been identified for an HV-wide input APS for an MMC-based AWG. The topology provides suitable HV isolation between the SM capacitor and the APS output, as well as between the gate pulse generator circuit and the APS power stage. However, the transformer winding design led to a mismatch in leakage inductance between the different converter primary side stages. This resulted in a slight discrepancy in the voltage shared between the series-connected MOSFETs during the converter's turn-off stage. Future work should focus on employing a winding design that ensures equal coupling of all primary windings to the secondary winding in the flyback transformer. Additionally, it is recommended to use a coupled inductor-based passive LC snubber rather than an RCD snubber across the primary windings to introduce an active IVS process during the turn-off stage of the converter as well.

This thesis investigates the design, analysis and testing of medium-voltage planar transformers for modular Solid State Transformer applications, with a particular emphasis on insulation for medium-voltage level. The renewable energy and microgrid implementation in the generation and distribution segments of power grids is motivated by the global urgency of the energy transition, particularly in response to the challenges in the Netherlands related to the phase-out of natural gas and grid congestion. Central to this work is the development of modular Solid State Transformers (SSTs) tailored for microgrid applications which are one of key components in the transition to more sustainable energy, for which planar transformers in high-frequency operations have to be designed.

A thorough review of high-voltage planar transformer design is conducted, focusing on core and winding losses, parasitic elements, and the complexities of achieving sufficient insulation within compact designs. The thesis outlines the design of a high-frequency planar transformer optimized for modular SSTs, highlighting key aspects such as core selection, loss minimization, and insulation design. Simulations are performed to assess the impact of high-frequency effects on transformer performance, with particular attention to the optimization of winding and shield configurations for improved field distribution and reliable insulation.

Multiple critical aspects in the design of insulation are pointed out and the experimental work is dedicated to testing these aspects.The PCB-based insulation under high-voltage conditions, with a focus on partial discharges and breakdown voltage in scenarios involving parallel conductors within a PCB layer. The results demonstrate the robustness of PCB insulation while revealing challenges posed by surface discharges and the effects of external conditions such as oil and air. These findings provide critical insights into the dielectric performance of FR4 laminates, contributing to the enhancement of insulation system reliability in planar transformers.

The outcomes of this research offer valuable contributions to the design and optimization of high-voltage planar transformers, underscoring the importance of balancing compactness, efficiency, and reliability. These advancements are essential for the development of resilient and sustainable energy systems in the context of the global energy transition.

One fifth of the global carbon dioxide (CO2) emissions can be attributed to the transportation sector. Consequently, the growing desire to participate in the energy transition begs for the exploration of alternative modes of transportation. This endeavor is particularly challenging in the field of aviation and aircraft propulsion, where the injection of electrification and electric motors (EMs) pose many hardships such as the need for high autonomy, light weight, high power density and reliability. This work will mainly focus on the latter.

Unlike the low voltage machines used in road mobility, the motors necessary to propel an airplane demand substantially higher voltages due to their power requirements. Despite the availability of dielectric materials and insulation techniques, an improper design of the insulation system may make it susceptible to electrical stress, possibly reaching its breakdown voltage, generating partial discharges (PDs) and degrading the insulating material overtime. This will eventually lead to the electrical failure of the machine.

This danger is further increased by the recent rise in popularity of new wide-band gap power electronic devices. These devices offer many advantages: reduced size and weight, the ability to operate in higher temperatures, and the improved efficiency due to the reduction in power losses, caused by their steep switching speed in the order of 10−100 ns. However, the steep voltage transients (dv/dt) they produce create harmful side-effects to the machine. Firstly, the feeder cable connecting the voltage inverter to the machine terminals suffers Reflected Wave Phenomenon (RWP), where the voltage pulses are reflected at both ends of the line generating intereferences, which in turn generate overvoltages that can rise to up to twice the original voltage pulse. Secondly, the inherent leakage capacitance between the cable turns and the core of the stator slots produce an uneven voltage distribution along the wiring turns. These effects could increase the voltage stress in the insulator and momentarily reach the breakdown voltage in certain points of the geometry, producing Partial Discharges (PDs) that degrade the insulation over time.

To avoid this degradation, the aim of this thesis is to study these harmful effects in depth, back-up the literature review with accurate simulations and experimentation and realize the worst case scenario considering the geometry of a given motor that is currently under design. Once the phenomena is understood, some mitigation techniques are proposed to lower the chance of any discharge occurring.

The thermal rating of power cables is a critical aspect of electrical cable design that determines their safe operational limits. Traditional rating methods rely on worst-case assumptions for weather conditions, such as constant ambient temperatures or steady-state loading with a 100% load factor. Given the significant thermal masses and variations in ambient conditions, these assumptions often do not reflect actual operational scenarios. As the integration of fluctuating renewable energy sources into power systems increases, the dynamic nature of these systems becomes more pronounced. Consequently, the thermal rating of the power cables must be considered dynamic to accommodate these changing conditions. In this thesis, an improvement of dynamic cyclic rating predictions is explored for cable temperatures using the Finite Element Method (FEM). This improvement is performed by incorporating detailed weather and environmental data into the model. The study begins with a comprehensive literature review that provides an overview of existing dynamic rating systems and their applications. Subsequently, an analytical and numerical thermal model was developed and compared with COMSOL and CIGRE standards. It is concluded that the developed numerical model demonstrates greater accuracy and usefulness. This numerical model is integrated into a web-based tool called Ampwise, which serves as the basis for all future improvements. Cable data from the Windpark Fryslân project is used to validate the predicted temperature against the measured cable temperature. The research demonstrates that incorporating weather data significantly improves the precision of dynamic cyclic ratings, especially the inclusion of the real external air temperature and solar radiation of the location. Validation of the predicted conductor temperatures against measured DTS cable temperatures from the Windpark Fryslân project showed a mean absolute error (MAE) of 1.9 °C and a root mean square error (RMSE) of 2.3 °C. These results show the enhanced accuracy and reliability of the dynamic cyclic ratings when real weather data are incorporated. However, the rate of change in the real temperature is significantly higher than the predicted temperature, leading to short-term differences between the predicted and actual temperatures. In addition, a sensitivity analysis is conducted to assess the impact of various factors. The analysis highlights the importance of correct initial conditions, particularly ground temperature gradients and seasonal environmental variations. The thesis concludes that integrating weather and environmental data into dynamic rating models is crucial to achieving reliable and precise cable performance predictions over an extended period, thus supporting better operational decisions and infrastructure management.

Unwanted breakdown of insulation is one of the primary challenges affecting the reliability of power systems. Liquid insulation is commonly used in power grids and subsea installations due to its preferred qualities, such as heat transfer and safety. Some insulating liquids have been available since the earliest times, and some have been proven to be harmful to humans, animals, and the environment. Consequently, the search for better candidates has always been ongoing. Unfortunately, the knowledge of the quality of the insulating liquids is insufficient since numerous phenomena related to electrical discharge and breakdown in insulating liquids happen unexpectedly, often surprising the personnel involved. Therefore, gaining a deep understanding of the breakdown physics of these liquids and mechanisms leading to the degradation of their dielectric properties is crucial. This understanding ensures that the new replacements are suitable for specific equipment. Partial discharge (PD), as one of the initial stages before breakdown and a major player in ageing the insulators, is the main focus of this work. One way to monitor a liquid’s insulation quality is through the Partial Discharge Inception Voltage (PDIV)
level. Based on PDIV measurements, there are defined standards, such as IEC 61294. This work is focused on investigating the behavior of different types of liquids under the AC voltage stress above and below the PDIV level to determine if the defined standards reflect the dielectric characteristics of each liquid properly. Three liquids have been examined, including two candidates from commonly used insulating liquids in electrical installations. Tests are performed in a needle-plane geometry under high-voltage AC stress. The same liquids and geometry have been used multiple times in previous studies under lower frequency AC voltage, yielding unexpected results. Therefore, this study aims to investigate deeper by testing the liquids at higher frequencies and comparing observations with previous results. The experimental setup used for this work includes a 20 kV resonance voltage source, and the needle-plane gap distance is 20 mm. The current flow from the plane to the needle is mostly capacitive in the range up to 100 nA, but space charges are proved to play a major role in the conductive component of the current. PD behavior has been found to relate to molecular structure and, in some cases, to the previous stresses, likely due to residual ions remaining from previous cycles. The presence and nature of space charges have been investigated carefully by testing liquids at higher frequencies below and above the PDIV level. Further, the test setup was facilitated, and half-cycle voltages were applied to involve only one polarity of space charges, and the differences were observed. Another part of this work involved the optical PD measurement method, which utilized two photomultiplier tubes (PMTs) and a coincidence circuit to filter out ambient dark noise. This method was employed alongside the well-known charge acquisition method to detect the initial PD pulses in the liquid. The results demonstrated a noteworthy difference between the two methods, prompting further investigation.

The integration of renewable energy systems into the grid brings new and unique propositions involving the integration of power electronic inverter based solutions into the High Voltage (HV) grid. Thus a need arises to test the HV components with unconventional waveforms which can be satisfied by the Arbitrary Waveshape Generator (AWG). Two such topologies have been shortlisted for the AWG, ie: Modular Multilevel Converter (MMC) & Cascaded H Bridge (CHB). The MMC requires a single insulated DC source while the CHB requires multiple insulated DC sources with isolation.

Developing insulated DC sources using traditional transformers requires oil, epoxy or other complex insulation materials. The insulation requirements are further exacerbated when HV > 5 kV is required at power levels greater than 1 kW. For this reason, Wireless Power Transfer methods are studied in this thesis to develop a DC Power Supply based on Inductive Power Transfer (IPT). The developed supply is capable of conducting DC breakdown tests and high frequency (>100 kHz) AC discharge tests on dielectrics.

The key requirements for developing such a system are Load Invariant Voltage, High Voltage Gain and Soft Switching. Considering these requirements the Series Parallel (S-P) topology is chosen to deliver 1.5 kW at a DC voltage of 5 kV from a 350 V input, with a diode rectifier. Using the Greinacher rectifier, 10 kV DC output is obtained. Different parameters of the developed resonant converter such as coupling coefficient, switching frequency, deadtime and overcurrent protection are analysed to find a suitable configuration for the DC Power Supply. An efficiency of 88% at rated load and load regulation of 18% is achieved for the developed power supply.

For the CHB, a multi-receiver IPT system is desirable to provide multiple isolated DC outputs. A 2-receiver and a 3-receiver IPT system is constructed and their coupling behaviour is studied. A conclusion is drawn that an ”n” receiver system has a coupling coefficient 𝑘𝑛 = √𝑛𝑘1 and accordingly a S-P IPT system can be developed to obtain multiple (n) isolated DC voltage sources.

The urgent need to decrease greenhouse gas emissions and enable clean energy transition has driven the power industry to seek compact and eco-friendly solutions for power transmission. Gas Insulated Switchgear has been a key technology in this regard, using Sulphur Hexafluoride gas as an insulating medium. However, is a potent greenhouse gas with high global warming potential, necessitating the search for more environmentally friendly alternatives.
Siemens Energy has proposed "Clean Air" as a substitute for inside high voltage. Clean Air, also called Synthetic Air is a homogeneous mixture of 80% N2 and 20% O2. However, due to its lower electrical strength compared to SF6, the GIS design with Synthetic Air as the insulation gas requires optimization and newer insulation techniques to maintain compactness and reliability.
The goal of this thesis is to analyse the breakdown strength of Synthetic Air as a substitute for SF6 inside the High Voltage GIS and examine the performance of its breakdown strength with dielectric coatings on the surface of the electrodes.
Initially, experimental studies were conducted to investigate the breakdown strength of Synthetic Air under various pressure ranges. Comparisons with compressed air reveal that Synthetic Air demonstrates similar electrical breakdown strength.
Next, the study focused on understanding the influence of dielectric coatings on the breakdown behavior of Synthetic Air for applications in GIS. Methodical optimization of coating thickness and air gap distance were done to effectively minimize the electric stress inside the air gap. The test setup was designed with electrodes coated with neat epoxy and the AC breakdown tests were performed. Compared to the uncoated electrodes, the coated electrodes showed significant improvement in breakdown strength.
Though the coated electrode did not breakdown at the voltage at which the uncoated electrodes broke down, with higher voltages, a new phenomena of surface flashover was observed. The observations of surface flashovers across the dielectric coatings demonstrated the ability of these flashovers to travel longer distances at remarkably low voltage ranges. This research concluded by postulating two hypotheses to explain the observed surface flashover phenomena. These hypotheses were researched and analyzed using FEM simulations and experimental investigations.

High Voltage (HV) power cables need to be connected to other HV equipment such as transformers or overhead lines. To safely connect the cable to such equipments a cable termination is used. The cable termination is usually filled with a type of dielectric fluid and is fitted and sealed at the customer site. However during the installation of the cable termination man made errors can occur and in some cases, it can result in ineffective sealing of the termination. These may act as points from where moisture or water droplets can enter inside the termination. The moisture which enters into the insulating oil would move towards the regions in the termination where there is high electrical stress due to the dipole nature of the water molecule. This area of high electrical stress is usually the surface of the stress cone. Depending on factors such as the amount of moisture entering the termination, it could take months or a few years to effect the functionality and eventual failure of the cable termination. The preliminary failure investigation shows evidence that the failure is caused to the stress cone, which is usually in contact with the mixture of water and the oil. The electrical activity on the surface of the stress cone due to the combined effect of the oil and water causes some discolouration and tracking of the rubber material. There are two main tests that are conducted in this research, one of the test is focused on the oxidation process of silicon rubber by thermal stress. The second test is a small scale test setup with oil and silicon rubber electrically stressed in presence of water to understand the long term effect on the surface of the material. Both the experiments conducted produced partial results which could be used to explain the degradation of silicon rubber, also information of partial discharge patterns which could be useful in detecting such moisture induced faults in terminations.

Insulated metal substrate (IMS) printed circuit boards (PCBs) are an attractive alternative to conventional FR4-based PCBs and power modules due to their good thermal performance, low cost, and compatibility with the reflow soldering processes. In this thesis, the dielectric performance of IMS PCBs is characterised to validate their use in high-voltage power electronic applications and ensure that an acceptable lifetime and reliability can be reached. In particular, the high-frequency degradation of the dielectric is investigated.

The IMS dielectric was characterised using breakdown tests with various voltage waveforms and frequencies to closely approximate their use-case in power electronic converters. Two high-voltage generators were designed and realised to generate high-frequency square-wave and sinusoidal waveforms. In addition, diagnostic tests were performed to identify the effect of electrical ageing on the dielectric properties.

he designed high-voltage test sources allowed for ramp breakdown and lifetime tests up to 100 kHz and 10 kVpk. These tests showed that the dielectric degrades extremely fast under high-frequency voltage stress, most likely due to localised high partial discharge activity and electrical treeing. As a result, no reasonable lifetime can be expected when operating above the discharge inception voltage. For thick dielectrics (>100 µm), surface discharges are the dominant degradation mechanism with an inception voltage of 2.4 kVpk. The maximum nominal voltage should be limited to approximately 1.5 kVpk to ensure the reliable operation of the IMS PCBs.

Remotely operated vehicles (ROV) are used for the inspection, maintenance and repair of submarine and offshore cables, subsea exploration, and rescue operations. ROVs are powered by umbilical cables, which have to withstand electrical, mechanical, and thermal stresses. This thesis investigates the effect of repetitive bending on the insulation properties of ROV umbilical cables for medium voltage.

Umbilical cables with XLPE, polypropylene, and HDPE insulation were mechanically aged using a cyclic bend-over-sheave setup, after which the dielectric properties were analysed using electrical breakdown tests and several diagnostic tests, such as partial discharge analysis.

The cables with XLPE and HDPE insulation showed delamination of the conductor–insulation interface, which resulted in increased partial discharge activity, a reduced breakdown voltage, and a significantly decreased lifetime. For HDPE, the lifetime power law exponent 𝑛 dropped from 18 to 9 after 23,000 cycles of mechanical ageing. Polypropylene, on the other hand, did not show delamination but an increase in dielectric-bounded cavities with mechanical ageing.

High-voltage circuit breakers protect and control the electrical power grid, making their reliability of the essence. Robustly assessing the technical `health' of every circuit breaker in operation is key for an optimal maintenance strategy and safety measures, but is traditionally not performed in a data-driven, statistical fashion. The goal is to design an indexation method for the health of the high-voltage SF6 circuit breakers of TenneT TSO NL using its available (monitoring) data. This indexation is performed through modelling the hazard rate of major and minor component failures with the Cox proportional hazards model. Failure modes and their causes are investigated using large-scale surveys on historical failure data and covariates for the model are chosen accordingly. The hazard rate for minor failures is found to be increasing with the asset's operating voltage level, its average switching frequency, certain manufacturers, and its relative SF6 leakage rate, of which the last relation has not been modelled before to our knowledge. Using the model output, the circuit breakers' probabilities of future failure are estimated, shown to have some predictive power, and incorporated into a color-coded scoring system. Using this quantitative measure of their condition, the TSO should be able to better manage its asset portfolio and risks. However, data quality, especially for major failures, is found to be low and poses a limitation, as well as a possibility for future research.

Power Electronics in association with the high voltage technology has the potential to bring out new generation of electrification with well-integrated renewable resources. Various high voltage applications of power electronics include Solid State Transformers (SST), Photovoltaic Inverters (PV), and Shore-to-ship connections. A demand for faster and smarter testing of high voltage components led to the development of an arbitrary waveform generator. It is possible to generate voltage waveforms with varying magnitudes and frequencies using a modular multilevel converter (MMC). Based on the voltage withstand capacity of semiconductor switches and the control complexity of MMCs, the number of levels and maximum voltage at each level in MMC are determined. The commercially available semiconductor switches are capable of withstanding 1.7kV (SiC-MOSFET) and 6.5kV (Si-IGBT). The low current and fast switching requirements of high-voltage (HV) tests argue in favor of SiC MOSFETs.

With the ability to connect commercially available and mature but lower breakdown voltage semiconductor transistors in series, medium-voltage (MV) and high-voltage (HV) converters can be built with a significant reduction in costs. Using the proposed technique, a modular multilevel converter (MMC) can be equipped with a HV switching module. When semiconductor switches are connected in series, the key challenge is to ensure equal voltage sharing in both static and dynamic switching states. There is a clear connection between the asynchronous gate driving signals and the voltage imbalance. Different types of intrinsic and extrinsic capacitive couplings also affect the voltage shared among series-connected SiC MOSFETs. The factors responsible for voltage imbalance are listed below from a dominant influence to a subservient one.
1. Gate delay introduced by the gate driver circuit and associated power supply.
2. Resistance and capacitance to ground introduced by the measurement probes.
3. External capacitance to ground introduced by the heatsink.
4. Parasitic inductance introduced by the interconnects between series-connected devices.
5. Process variation in intrinsic parasitic capacitances.

Stacks of SiC MOSFETs can be controlled synchronously and isolated from the gate driving circuit using a pulse transformer. A common primary winding is made of a HV insulated cable passing through ’N’ toroidal cores carrying ’N’ secondary windings for respective gate terminals of the series-connected SiC MOSFETs.
By resolving the frequency dependency between pulse transformer and stack duty cycle, the proposed technique achieves arbitrary switching function for the series-stack. There are two sets of such pulse transformer arrangements that ensure the fail-safe switching transitions for series-connected SiC MOSFETs. An experimental verification of the proposed technique is performed with a 6kV prototype of five 1.2kV SiC MOSFETs in series. During differential probe measurements across drain-source terminals of the individual series-connected SiC MOSFETs, a significant challenge has been encountered. It is demonstrated that external resistive and capacitive couplings induced by the measurement probes can affect switching function.

The objective of the thesis is partially met, and a low-cost approach to achieve high voltage switching modules with commercially available devices in series connection is proposed. With the conducted research it is possible to build high-voltage isolation between gate driving circuit and series-connected SiC MOSFETs.
The proposed method is an open loop voltage balancing technique which significantly reduces the required number of components by eliminating the need for feedback and its isolation challenge. Furthermore it achieves synchronous switching with the ability to set the duty cycle for the series-connected switch.

Emerging PV technologies like Perovskite has lead to the development of Perovskite/Silicon (Pvk/c-Si) tandem device (Multi-junction device) and has now gained a lot of momentum and attention due to the fact that the tandem device can reach a Shockley-Quisser limit of about 44.1% efficiency. But to speed up its development, a lot of emphasis is made on conducting tandem solar cell device simulations to have get a good idea on the experiments that can be carried out. Solar cell simulations for tandem devices are carried out in advanced 2D/3D solar cell simulators like SETFOS or Synopsys TCAD Sentaurus. These softwares have large computational time and have complex user interface making simulations challenging. There are 1D solar cell simulators like ASA (Advanced Semiconductor Analysis) developed at TU Delft that has less computational time with an easy and intuitive interface. But ASA does not have the appropriate tunnelling models to simulate the tunnel recombination junction characteristics of perovskite silicon tandem devices.

To make ASA a complete software suite for simulating perovskite silicon tandem device, tunnelling models proposed by Ieong et al. is used for both direct and band to band tunnelling. These tunnelling models use basic inputs like band energy data, electrostatic potential data and electric field data for a particular device and outputs the tunnelling generation rate that can simply be added to the continuity equations for electrons or holes during the simulation of the device. A fully fledged algorithm was designed for the automation of the direct tunnelling and the band to band tunnelling process with limited input from the user while using the models proposed by Ieong et al. In simple terms, the algorithm takes in the required input and scans through the device for every interface to check for tunnel contributions and performs calculations when necessary. All necessary conditions are included inside the algorithms to ensure that the simulation is accurately solved.

The validation of the newly developed tunnelling algorithm was commenced for PN junction, Silicon hetero-junction and finally the Perovskite-Silicon tandem device. The JV curve for the above device simulations with and without incorporated tunnelling models was extracted and a comparison was made with literature results and simulation results from other software suites. The newly developed tunnelling algorithms was concluded to be accurate. The JV curve obtained from the ASA simulation showed good agreement with literature results and simulation results from other software suites with error percentages of less than 3% for open circuit voltage, short circuit current and fill factor. With proper tuning of the device layer thickness, texturing on multiple layers for the tandem device and enhancing the mobility of the non-absorber layers for the perovskite top cell, further improvement can be expected in the simulation. These methods can eventually lead to reaching the Shockley Quisser limit of about 35.7% efficiency for the perovskite silicon tandem solar cell even when general opto-electrical losses are taken into account.

Energy consumption keeps increasing, and more than three-quarters of the EU’s green-house gas emissions stem from it. Hence, it is important to reduce or stop burning fossil fuels to limit the temperature rise to 1.5∘C according to the Paris agreement target. Nowadays, renewable energy sources and distributed generation are increasingly integrated into the electric power system to achieve the ambitious target of the energy transition. Renewable energy sources are inherently fluctuating in their power. Whereas the grid is stable. The interface between these two can be made smooth based on a medium-frequency DC-DC converter. The advantages of increasing frequency are high power density, significant weight and size reduction, dynamic power flow control, and power quality management. All these are possible mainly because of advancements in power electronics in the recent decade. However, these power electronics induce harmonics across insulations, which are fast rising voltages and are repetitive. The dielectric strength of insulations under these fast-rising pulses is to be understood. This thesis project studied oil impregnated paper behavior under pulsed stresses.

To test the Oil Impregnated Paper samples, a source capable of generating repeating pulses with fast rising and lower overshoots is required. This source comprises an AC- DC converter that will power the H-bridge. The pulses from the H-bridge are then fed into a 1:50 pulse transformer to step up the voltage, which is then delivered across OIP (Oil Impregnated Papers) through test components that regulate the pulse’s rising time and overshoot. The process began with identifying critical factors to consider while developing a pulse generator. Following that, a random transformer prototype is created along with another transformer to extract the parameters, which are then modeled in the circuit simulator and COMSOL to predict the pulse. An algorithm was created to optimize the transformer design. Finally, this design method creates a pulse transformer that generates the necessary output.

The OIP samples are then tested at frequency 50𝑘𝐻𝑧 and at various voltages with a rise time 𝑇𝑟 ≈ 1.3𝜇 𝑠. A lifetime curve was obtained using the data of ageing tests at five field strengths. The results showed that the lifetime was reduced with the increase in frequency. At lower fields, a transition was seen, indicating a shift in the ageing process.

With the increasing growth and penetration of power electronic devices in the power system, it is essential to understand the behaviour of insulation materials and systems under the varied signals that are produced with power electronic systems. These signals could be square pulses, sinusoidal or even superimposed. The high-frequency sinusoidal excitation in the range of 50-100kHz is the focus, of this project. The project aims to design a resonance-based test system to measure the lifetime curves of dielectric materials such as oil-impregnated paper and epoxy under high frequencies.

The major components of this project are the H-Bridge Circuitry found on the low voltage side and the resonant transformer which is tuned with the test sample to be in resonance. The design techniques and critical factors for the design of gate driver circuits have been provided. Moreover, through simulations in COMSOL Multiphysics and MATLAB along with practical measurements using vector network analyzers the behaviour of the transformer equivalent circuit parameters with changes in geometry is discussed. Through this understanding, the critical parameters to design a test system to obtain the highest gain while in resonance have been highlighted.

The final test setup is designed and is used to perform lifetime tests on oil-paper and epoxy samples, both materials exhibited different behaviour during the tests. Due to the high gain of the system, it is very sensitive to changes in the capacitance of the test object and hence a constant voltage is not applied to the oil-paper sample. Therefore the breakdown times cannot be considered for the lifetime curve. Recommendations are provided to improve the test setup. Epoxy showcased a different behaviour, it is observed that the linear voltage gain of the resonance system is lost once partial discharges are initiated, and the voltage across the test sample becomes constant even upon a further increase in the input voltage. Based on these observations recommendations are provided to improve the test setup along with further topics that need to be studied.

Modelling of power transformers to predict and examine its performance in the presence of Geomagnetic Induced Currents (GICs) is of particular interest in research since these currents that are quasi-DC in nature drive the transformer into half-cycle saturation. During the half-cycle saturation, the increased leakage flux and harmonics have undesirable effects on the windings and the structural components of the transformer, which can possibly lead to their permanent damage. This prompts the need to perform a precautionary study of the transformer validating its robustness when subject to GICs. This thesis proposes a method to model the transformer and its electromagnetic (EM) behaviour under GICs that combines the accuracy of Finite Element Modelling (FEM) with the speed of Magnetic Equivalent Modelling (MEM) to produce a fast-computing yet detailed modelling method using simulation tools, COMSOL and MATLAB Simulink, respectively. The transformer under study in this thesis is a single-phase 280MVA, 500/230kV auto transformer designed by Royal Smit Transformers. It was first modelled using FEM in COMSOL, providing details of the equivalent 2D axisymmetric geometry of the core and the windings and the material properties to each component. The model was then computed and validated against factory measurements done by Royal SMIT for the transformer when not subjected to GICs. After which, the model was run for the condition in which the transformer is subject to a GIC. The main problem of failure of the model to converge before reaching steady-state condition under GIC was overcome after optimizing the solver settings of the FEM software. The difference in the EM behaviour of the transformer with and without GIC were studied. After ensuring that the FEM model was robust and accurately represents the transformer under GIC as well, the time-consuming part of computing the FEM model till it reached steady-state under GIC was then made to be taken care of by a MEM model developed in Simulink. Induced flux versus magnetizing current characteristics of the transformer were obtained from the FEM model, and fed to the MEM model, which only took a few seconds to run. The magnetizing current waveform obtained for a few cycles after reaching steady-state under GIC was then transferred to the FEM model, which could finally compute - for those values of magnetizing current - the detailed EM effects of GICs on the transformer, spatially. An important EM study needing the spatial distribution of the EM properties within the transformer is the calculation of eddy current losses in the windings. In this thesis, a method that only considers the envelope of the set of windings as opposed to modelling every strand to calculate their losses is developed using an analytical formula found in literature. This is validated against the losses calculated by Royal SMIT as well as the FEM software for a detailed model of the windings. The timesaving method to study the EM behaviour of transformers subject to GIC proposed in this thesis accurately models the transformer in the saturation region that should enable conducting EM studies furthermore to winding loss calculation with ease.

The ambitious transition towards a renewable future is being made possible by power electronic systems that facilitate effective control and efficient conversion of energy. One such system is the solid-state transformer (SST) which aims to replace conventional transformers by offering larger energy densities and flexible power flow. However, these benefits come at the cost of severe mixed-frequency stresses experienced by the medium frequency transformer (MFT) of the SST. These stresses are characterised by fast-rising pulsed waveforms of several tens of kilovolts repeating at frequencies of up to a hundred kilohertz. In contrast to pure sinusoids, the behaviour of dielectrics undermixed-frequency stresses is to a large extent unknown. This research gap forms the core motivation behind this thesis project. To analyse these exciting yet extreme phenomenon, the first objective was to build a pulse modulator for testing cellulosic dielectrics under voltages up to 10 kV with rise-times ≈ 2 μs at frequencies between 10 to 50kHz. The topology consists of a rectified DC supply feeding a SiC H-bridge pulse generator connected to a 4:200 pulse transformer and other variable test elements. The work began with selecting the switch and gate driver, simulations in LTspice and TINA-TI, and the fabrication of the PCB pulse generator prototype using Altium Designer. Next, a novel third-order PQR equation was derived for a PT with capacitive load involving the parasitic leakage flux Ls and distributed capacitance Cd. The influence of bobbin geometry on parasitics was studied with 20 different 3D printed iterations designed in Fusion 360. A failure mode analysis was conducted to identify weak points in a transformer, and the solutions detailed in this report. The final objective was to apply the produced waveforms of T_r ≈ 1.8 µs across single-layer OIP samples at 10 kHz and 50 kHz. The sample strength was determined through ramp tests with 1 kV/s slope. The lifetime curves were obtained by performing ageing tests at 10 field strengths each for 21-41 samples, and fitting the median failure time into an inverse power-law model. The results show a clear reduction in lifetime at higher frequencies hinting at the unsuitability of OIP in its current form as an insulation for MFTs. A transition point was observed indicating a shift in the ageing mechanism at lower fields. The report ends with a discussion on future scopes of research.