High-voltage direct current gas-insulated switchgear (HVDC GIS) uses a grounded enclosure and gas insulation to provide compact high-voltage equipment for DC transmission. Solid spacers support the high-voltage conductor and form the gas–solid insulation interface. Under DC conditions, the electric-field distribution is governed mainly by the electrical conductivity of the insulation materials. The conductivity of epoxy is strongly temperature dependent. A temperature gradient across the spacer therefore changes the conductivity distribution and redistributes the electric field. The field also evolves during DC pre-stress as charge redistributes. This motivates investigation of how temperature gradient, gradient direction, voltage polarity, and DC pre-stress duration affect surface-flashover behaviour.

This thesis develops and numerically assesses a reduced-scale gas–epoxy test bench for future DC surface-flashover measurements. The design operates in atmospheric air near 1 bar with a maximum operating voltage of 50 kV DC. Either electrode can be heated to impose a temperature difference of 40–50 K without changing the field-defining electrode–spacer geometry. Two heating concepts were developed, together with a transparent chamber, diagnostic access, qualification requirements, and a test method. The electrode–spacer geometry was numerically verified against a reduced-scale coaxial GIS reference. The scientific contribution is an integrated methodology that combines the common test geometry, controlled thermal and electrical conditions, time-dependent electrothermal calculation, and a defined basis for future flashover measurements.

At 60 kV, the final electrode–spacer geometry reproduces the coaxial reference with differences of 0.8% in representative peak interface field and 2.2% in mean interface field. For the hot-oil numerical screening case with the HV electrode heated and the grounded electrode colder, the thermal gradient decreases the immediate hot-HV-edge field by about 13%, while the central minimum and grounded-side regional maximum increase by about 38% and 30%, respectively. The HV-to-GND regional-maximum ratio decreases from 1.77 to 1.34, showing a substantial redistribution of the DC interface field while the largest regional field remains on the HV side.

The time-dependent calculations further show that DC field formation is strongly position dependent. Between 1 and 12 h, the interval-based apparent relaxation times are about 3.2 h on the hot HV side and 15.8 h on the colder grounded side. The HV-side field is close to its stationary value after 12 h, whereas the grounded-side deviation remains about 3.9% after 24 h. Together with the non-monotonic central response, these results show that one common relaxation time cannot represent DC field formation along the complete spacer interface. Thermal condition and DC pre-stress duration must therefore be controlled in future flashover measurements. The developed test bench and test method provide the basis for the experimental investigation of RQ1–RQ3 after manufacture and physical qualification. ... Read more

The increasing adoption of High Voltage Direct Current (HVDC) transmission in recent years has been driven by the growth of long-distance bulk power transfer. This expansion is increasing the deployment of DC Gas-Insulated Switchgear (GIS) and Gas-Insulated Lines (GIL), which use pressurized SF6 for greater compactness than existing air-based insulation systems. DC operation, however, introduces charge accumulation on spacers, resulting in greater electrical stress on the insulation. The effect of the accumulated charges becomes particularly important during Lightning Impulse (LI) overvoltages and may cause spacer flashovers. Although this phenomenon has been studied for SF6, regulatory pressure is now driving a transition to alternative gases for which this behavior remains poorly understood.

The outcome of the thesis is a fully manufactured test cell with a camera arrangement to capture flashover across the insulator surface. A reduced-scale bowl plate electrode geometry is designed using Finite Element Method (FEM) simulations using an ion flow model to reproduce the quasi-homogeneous field distribution and surface charge accumulation behavior of HVDC GIS disc spacers, and the design is validated for operation at pressures up to 3 bar with technical air. A pressurized floating enclosure was also designed, dimensioned, and manufactured to safely contain alternative gases at a maximum operational stress of 30 kV DC with a superimposed ±150 kV LI. A composite voltage test circuit was then designed from available laboratory equipment and validated in LTspice.

Verification in the lab revealed repeatability issues with the spacer electrode contact, which were resolved by redesigning the spacer electrode to use a screw arrangement. Surface charge measurements showed poor agreement with the initial simulation, with additional ionization sources in the test setup not being accounted for in the model. Comparisons with external data, however, showed good agreement with measured surface potentials. The composite test circuit using a spark gap and a coupling capacitor has been tested in the TU Delft HV lab, and the test experience is presented. Further work must be pursued on charge measurements under DC application in technical air, and validation of the test circuit with higher applied voltages and LI breakdown tests with pressurized gas. With this remaining work completed, the cell can be used to systematically measure and visualize spacer flashover under composite DC/LI voltages in alternative gases.
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Dutch urban distribution grids face growing congestion, with many areas unable to accommodate new connections until reinforcement is completed years from now. Light-rail traction power systems, dimensioned to power trams and metros, represent an underexplored asset on the same grid: their substations are sized for peak vehicle loads, leaving capacity that could in principle host additional external loads such as fast chargers or local industry. This thesis develops a method to quantify and allocate that hosting capacity. Train traffic makes the demand on each substation vary unpredictably from second to second; the network’s thermal limits depend not on instantaneous load but on how long an overload persists; and a load added at one substation shifts power flows across the entire line. A worst-case assessment returns conservatively small numbers; a single-snapshot average ignores the events that actually constrain operation. Treating the substations collectively, as a single aggregate capacity, hides the spatial trade-offs that determine where loads can actually be sited. The method addresses this in three stages. A simulation of the network generates a large pool of realistic operating scenarios. The pool is then reduced to a representative subset using a new clustering approach that preserves the scenarios most relevant to feasibility. The reduced set feeds a multi-objective optimisation, formulated with one capacity objective per substation, that exposes the spatial trade-offs between total hosting capacity and how evenly the load can be distributed. Probabilistic operating limits replace deterministic worst-case bounds where the underlying engineering standards admit them, so that brief admissible transients do not subordinate the result. Of the two metaheuristics evaluated, swarm optimisation handles the search space most effectively at moderate confidence levels, though its advantage narrows as the reliability requirement tightens. Applied to the Calandlijn of the Rotterdam metro, operated by Rotterdamse Elektrische Tram N.V., the method identifies a hosting capacity of 2 to 5 MW across the line’s twelve substations, with the achievable value depending on how evenly the load is distributed. A more important finding emerges from the analysis: the binding limit is not uniform across the network. On some feeders it is the contractual cap on utility intake rather than the traction infrastructure itself, while on others the smaller substations run close to their thermal limit. The distinction matters for the operator, since the two cases call for different remedies: contract renegotiation in one, physical reinforcement in the other. ... Read more

In the coming years, a large portion of the 110 and 150 kV substations in the Netherlands will be replaced. For this, the Dutch TSO TenneT has introduced the Bay Replacement Program (BRP), in which the old bays are being replaced completely by standard, modular, and compact skid-mounted bays. These bays are, in principle, plug-and-play, except for the busbar disconnector, for which the pantograph has to be aligned to the overhead busbar. To make substations even more compact in the future, this thesis aims to investigate the switching impulse breakdown strength of short air gaps relevant to maintenance of the 110 kV BRP busbar disconnector, in order to determine the minimum safety clearances.

The validity of the Schneider and Weck method for simulating the gap factor of short gaps (gap distance smaller than 2 meters) is investigated. Electric field simulations in COMSOL were compared with the original results from Schneider and Weck. The simulations reproduce the original results with deviations below 5% for gaps larger than 2 meters. For gaps smaller than 2 meters, the results show irregular behaviour. Experiments on a rod-plane and conductor-rod gap show that the simulated gap factor deviates significantly from the experimentally found gap factor, and the Schneider and Weck model is therefore considered to be invalid for air gaps smaller than 2 meters. The experiments on a rod-plane and needle-plane show that the Feser equation best describes the breakdown strength of short rod-plane gaps, while the CRIEPI equation provides a conservative value suitable for clearance determination.

Experiments on the BRP busbar disconnector were conducted in the TU Delft high voltage laboratory to investigate the gap factor that may occur during maintenance. Four different gaps were tested: conductor-rod, pantograph-rod, pantograph-needle, and earthing contact-needle. The results show that the earthing contact-needle gap has the lowest gap factor of 1.20, and hence is the determining gap for the critical clearance. This critical clearance is found to be 45.6 cm, based on a worst-case risk evaluation. The currently enforced critical clearance by TenneT of 47.9 cm is considered to be adequate. A simulation is performed to study the minimum clearances related to the electric field. ... Read more

The transition away from SF6-based insulation in high-voltage equipment has accelerated interest in alternative gas mixtures with lower environmental impact. One such mixture is CO2/O2 (70%/30%), which is used as a carrier gas in C4-FN based mixtures but is also being explored for stand-alone insulation applications. To enable detailed characterisation of partial discharges (PDs) in such alternative gases, a high-bandwidth measurement setup was developed. The setup is based on a custom designed test compartment that uses a needle-to-plane electrode configuration to generate corona discharges and applies a high-frequency current transformer (HFCT) as a measurement impedance.
Initial measurements using a conventional IEC 60270 configuration revealed a resonance frequency of 3.5 MHz, which limited the effective bandwidth of the system. By eliminating the traditional coupling capacitor and instead using the inherent capacitance between the conductor in the bushing and the grounded enclosure as a coupling path, the resonance frequency was shifted to 144 MHz. Aside from the significant increase in bandwidth, this also improved sensitivity. The frequency response of each segment of the measurement circuit was characterised, allowing the derivation of the PD current from the measured voltage and enabling calibration-free charge estimation. The setup was used to study the PD behaviour in a CO2/O2 (70%/30%) mixture at pressures ranging from 0.2 to 0.4 MPa and voltages up to 1.5 times the PD inception voltage. A novel phenomenon was observed: certain PDs were rapidly followed by another discharge, after which a longer interval was required for the next event.
Multiple charge estimation methods were adapted to suit the measurement circuit. After evaluation, the modified frequency domain method demonstrated the strongest correlation with peak current, especially at low discharge magnitudes. These results demonstrate that the developed measurement setup is suitable for detailed PD analysis in alternative gases and that it is able to offer new insight into the behaviour of alternative gases. ... Read more

The increasing integration of renewable energy sources and power electronic devices is changing the electricity grid, leading to widespread harmonic and supraharmonic distortions. Accurate measurement and calibration of current transformers (CTs) up to the 150 kHz range are essential for reliable power quality assessment and grid monitoring. However, traditional calibration approaches are limited in both bandwidth and practicality, particularly for high-current and high-frequency conditions.

This thesis develops and validates a broadband calibration methodology for CTs, enabling ratio and phase error characterization from 50 Hz to 150 kHz using a high-precision digital sampling ammeter (from a power analyser) as the core measurement instrument. The proposed system eliminates the need for auxiliary equipment and thus reduces component count, ultimately allowing for simplified broadband calibrations. An uncertainty budget is established with combined expanded uncertainties (k=2) for the measurement system of less than 10 ppm up to 10 kHz, and less than 100 ppm at 150 kHz for the secondary-to-secondary comparison method. This is an improvement over the previous state of the art for this setup, which had an uncertainty of 50 ppm and a maximum frequency of 10 kHz. For primary-to-secondary calibration, uncertainties remain below 110 ppm at the highest frequency, allowing for the further development of a reference current transformer.

The thesis systematically examines the influence of critical experimental factors, such as grounding configuration, shunt selection, conductor positioning, cabling, and measurement duration, on overall calibration accuracy and repeatability. Key findings include the importance of instrument warm-up, the impact of earth-loop currents, and practical considerations for shunt and cable selection for high-frequency application. The demonstrated approach provides a metrological foundation for future implementation of wideband CT accuracy classes and supports ongoing international efforts to establish traceable measurement infrastructure for power quality applications.

This work, carried out at the Dutch national metrology institute (VSL), aims to contribute to the goals of the European ADMIT project. ... Read more

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. ... Read more

In this thesis the lifetime assessment of low-pressure oil-filled (LPOF) cables is investigated. Most of the LPOF cables are already quite old but it is unknown if the degradation of these cables matches the age of the cable. Due to energy transition and the fact that not all cables can be replaced simultaneously, an estimate of the lifetime of the cable needs to be defined. This is done in this research using the following three research questions:
•What is the health level of the LPOF cables based on their historical loading profiles and their health index?
•Which LPOF cables are suitable for more loading?
•What is the remaining lifetime of cables that are suitable for more loading on this new loading levels?

These questions are answered using a thermal ageing model based on the historical loading data. From this model it could be concluded that the thermal degradation of the insulation of TenneT’s LPOF cables has been very limited even after 50 years of service. This means that it might be possible cable insulation wise to let it be run continuously on the nominal loading level (maximum allowed temperature) for another 47 years. Although it must be mentioned that the accessories remaining lifetime and loading capabilities are not investigated in this research.

There are done tan delta measurements on an old cable to see if there would be any degradation visible from the 50 years of ageing of the cable. From this testing there was no noticeable degradation in the insulation paper visible. However, from this testing it could not be fully conclude that no ageing would be visible from this cable. Therefore, more testing needs to be done which could be done in the following way.
... Read more

The emergence of electric mobility in urban public transportation, with a particular focus on electric buses, presents a promising solution to address emissions and environmental concerns. However, a significant challenge lies in ensuring continuous bus operation without the need for frequent charging. In-motion charging (IMC), often referred to as dynamic charging, is a concept engineered to overcome this challenge.

A comprehensive study was undertaken to know the potential and practicality of intercity IMC buses. A bus model was developed to assess the power traction capabilities of these vehicles. This model served as the foundation for exploring four distinct charging scenarios, each characterized by varying charging powers and strategies. An investigation into the battery load profiles observed in intercity IMC buses across the different charging scenarios was done. This was essential for understanding the intricacies of power demand, especially in scenarios where the bus relies heavily on catenary charging. Notably, the introduction of in-motion charging in the first scenario (IMC only) underscored the critical role of catenary charging power in meeting operational demands. Subsequent integration of a stationary charging system at Arnhem Central showcased the potential to reduce catenary charging power, offering prospects for enhancing battery health. IMC with opportunity charging and overnight charging were also explored.

The second part of the thesis delved into the comparative effect of various charging scenarios on the aging of commonly employed battery chemistries in IMC vehicles. Using comprehensive battery models, the aging dynamics under diverse conditions were studied. It became evident that each scenario held distinct implications for battery aging.

Lastly, the study addressed the pivotal question of cost-effective battery selection for intercity IMC buses. An exploration of four distinct scenarios, in conjunction with different battery chemistries, yielded valuable insights into their respective performances. Lithium-titanate (LTO) batteries consistently emerged as the preferred choice. Their extended lifespan, reduced replacement frequency, and overall cost-effectiveness positioned them as the frontrunners in the context of intercity IMC bus systems. This consensus held across most scenarios, underscoring the practicality of LTO batteries.

In conclusion, the transportation sector's substantial impact on greenhouse gas emissions and urban air quality necessitates innovative and sustainable solutions. Intercity IMC buses, along with optimal battery selection and charging strategies, represent a promising avenue for sustainable urban transportation. Among battery chemistries, LTO batteries was proven to be the most cost-effective choice for powering intercity IMC bus operations. ... Read more