High-voltage direct current (HVDC) has established itself as the leading technology for long-distance transmission, particularly for interconnections between countries and offshore wind farms. Sulfur hexafluoride (SF6) has traditionally been the preferred insulating medium in gas-insulated substations (GIS) due to its excellent dielectric properties; however, its high global warming potential (GWP) remains a significant drawback. Partial discharge (PD) detection serves as a critical diagnostic tool for ensuring the operational reliability of GIS systems. This study investigates the long-term PD behavior of protrusion and free metallic particle defects in HVDC GIS filled with technical air. The PD apparent charge magnitude and repetition rate evolution are analyzed using pulse sequence analysis (PSA) plots. Results indicate that PSA plots evolve and vary depending on the defect type, posing challenges for human experts and machine learning models in defect classification. Furthermore, most existing PSA plots are derived from test conditions using SF6, highlighting the need for research in alternative insulation gases such as technical air. Both conventional and unconventional PD detection methods were employed within a full-scale GIS test cell. The two defect types were subjected to voltage application for one week. The free metallic particle defect exhibited a 20% change in PD apparent charge magnitude over the test duration but showed minimal alterations in weight and physical structure. In contrast, the protrusion defect experienced a 30% increase in PD apparent charge magnitude, accompanied by significant physical changes, as revealed through microscope imaging. The observed changes in PD behavior after just one day of voltage application suggest that long-term testing in technical air is unnecessary. Similarly, PSA patterns from SF6 were successfully used to classify defects in technical air, demonstrating that knowledge transfer is possible. Finally, the similarities between the certain patterns of free metallic particles and protrusion defects in technical air highlight the need for further investigation in different test environments to refine defect classification in future studies.

AC power transformers are one of the most sophisticated technologies in electrical engineering. However, they have some drawbacks facing volume-sensitive or weight-sensitive applications and energy conversion situations. The solid-state transformer is a promising power electronic technology that can solve the requirements of these applications. It utilizes power electronics and medium frequency (MF) transformers to achieve high power density and multiple functions in energy conversion. With the development of semiconductor switches, it is possible to handle medium voltage in the MF range (1kHz to 1MHz). Therefore, as an essential part of solid-state transformers, medium voltage high power medium frequency transformers require validated tools to model and optimize their design, which is the goal of this thesis. There are several challenges in designing such transformers. They include multiphysics models, Litz wire models, insulation design and design optimization, which are addressed in the thesis....@en

The future power grid must accommodate large-scale integration of variable renewable energy sources. Power electronic (PE)-based components, e.g. onverters, will play an essential role in the operation of the power system. However, the harmonics and transients generated by these PE-based components can significantly affect the lifetime and reliability of various power system components. These disturbances, particularly harmonics and transients, are challenging to eliminate. Thus, a more effective strategy is to assess the effects and enhance the durability and performance of the system components. To ensure the reliability of critical system components, these should meet stringent specifications and undergo rigorous testing prior to installation. Using a modular cascaded H-bridge (CHB) based high-voltage arbitrary waveform generator (HV-AWG), capable of replicating the dielectric stresses induced by PE components, enables a more accurate assessment of component resilience under operational conditions.
The CHB-based HV-AWG can be divided into several submodules; each module consists of three key components: a driver, a medium-frequency transformer, and an H-bridge equipped with HV rectifiers. Although various types of HV-AWGs exist, the modular CHB-based HV-AWG excels due to its superior high-voltage capability, broad operating frequency bandwidth, simple topology, compact size and low manufacturing costs. To successfully realize the CHB-based HV-AWG design, several technical challenges must be addressed, including the development of the insulation system for the medium-frequency transformer and the design of the highvoltage switch within the H-bridge....@en

This thesis investigates the impact of geometric, material, and operational parameters on the electromagnetic fields (EMFs) emitted by submarine power cables, particularly those used for offshore wind power transmission. The study is essential due to the growing deployment of offshore wind farms and the corresponding need for efficient submarine power transmission systems, combined with ecological concerns. The primary focus is on high-voltage alternating current (HVAC) cables, commonly used to connect offshore wind farms to the onshore grid. The rapid expansion of offshore wind power has highlighted significant ecological and environmental concerns, especially the effects of EMFs on marine life. Species such as elasmobranchs (sharks, rays, and skates) are highly sensitive to EMFs, making this an important area of research. The objective of this thesis is to develop guidelines for modeling EMFs from submarine power cables to aid ecological research, focusing on establishing effective parameters for EMF emission models. A literature review has shown that EMFs can impact marine animals’ navigation, predator-prey relationships, and embryo- genic development. Elasmobranchs, in particular, are vulnerable due to their sensitivity to EMFs. The review highlights the necessary accuracy levels for EMF models, focusing on HVAC cables. Starting with the basic physics including Maxwell’s equations, the Biot-Savart law, and the Lorentz force, followed by an outline of HVAC cable and transmission system design parameters, an understanding was created of the electromagnetic phenomena occurring in a submarine power cable. It was concluded that modeling EMFs requires considering current distribution along the conductor and metallic sheath, and the local interactions between cable components that result in shielding of the EMFs. A method from literature was used for predicting the longitudinal distribution of current along the conductor. The current distribution was affected by voltage and current transmission requirements, impedance, and the capacitive and inductive properties of the phases as well as reactive power compensation. Analyzing these parameters showed that all are crucial for accurate EMF modeling, as parameter changes within realistic ranges could result in differences over 10%. A significant part of this thesis examines the intensity of the metallic sheath currents. Modern HVAC cable designs use conductive polyethylene layers around the metallic sheaths, creating an electrical interface between them. This was conventionally assumed to dissipate circulating currents in the sheath, but this thesis questioned this belief. It was proven that some circulating currents remain and that induced currents were even unaffected by the conductive interface. The induced currents in the metallic sheath are shown to only be influenced by the conduction current and its design parameters, inductance, and resistance per unit length. Testing on the Borssele Alpha cable showed that sheath current for all standard operations was more than 12% of the conduction current. An analysis of the local impact of various design and operational parameters on EMF emissions was conducted using COMSOL Multiphysics. The Borssele Alpha 1 cable design served as a standard test case. The analysis highlighted the importance of the metallic sheath and even more so, the armor layer. The steel armor layer created a path of low reluctance, with geometric parameters and permeability playing significant roles. The effect of twist in the phases and armor wires on field emissions was also shown. The lay-length of the phases played a large role due to its effect on destructive interference between emissions of different phases. The armor lay-length had a considerable impact as well, possibly affecting the reluctance of the armor layer. The parameter analysis also confirmed the validity of an ultra-shortened section length in COMSOL, greatly reducing simulation time without impacting results. This research provides insights for developing accurate EMF emission predictions, aiding biologists and ecologists in evaluating the environmental impacts of offshore wind power infrastructure. This can guide the development of mitigation strategies and support sustainable expansion of renewable energy sources. Recommendations for future research include experimental testing of the sheath current model, further development of the transmission line method for the metallic sheath and developing better mitigation strategies. Unrelated to the main question is that in thermal analysis of comparable submarine power cable designs, the induced current must be incorporated, as it was unclear if this is the case. The findings of this thesis can significantly contribute to the sustainable growth of offshore wind power by addressing ecological concerns and improving the understanding of EMF effects on marine life.

The integration of wind and solar energy through power electronic converters has introduced new challenges to High Voltage (HV) equipment in the electrical power system. Switchgear, cables, and transformers are now subject to higher dV/dt stress and complex wave shapes due to solid-state switching. This poses a threat to the reliability of the grid by weakening the dielectric material of these assets. Existing HV test sources face limitations in generating complex wave shapes and have restricted current capabilities. Building a customized test setup is time-consuming when combining multiple HV test sources for complex waveforms.

To overcome these challenges, an Arbitrary Wave shape Generator (AWG) for dielectric testing of HV grid assets is proposed. The Modular Multilevel Converter (MMC) topology is chosen for its modular structure, low harmonic content, and scalability to higher voltage levels. The initial focus is on dielectric testing of Medium Voltage (MV) class equipment, with the ultimate goal being the development of a modular prototype as part of a PhD project.

HV test requirements and procedures for conventional tests of MV class equipment are compiled, along with specifications for non-standard wave shapes in consideration of the hybrid grid. Two main HV test requirements are addressed in the PhD thesis: the output voltage range of 10 kV to 100 kV with a load capacitance range of 50 pF to 10 nF and a large-signal bandwidth up to 2.5 kHz. The second requirement involves generating steep pulses with a rise time of a few microseconds for a voltage magnitude of 250 kV across a capacitive load of 10 nF.

Despite the maturity of MMC technology for HVDC transmission, adapting it for HV AWG applications presents unique challenges. The thesis explores design trade-offs related to MMC parameters such as the number of Submodules (SMs) per arm, arm inductance, arm resistance, modulation technique, SM capacitance, and control system. Design criteria are developed and demonstrated through simulation models and a scaled-down prototype.

The control hardware of the HV AWG is addressed using a commercially available Real Time Simulator (RTS) named Typhoon-HIL. This choice is based on its flexibility to program arbitrary waveforms in the FPGA without coding in any special hardware description language. The performance is demonstrated in the scaled-down prototype, achieving sinusoidal waveforms up to 5 kHz reference frequency with THD less than 5%.

The second HV test requirement, steep pulse generation, is investigated with the MMC topology. It is found that the series-connected SMs of MMC make it challenging to obtain a short rise time across a large capacitive load. To address this, an integrated hybrid circuit of MMC and Marx generator circuit is proposed for complex waveforms with a rise time faster than 100 μs. Proper guidelines for choosing circuit parameters are provided and experimentally validated with a scaled-down prototype.@en

In this master thesis, the design and implementation of a high-voltage switch using series-connected Metal Oxide Semiconductor Field Effect Transistors (MOSFETs) is discussed. A high-voltage switch is needed for applications such as high-voltage DC converters. Multilevel modular converters (MMCs) can also aid in the voltage-blocking capabilities of converters. MMCs use multiple submodules to block the full voltage. Every submodule thus only blocks a part of the full voltage. Each submodule needs multiple components that can be expensive, like voltage sensors and gate drive circuitry. By having the submodules block a higher voltage, fewer components are needed. By implementing a higher voltage blocking switch into such a submodule, cost and components can be spared by having a lower total amount of submodules. Current technologies of high-voltage switches are MOSFETs, insulated gate bipolar transistors, thyristors or electro-mechanical switches. MOSFETs have the advantage of being faster and have lower losses than the other technologies. The downside of using MOSFETs is that the voltage-blocking capability is lower than the other technologies. This could be solved by connecting multiple MOSFETs in series for a higher blocking voltage. To do this, care has to be taken that the MOSFETs turn on- and off at the same time. They also need to share the voltage equally to take full advantage of the blocking voltage of each MOSFET. A cost-effective way to implement a high-voltage switch using series-connected MOSFETs is to use capacitive coupling. In this method, the gate charge needed to turn the MOSFETs on is stored in capacitors. When the first MOSFET is turned on, the other series-connected MOSFETs will also turn on. This method has the downside that the switch can only be turned on for a limited amount of time (a few microseconds) and that the voltage balancing is dependent on the load, the switching frequency and the parasitics of the circuit. For this reason, a single high-voltage MOSFET was used in this project, even though it can be slower and more lossy than the series-connected switch. An MMC submodule was made using a full-bridge configuration. Additionally required components such as a gate signal generator, power supply, voltage- and current sensors, and fault protection were also designed, built and tested. In this way, a functioning MMC submodule was created that can be used with a capacitor or an isolated transformer as its source.

The energy transition requires massive expansion and reinforcement of the transmission grid which is challenging because of its high utilization, and regular maintenance, replacement, and repair. The difficulty of performing maintenance on high-voltage equipment and especially the substations lies in the fact that a big part of the substation needs to be taken out of service for the maintenance crew to work safely. When a part of a substation is taken out of service this influences the capacity and redundancy and thus the reliability of the grid. A planned outage is required to take a part of a substation out of service, these planned outages are limited in frequency and duration due to a shortage of especially trained and skilled people in combination with the high utilization of the grid.

The objective of this master thesis is to investigate important design aspects for mobile transmission stations to reduce/prevent planned outages during maintenance, replacement, and expansion projects on TenneT's (the Dutch transmission system operator) permanent substations. A mobile substation is a fully equipped substation mounted on one or multiple trailers to be easily transported. The mobile substation provides a bypass during maintenance on permanent substations allowing continued service without requiring planned outage.
A case study in this work shows that using a mobile substation during maintenance, replacement, and expansion projects on substations can possibly save TenneT several hundred million euros per year. Using the mobile substation on a 380 kV or 220 kV substation would make it possible to work on multiple 380 or 220 kV substations at the same time. It would also be easier to get a planned outage permit for the 150 kV and 110 kV substations. The use of a mobile substation would thus increase the efficiency of the projects and would reduce the amount of required critical resources.

Besides the case study, this work investigates the design requirements for mobile substations. The mobile substation should be able to (partly) bypass all TenneT substations and should contain mobile power transformers. The weight of the mobile substation is limited by road regulations. The transformers will be the heaviest components in the substation thus special attention is needed for their design. For the power transformers, a shell-type core design would be preferred with (high temperature) hybrid-insulation, due to its compact design and robustness for transportation. To comply with road regulations without special permits six 83.33 MVA single-phase transformers with an approximate weight of 36 tonne need to be connected in two banks of three single-phase transformers to reach the standardized capacity of 500~MVA.

The insulation distance in the mobile substation will greatly influence the size of the substation when deployed and during transport. Therefore, this insulation distance is an important design aspect which is covered in this work. The standards for insulation distances were analyzed. This analysis gave rise to doubts if these distances would apply to compact substations with large electrodes. An experiment was conducted at the TU Delft Electrical Sustainable Power Lab to determine the dielectric strength of large electrodes with sharp points in compact substations, and to verify if the insulation distances as specified in the standards should be applied to mobile substations.
It was found that the large electrodes with sharp points have similar breakdown voltages as a rod-rod gap setup. A higher breakdown voltage was found for the large spheres with protrusions compared to the rod-conductor gap which is used by the IEC standard. This indicates that the IEC standard insulation distances would be sufficient for use in a compact substation. Whether the substation can be made more compact by reducing the insulation distances should be investigated by executing experiments on an actual skid in a future study.

Medium voltage cable joints are expected to function for 30-40 years after successful installation. To optimize and improve the design of these joints, it is essential to find out the changes that the components inside a cable joint go through. In this thesis, the effects of accelerated thermal ageing on electrical and mechanical properties of medium voltage cable joints have been investigated. For this research, three cable joint components were selected, that are being used and developed at Lovink Enertech. These components are silicone grease (insulation used at cable insulation-deflector interface), Lovisil (main insulation liquid filled inside the joint) and deflectors (geometric field grading to distribute electric field). These components were thermally aged and then tested, with the aim of observing the extent of change in physical and electrical properties.
Silicone grease was tested for its breakdown strength, under normal and tangential voltage application. Normal breakdown strength of unaged grease was measured at different temperatures, and of aged samples was performed at 50 °C, as that is the average operating temperature inside a cable joint. Tangential breakdown strength test was performed on XLPE and unaged silicone grease interface, at room temperature. It was observed that the breakdown strength under normal electric field of silicone grease reduced with ageing and with operating temperature. Also, the tangential breakdown strength is approximately half of the normal breakdown strength. For the deflectors, experiments to determine hardness of the material, young’s modulus, conductivity and weight of silicone grease absorbed into the deflector, were performed to characterize and compare the material properties at different stages of ageing. It was observed that silicone grease got absorbed into the defector at all ageing temperatures, as the weight of the deflectors increased. Hardness of the inner surface also increased with ageing. Whereas there was not a considerable change in young’s modulus and conductivity of deflector samples. Lovisil was thermally aged and tested to observe changes in its breakdown strength and influence of crosslinker polymer on the same. The measurements were made using Baur vessel, sphere-sphere electrode configuration and under two voltage applications: constant ramp and step voltage. Lovisil got darker with ageing and a decrease in the breakdown value was observed. Measuring breakdown strength under constant ramp voltage gave more reliable and repeatable results in comparison to step voltage. It was also noted that crosslinker enhances the dielectric properties of the Lovisil and is responsible for its curing and discoloration.
Three joints of 12/20(24) kV were prepared and aged to see if silicone grease helps in avoiding degradation at the cable insulation-deflector interface. It was observed that not applying silicone grease at the interface led to a degradation, whereas applying it did not cause any damage even after overstressing the joints. These tests also validated the newly developed silicone grease as an insulation at the interface.

The penetration of power electronics into the grid is growing rapidly due to the increasing number of renewable generation systems. These power electronic converters introduce electrical stresses in the form of complex waveforms into the grid caused by their switching operations. The High voltage (HV) equipment is subjected to these stresses. To maintain the reliability of the power system it must be able to withstand these kinds of complex stresses. To test the HV equipment with such stresses and assess its correct functioning, it is important to be able to reproduce such complex waveshapes. A new Modular Multi-level Converter (MMC) based arbitrary waveshape generator with 67 sub-modules in each arm rated for 100 kV is being designed to target the testing of medium voltage (MV) class equipment. Since the MMC has its own limitations in generating superimposed Lightning impulse (LI) waveforms, an integrated hybrid configuration combining the conventional Marx generator circuit and MMC is needed to generate the superimposed LI. This thesis intends to design and demonstrate the feasibility of such an integrated hybrid configuration focusing on two major goals. Firstly, to investigate the capabilities of the Typhoon HIL to implement the control of MMC and compare this with the existing OPAL-RT controller, and secondly, to design an integrated hybrid configuration to produce superimposed waveforms. To achieve the first goal the Typhoon HIL hardware, software, MMC topology, and the various modulation techniques were studied in depth. Phase shifted carrier (PSC) modulation-based open loop control was implemented on the CPU and FPGA of the HIL device. The results were compared with the offline simulations, and it was found that the control implemented on the CPU is suitable for up to 1 kHz, and the FPGA control is recommended for higher frequencies. The performance of Typhoon HIL is better than OPAL-RT regarding generating accurate gate pulses at higher fundamental frequencies. The approach for the second goal used is to study the literature on existing composite test setup to understand the requirements to design the decoupling circuit between impulse generator and MMC. A downsized model of an integrated hybrid configuration was designed and validated by offline simulation. Next, a 300 V hardware prototype was implemented in a step-by-step manner. First, Rapid control prototyping was performed separately on the MMC comprising 2 sub-modules in each arm with the FPGA control developed using the Typhoon device. Thereafter, the impulse circuit was coupled with the MMC to test for both power frequency and DC superimposed impulses. It was observed that the real-time simulation results were identical to the offline simulations and the superimposed impulses could be generated with high accuracy. Finally, the output of the MMC for higher signal bandwidth was tested using the Typhoon FPGA control. It was found that the MMC could generate sinusoidal waveforms up to 3 kHz.

With the aid of the analytical formulas of the disc pair model inter-shielded by different type of shield wires, the total series capacitance of the disc pair could be calculated. After introducing a number of inter-shield pairs to the disc pair model, the influence on the mutual inductance between discs has been thoroughly discussed. Last but no least, after comparing the figures for the voltage response distribution among different configurations and different type of inter-shield pairs, the case which could contribute to the best voltage response performance is selected.

Partial Discharge (PD) measurements are of great importance to enable the monitoring and diagnostics of HV systems. The requirements of the Paris Agreement and climate goals have fuelled the increase in penetration and demand of HVDC for offshore wind. The HVDC Gas Insulated Switchgear (HVDC GIS) is a reliable technology to support the necessary electrical infrastructure. Nevertheless, some in-service failures may occur. These failures can occur in the insulation system and thus developing a measurement system for PD detection is essential for monitoring and diagnostics. To monitor and diagnose the HVDC GIS, a novel Magnetic Antenna (MA) is being developed to operate in the high-frequency (HF) (30-300 MHz) range. The well-established UHF method for the GIS is typically used due to its high sensitivity and resilience to electromagnetic interference. However, the UHF method is unable to calibrate to apparent charges as this information is in the low frequency (up to 30 MHz) until HF range. The knowledge of charge calibration indicates the discharge type which is important in DC as DC does not have phase-resolved information as with AC. The appropriate frequency range of the MA should enable the measurements of the apparent charge and localize the defects when monitoring and diagnosing a HVDC GIS setup. The overarching goal is to develop a measurement system to measure PDs in the HF range in a GIS setup. For this purpose, MAs are created and investigated. A workbench has been built and developed to characterize the MAs and measure its frequency characteristics. A 380 kV GIS measurement setup has been developed. This enabled the measurement and acquisition of data of the discharges using the MAs. The Threshold Peak detection (TPD), Energy Criterion (EC), and Phase Method (PM) localization methods are investigated and implemented for localization of the source of defects. The PM is unable to localize the pulse due to its sensitivity to noise and reflections. The TPD and EC are both suitable with the TPD being the preferred method due to its 95% accuracy of localizing the defect within ± 1.5 m.   

Power electronics technology has been widely used in industry. High-frequency harmonics caused by power electronics related products will have a huge influence on the insulation characteristics of the electrical devices. In order to study these influences, an arbitrary waveform high voltage generator is needed. The multilevel modular converter is a promising topology to build such a generator. The thesis goal is to design and build a prototype DC-DC converter which can step down the MMC submodule capacitor voltage to a reasonable low value for driving the submodule IGBTs. In chapter 1, an introduction is given which shows the background and research questions of this thesis. Chapter 2 is literature review studies. In this chapter, different possible converter topologies are analysed. Finally, the flyback topology with series-connected MOSFETs is selected. High voltage switch constructions and voltage balancing methods are also discussed and some of these methods are simulated in chapter 3. In chapter 4, an open-loop prototype was built. In this chapter, flyback transformer and driving transformer design methods are presented. After selecting proper values for the RC snubber, satisfactory voltage balancing effects can be achieved. Finally, in chapter 5, a DC-DC converter with 500V-1500V input voltage was built and it is designed at 30W. Recommendations and future work are given in chapter 6. They include necessity of temperature rise measurement, PCB design and so on.

HVAC cable is normally installed in a PE (Polyethylene) pipe underground. The pipe is placed in the underground path formed by horizontal directional drilling (HDD). During power transmission, the generated heat from the cable is dissipated by natural convection of water in the gap between cable and pipe. In this project, the first part focuses on the analysis of natural convection in water in the annulus space formed by a cable-pipe boundary. The second part simulates the heat dissipation process of the underground cable-water-pipe system with the influence of the layered soil. The temperature distribution along the HDD path shows an abnormal variation due to different thermal conductivity of soil layer, which causes some hotspots to form on the surface of the pipe.
Heat transfers by conduction, convection and radiation. In this project, only conduction and convection are considered. The thermal radiation analysis is not considered and reserved for future study. Convection can only occur in fluid (liquid or gas). In convection mode, the fluid near the heat source which gains energy become less dense and rises forming a convection flow. The thesis covers two main sections. The first section covers analysis of two problems: the first one is the study of natural convection in the concentric annulus (corresponds to the geometry of cable at the centre of the pipe) and the second one is the natural convection analysis in the eccentric annulus (corresponds to the geometry that cables touches the pipe bottom due to gravity). For the concentric annulus, the highest temperature occurs in the top region of the annulus. For the eccentric annulus, the highest temperature occurs in the narrow region where the inner cylinder touches the outer cylinder. Generally, in the laminar flow regime, the eccentric annulus has a poorer heat transfer rate compared to the concentric annulus. In the second section, a time-dependent model based on the HDD scenario is built in COMSOL, taking into account the influence of natural convection in water and inhomogeneous thermal conductivity of layered soil. The heat transfer process from the cable to the soil along the HDD path is simulated. The heat dissipation rate from the pipe to soil varies due to different thermal conductivity of each soil layer, which causes unpredictable temperature changes at different position of the pipe surface. Hence, some hotspots form on the surface of the pipe. The location of hotspots is detected by numerical modelling in COMSOL. The simulation model can be used in the future for localization of potential hotspots and cable life calculation.

Destructive consequences of trapped space charges within HVDC coaxial cable insulation, such as accelerated aging, degradation, and breakdown, are some of the most significant challenges that manufacturers and users usually come up with. Pulsed Electro-Acoustic (PEA) method is a widely-used method for space charge detection and measurement, in which the space charge profile is mapped through acoustic waves that are excited by applied pulsed-voltage. There are a few practices for pulse application into PEA circuit. Pros and cons of the different injection practices have been remained unknown due to limited accessibility to the critical points like the core conductor. Modeling of the pulse injection setups can help us with simulating the essential parameters, like voltage and current, at the unreachable spots. The way how the circuit can be modeled, selecting adequate software and transmission line model, strengths and weaknesses of the injection practices and the changes required for possible optimization in the practices are challenging questions that need to be answered. This thesis develops frequency-dependent models for all the PEA injection setups that are usually deployed for Medium Voltage (MV) mini-cables geometry. It shows, through a validation
process, that PSCAD/EMTDC phase model can yield sufficiently accurate results with reference to the lab measurements. In the next stage, the questioned strengths and weaknesses of each original design setup are determined through a series of lab tests and computer-aided simulations by utilizing of which the best setup is recognized. Seeking some approaches for further optimization of the setups is also another aim of this research. This thesis proves that the PSCAD/EMTDC phase model is capable to present an acceptably exact model for the circuits, despite having some limitations that will be explained. Operational
versions of all setups, in which the fundamental problems are cleared up, will also be proposed in this thesis. Among all the setups, Core Pulse Injection (CPI) and Table Pulse Injection (TPI) practices offer the best non-optimized (original design) and optimized results, in terms of quality of the delivered voltage across the target dielectric respectively, while the original design of Double Side Pulse Injection (DSPI) and Single Side Pulse Injection (SSPI) seem to be unreliable due to severe oscillatory behaviors. Nevertheless, two modified versions for DSPI and SSPI show quality results.

There is an increasing demand for compact onsite dielectric test systems of medium and high voltage equipment. Though there are commercially available mobile high voltage (HV) test sources, they have some important drawbacks such as limitation in voltage capability, complexity in building up the test setup and lack of versatility to test different equipment. Alternatively, the present challenges in these conventional test sources could be resolved by constructing a power electronics based test source. Amongst various converter topologies, Modular Multilevel Converter (MMC) is considered as a suitable topology for this application because of its high efficiency, modular structure, and reduced filter requirement. In order to achieve a compact solution, an oil immersed design of MMC, instead of the typical air insulated converter design is proposed in this master thesis. Oil can act both as a coolant and an effective insulating medium, thereby reducing the dimensions of the test system.
This master thesis demonstrates the feasibility of oil immersed MMC based HV test source, covering both system level and component level aspects. The extent of compactness of the test source in oil was estimated at a system level. It was found that for the given trailer dimensions, the voltage capability of test source is around 4 times higher in oil when compared to air, This shows the effectiveness of oil as an insulating medium. The feasibility of oil immersed converter design depends on the degree of compatibility of its components under oil. Hence, experimental investigation was conducted on Insulated Gate Bipolar Transistor modules (IGBT) under oil as they are considered to be a pivotal component in MMC. Preliminary results show the penetration of oil into the IGBT chip surface. Despite oil migration, the operation of IGBT was not hindered during the test. Based on the results obtained from experimental work, a road map of future tests is suggested that need to be performed to realize an oil immersed HV mobile test source.

The safety in the high voltage substations rely on the integrity of its grounding grid. A single discontinuity at any part of the grounding grid may cause a swell in touch or step potential that may expose the personnel to a hazard of critical electric shock. It may also lead to malfunction of the protection devices leading to erroneous tripping of circuit breakers. This report presents a cost-effective, reliable and precise way to investigate the integrity of grounding grid. An experimental setup was performed in the laboratory to inspect and corroborate the results; thereafter repeating and validating the experiment in a substation.

Increase in demand for electric power and renewable sources of energy drives the need for more transmission & distribution capacity. When using DC, as RMS and peak voltage are the same, transferring more power can be economical but at the same time erecting new transmission lines is often associated with demanding approval procedures, conflicts of interest and mainly with more cost. Thus a more cost-effective option to cater the demand is to share HVAC and HVDC lines on the same tower (called as hybrid lines). Charges due to corona creates problems like radio interference, audible noise and power losses. Also electric field (below transmission lines) at ground level causes head hair sensation resulting in public annoyance. Thus electric field and ion current density with the effect of space charges at ground level needs thorough investigation.This thesis focuses on calculation of electric field (affected by space charges) and ion current density on ground for HVDC and hybrid transmission lines using COMSOL multiphysics software. In HVDC, several configurations like monopolar, bipolar and double bipolar were examined. Effect of wind and ground conductors on space charges were also studied for monopolar models. A verified HVDC model is presented in this thesis which can be used to study the effect of space charges from DC conductors on electric field and current density. This work has been further extended to build a model for hybrid (HVAC+HVDC) configuration.

Conventional high voltage test sources, i.e., transformers (cascaded and resonant), impulse generators, and rectifier circuits generate 50 Hz/60 Hz sinusoidal, lightning and switching impulses, and DC respectively. For arbitrary wave shapes, function generator and HV amplifier setup are available up to 50 kV and few kHz signal bandwidths. Superimposed signals (impulses or transients on a sinusoidal or DC) are generated from a careful design of superposition of two voltage sources. The demand of these unconventional test sources is increasing as new electric stresses are introduced in the current electrical power network due to the rise of power electronics. This motivates the search for a novel high voltage test source which can generate arbitrary wave shapes with larger signal bandwidth at higher voltage level. Modular multilevel converter (MMC) topology is a promising solution for high power applications because of its high efficiency, modular structure, and reduced filter requirement. Hence, the idea of a MMC-based high voltage test source is proposed for arbitrary wave shape generation. Such a converter-based test source can also be adapted as a mobile test system for site acceptance and diagnostic testing and opens an entire new high voltage research area.

This master’s thesis demonstrates the proof of concept of a MMC-based high voltage test source using a mathematical and simulation model. The first step is to identify critical differences between existing MMC applications versus high voltage testing applications. Based on this analysis, control methodology and converter parameter design are adapted for high voltage testing application. Since a power electronic converter generates an output signal with harmonics, a filter is required to obtain a smooth signal that can be applied to the test objects. The quality of the generated signal is measured in terms of the peak value and the slope of the output voltage. Also, the protection system and DC source requirements are investigated briefly for a comprehensive picture of the MMC-based high voltage test system. The thesis demonstrates the feasibility of the MMC-based high voltage test source with a proof of principle on paper and with simulations. Recommendations are given for the next steps to realize a high voltage test source for arbitrary wave shape generation.

The electricity demand is increasing with the development of industry. Nowadays, sustainable resources used for electricity generation are playing a more and more important role in the power system due to serious energy problems. Higher capability of power transmission and distribution thus becomes significantly necessary to keep the stability and reliability of the whole system. Instead of investing in new infrastructure, this thesis concentrates on existing HVAC power cables using XLPE insulation, trying to find out a way to improve their capability of power transmission without reducing their expected lifetime.
For the purpose of utilization improvement, two models based on a realistic 132kV-XLPE power cable are developed first and then analyzed for AC, DC and combined voltages. Inspirations for creating voltage waveforms at an increased level, and thus improved capability of transmission are obtained from different field distributions in AC and DC voltage situations. The process of space charge development and the formation of the corresponding charge induced field is used in this work.
Secondly, two kinds of hybrid voltage waveforms are created: trapezoidal and ones in shapes of sinusoidal and DC waves. Those types of waveforms are able to force the maximum electric field stress in XLPE to move within a small range, resulting in a higher utilization compared to the AC nominal voltage application. The optimal voltage waveform for the 132kV HVAC power cable is determined based on the highest rms value of voltage.
Thirdly, for the suggested voltage waveform, sensitivities are evaluated for changing load levels, XLPE conductivity and the thermal conductivity of soil which depends on weather conditions. It turns out that the load level has highest sensitivity, followed by XLPE conductivity. Once the ampacity of the power cable is determined, the soil property needs little consideration. These important factors must be taken into account when designing the combined voltage waveform.
Finally, cable terminations are modeled to check the feasibility to cope with the new waveform. The main conclusion is that: it is possible to apply the optimal waveform to existing high-voltage cables using SCT field grading technique in their terminals, while those terminations with stress cones may be endangered if higher voltages than the designed value are applied.

Keywords: cable insulation, space charges, electric field distribution, capacity improvement.