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.

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.

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.

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.

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.

The power system in Northern Europe supports a high degree of penetration of Wind Energy-based HVDC connections. Most of the modern HVDC connections will have Bipole with a dedicated metallic return (DMR) configuration. This thesis presents a basic guideline to increase the Levelized cost of operation of Bipole HVDC links by attaching the offshore poles to the point of common coupling and designing suitable control strategies. The results show that the control strategies can be applied under various real-life scenarios with slight overvoltages within the acceptable range.

The factory acceptance tests are extremely important for the deployment and service life of Extra-high voltage transformers (EHV) and reactors. An essential part of such tests is Lightning Impulse (LI) tests which are intended to ensure that the transformer insulation withstands the transient lightning overvoltages which may occur while in service. These tests are usually done with negative polarity to prevent air side flashovers. In recent years, there has been an increasing demand for additional positive polarity lightning impulse tests to secure the reliability and life of the transformers. The transformer, during its service life, is subjected to massive electrical stresses due to lightning strikes and switching impulses. As a consequence, this may lead to the degradation of the transformer insulation. During lightning impulse tests, ionisations might occur in the pressboard/insulating liquid insulation system, which would lead to the development of space charges. The space charges generated may have an influence on the withstand behaviour of the insulation corresponding to applied impulse voltage. There is also a possibility of field enhancements due to the effect of space charge, especially when tested consecutively with opposite polarity. During the acceptance tests on transformers at SGB-SMIT transformers, a waiting period of one hour is given between the negative polarity LI and positive polarity LI. This arbitrary waiting time is given under the assumption that this will allow any ’trapped’ charges to decay and provide relaxation time for the slower polarisation processes. So the main question is whether this waiting period allows sufficient time for the decay of charges in the transformer insulation. For this analysis, the relaxation time characteristics of the transformer insulation system were investigated by estimation of the time dependency of depolarisation currents in the oil, paper/pressboard insulation. In this thesis, the characteristic of the time-domain dielectric response and the time dependency of the currents due to depolarisation of charges under DC and impulse voltage was investigated by the polarisation-depolarisation current (PDC) measurements on the test samples. The PDC method is a non-destructive diagnostic method for evaluating transformer insulation in the time domain. A test set up was built to represent a simplified model of transformer insulation comprising of mineral oil, paper and pressboard. The measurements of discharge voltage over time were conducted on the test samples of oil, paper and pressboard to understand the time-dependency of polarisation-depolarisation processes occurring within the transformer insulation. Two dominant time constants of decay were estimated for oil, paper and pressboard samples. The results from the discharge voltage measurement were compared with the analytical solution of the output voltage of the equivalent R-C circuit of the test sample which revealed that there were more than two polarisation phenomena occurring within the composite test sample of oil, paper and pressboard. Later, to mitigate the inconsistencies with the simplified R-C model, it was extended and modified based on the linear dielectric response theory to study the dielectric response behaviour of transformer insulation under DC voltage for longer charging times (tc=10,000 seconds). The modified R-C model was envisioned and developed in PSPICE simulation environment. The model incorporates the effect of the individual polarisation processes occurring within the constituent dielectrics of transformer oil-paper-pressboard insulation. The dielectric properties like conductivity and dielectric response function f (t ) can be estimated reasonably accurately with this modified R-C model. A comparison of the maximum and minimum values of the polarisation currents of the composite test sample obtained from the simulated model and from dielectric testing was conducted. The results demonstrated that relative errors were limited to a maximum of 8 %. The time-domain polarisation and depolarisation behaviour of composite transformer insulation was analysed under DC and impulse voltages from simulations of PDC measurements using PSPICE simulation software. It was observed that for the same thickness of solid insulation, as the oil-gap increases, the magnitude of the depolarisation current at the end of the discharging period (10,000 seconds) also increases. For identical oil-gaps, as the thickness of the pressboard in the composite test sample of oil-paper-pressboard was increased, the depolarisation currents show a delayed response to decay to a minimum value at the end of discharging duration. The time dependency of depolarisation currents at the end of discharging time of one hour was realised for impulse voltages. The study of the depolarisation currents under the influence of impulse voltage revealed that the charge induced field at the end of the discharging period of one hour does not exceed the permissible threshold electric field of 2 kV/mm inside the transformer insulation. In the future, the model could be developed into a valuable diagnostic tool for studying the dielectric responses of complex transformer insulation under the influence of different parameters like moisture content, ageing products, geometrical configuration and temperature.

In this thesis, an algorithm that calculates the hosting capacity of independent PV, or EV charger systems was created. The hosting capacity is based on slow voltage variations and voltage unbalances caused by worst case PV generation and EV charging scenarios. The algorithm was run for four different commercial and industrial networks, and a range of network impedances. The outcome of the algorithm resulted into two figures for each of the networks, representing the possible installed capacity of PV and EV charger systems, respectively.

Detection of partial discharge (PD) in any high voltage equipment is of great importance to improve the reliability of the device and the power system. Characteristics of PDs, such as: rate of occurrence, magnitude, location and the type; gives information of the assets that must be considered. Nowadays, there are many different methods for sensing PDs in GIS (Gas Insulated Substation), however none of them can give fully information of the PD. The most important sensor is the antenna, with great advantages over other sensors, but still, with the limitation of not giving the magnitude of the discharge. The Delft University of Technology has patented a new method for measuring PDs in HVDC GIS. It is believed that this new method has great advantages over the current method, namely charge determination and improved sensibility.

The aim of this work is to find the measuring characteristics of this new method for sensing PDs in a GIS. These characteristics include resolution, attenuation and sensitivity. The most important feature of this new sensing method is the capacity of measuring the PDs magnitude. Therefore, a calibration method for charge calculation is provided. Finally, the new method and the antenna are compared with real measured PDs in a GIS.

To analyse the calibration of the new method; electrical characteristics of the sensor and GIS were simulated and compared with measurements. The results demonstrate that a PD can be calculated using the calibration method with low error values. This error value will depend in the measuring situation. The thesis also demonstrates some measuring advantages of this new method over the antenna.

The first commercial High Voltage Direct Current (HVDC) transmission link was installed in 1954 between the mainland of Sweden and the island of Gotland. Since then, there is continuous and remarkable development in the HVDC technology making it an efficient way of transmitting bulk power over large distances. Due to the transition towards low carbon electricity, more renewable energy sources are being integrated into the grid. This trend has increased the use of Direct Current (DC) in the electrical networks at low voltage levels.

In the past few decades, space charge phenomenon in HVDC insulation have been investigated. Many techniques have been developed for studying and understanding the space charge phenomenon in the HVDC insulation. Considering the growth of DC in electrical networks, further research into these techniques and development of novel dielectric materials suitable for DC is also progressing at a quick pace.

Earlier studies on space charge phenomenon were performed mainly on Polyethylene, Epoxy, Polymethyl methacrylate and Polycarbonate insulation materials. These studies have indicated that the space charge starts accumulating beyond a threshold value of the electric field and has a strong dependency on temperature as well as the electric field.

Silicon based insulation is also known as silicones. It is a highly stable and fire-resistant fluid. It is used in cable joints, traction transformers and increasingly in compact transformers where higher than normal temperatures are expected. However, it is known that when equipment is used with DC, they are expected to suffer strongly from space charge accumulation. As a result, considerable modifications in the electric field distribution with respect to the Laplacian field occurs, especially in case of voltage polarity inversion. This may cause insulation degradation and premature breakdown.

The conductivity behaviour and the electric field threshold for space charge accumulation are two of the most important parameters for the design of insulation systems under DC conditions. By limiting the electric field in insulation system below the threshold value, space charge accumulation can be minimised.

Reusing the existing AC cable joints under DC conditions could save considerable time as well as money in realising new DC networks. For that, conductivity behaviour and electric field threshold values of the insulation material should be investigated.

The main goal of this thesis is to investigate and characterise silicon insulation material with regards to its conductivity behaviour and the electric field threshold for space charge formation. This study is a first step towards the feasibility of using silicon fluid-based AC cable joints under DC conditions. The silicon insulation samples in liquid as well as cured(solid) state are subjected to conduction current measurements and space charge measurements using pulsed electroacoustic method.

Simulations are performed on a cable joint model in COMSOL Multiphysics. Presence of cured silicon layer results in a divergent current density. This resulted in high field concentration in cured silicon layer.