P.T.M. Vaessen
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1
Load Hosting Capacity of Urban Traction Networks
Case study of the Calandlijn metro network in Rotterdam
Master thesis
(2026)
-
Y.M. Dwarkasing, P.T.M. Vaessen, Dennis van der Born, A. Lekić, I. Diab, J. Verschoor
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.
...
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.
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. ...
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. ...
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.
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.
The global transition toward sustainable and electrified energy systems is fundamentally transforming electrical power grids. Increasing penetration of renewable energy sources, electrification of transport and heating, and widespread deployment of power-electronics-based interfaces are driving the evolution of today’s AC grids toward future hybrid AC/DC architectures. While this transformation enhances flexibility and controllability, it also introduces fast switching transients, non-sinusoidal waveforms, and steep voltage gradients that impose novel electrical stresses on existing grid assets. Conventional high-voltage test equipment, designed for sinusoidal AC, DC, and impulse testing, is no longer sufficient to realistically replicate these in-service conditions. This creates a clear need for compact, flexible, and high-voltage Arbitrary Waveform Generator (AWG) capable of emulating future grid stresses. Power-electronics-based AWGs using modular multilevel converter (MMC) architecture offer a promising solution due to its scalability and waveform flexibility. However, practical implementation of MMC-based high-voltage test sources is often limited by excessive system complexity, large numbers of low-voltage submodules, more points of failure, and high cost. Increasing the voltage capability of a single MMC submodule therefore emerges as a key enabler for reducing system complexity, footprint, and cost while maintaining high performance.
Within this context, this PhD thesis addresses the fundamental challenge of realizing high-voltage and high-speed switching, using commercially available low-voltage wide-bandgap semiconductor devices. The work investigates series connection of SiC MOSFETs and GaN HEMTs as a cost-effective and scalable approach to enhance voltage-blocking capability.
The thesis establishes a comprehensive understanding of voltage imbalance mechanisms in series-connected devices, identifying gate-drive signal mismatch as the dominant contributor to dynamic voltage imbalance, while also revealing the critical and often overlooked influence of measurement-probe-induced parasitics on voltage distribution across series connected devices. Based on these insights, a transformer-coupled, gate-current-synchronized driving approach is identified as the most effective voltage-balancing technique. To overcome the inherent frequency and duty-cycle limitations of conventional transformer-based drivers, a novel programmable dual-transformer gate driving architecture is developed. This approach decouples switching control from transformer constraints, enabling flexible, microcontroller-compatible arbitrary waveform generation while maintaining nearly uniform voltage sharing across series-connected SiC MOSFETs. Experimental validation demonstrates stable operation at kilovolt levels with nearly even voltage balance.
The work further extends and modifies the proposed new series-connection and gate-current synchronization concepts to ultrafast GaN HEMTs, addressing the challenges posed by nanosecond-scale switching. The developed open-loop, dual transformer gate driving strategy is shown to be well suited for GaN devices, and systematic optimization of transformer and excitation-stage parameters enables balanced voltage sharing at kilovolt levels while preserving the intrinsic speed advantages of GaN technology, confirmed by experimental validation on a hardware prototype under high-voltage, high-dv/dt switching conditions.
Overall, this thesis provides a simple, scalable, and experimentally proven high-voltage switching solution that enables low-voltage wide-bandgap devices to be used in medium-voltage systems. The proposed high-voltage switch has the potential to significantly reduce the complexity of MMC-based arbitrary waveform generator, enabling compact, cost-effective high voltage testing system capable of emulating realistic electrical stresses of renewables-rich hybrid power grid.
...
Within this context, this PhD thesis addresses the fundamental challenge of realizing high-voltage and high-speed switching, using commercially available low-voltage wide-bandgap semiconductor devices. The work investigates series connection of SiC MOSFETs and GaN HEMTs as a cost-effective and scalable approach to enhance voltage-blocking capability.
The thesis establishes a comprehensive understanding of voltage imbalance mechanisms in series-connected devices, identifying gate-drive signal mismatch as the dominant contributor to dynamic voltage imbalance, while also revealing the critical and often overlooked influence of measurement-probe-induced parasitics on voltage distribution across series connected devices. Based on these insights, a transformer-coupled, gate-current-synchronized driving approach is identified as the most effective voltage-balancing technique. To overcome the inherent frequency and duty-cycle limitations of conventional transformer-based drivers, a novel programmable dual-transformer gate driving architecture is developed. This approach decouples switching control from transformer constraints, enabling flexible, microcontroller-compatible arbitrary waveform generation while maintaining nearly uniform voltage sharing across series-connected SiC MOSFETs. Experimental validation demonstrates stable operation at kilovolt levels with nearly even voltage balance.
The work further extends and modifies the proposed new series-connection and gate-current synchronization concepts to ultrafast GaN HEMTs, addressing the challenges posed by nanosecond-scale switching. The developed open-loop, dual transformer gate driving strategy is shown to be well suited for GaN devices, and systematic optimization of transformer and excitation-stage parameters enables balanced voltage sharing at kilovolt levels while preserving the intrinsic speed advantages of GaN technology, confirmed by experimental validation on a hardware prototype under high-voltage, high-dv/dt switching conditions.
Overall, this thesis provides a simple, scalable, and experimentally proven high-voltage switching solution that enables low-voltage wide-bandgap devices to be used in medium-voltage systems. The proposed high-voltage switch has the potential to significantly reduce the complexity of MMC-based arbitrary waveform generator, enabling compact, cost-effective high voltage testing system capable of emulating realistic electrical stresses of renewables-rich hybrid power grid.
...
The global transition toward sustainable and electrified energy systems is fundamentally transforming electrical power grids. Increasing penetration of renewable energy sources, electrification of transport and heating, and widespread deployment of power-electronics-based interfaces are driving the evolution of today’s AC grids toward future hybrid AC/DC architectures. While this transformation enhances flexibility and controllability, it also introduces fast switching transients, non-sinusoidal waveforms, and steep voltage gradients that impose novel electrical stresses on existing grid assets. Conventional high-voltage test equipment, designed for sinusoidal AC, DC, and impulse testing, is no longer sufficient to realistically replicate these in-service conditions. This creates a clear need for compact, flexible, and high-voltage Arbitrary Waveform Generator (AWG) capable of emulating future grid stresses. Power-electronics-based AWGs using modular multilevel converter (MMC) architecture offer a promising solution due to its scalability and waveform flexibility. However, practical implementation of MMC-based high-voltage test sources is often limited by excessive system complexity, large numbers of low-voltage submodules, more points of failure, and high cost. Increasing the voltage capability of a single MMC submodule therefore emerges as a key enabler for reducing system complexity, footprint, and cost while maintaining high performance.
Within this context, this PhD thesis addresses the fundamental challenge of realizing high-voltage and high-speed switching, using commercially available low-voltage wide-bandgap semiconductor devices. The work investigates series connection of SiC MOSFETs and GaN HEMTs as a cost-effective and scalable approach to enhance voltage-blocking capability.
The thesis establishes a comprehensive understanding of voltage imbalance mechanisms in series-connected devices, identifying gate-drive signal mismatch as the dominant contributor to dynamic voltage imbalance, while also revealing the critical and often overlooked influence of measurement-probe-induced parasitics on voltage distribution across series connected devices. Based on these insights, a transformer-coupled, gate-current-synchronized driving approach is identified as the most effective voltage-balancing technique. To overcome the inherent frequency and duty-cycle limitations of conventional transformer-based drivers, a novel programmable dual-transformer gate driving architecture is developed. This approach decouples switching control from transformer constraints, enabling flexible, microcontroller-compatible arbitrary waveform generation while maintaining nearly uniform voltage sharing across series-connected SiC MOSFETs. Experimental validation demonstrates stable operation at kilovolt levels with nearly even voltage balance.
The work further extends and modifies the proposed new series-connection and gate-current synchronization concepts to ultrafast GaN HEMTs, addressing the challenges posed by nanosecond-scale switching. The developed open-loop, dual transformer gate driving strategy is shown to be well suited for GaN devices, and systematic optimization of transformer and excitation-stage parameters enables balanced voltage sharing at kilovolt levels while preserving the intrinsic speed advantages of GaN technology, confirmed by experimental validation on a hardware prototype under high-voltage, high-dv/dt switching conditions.
Overall, this thesis provides a simple, scalable, and experimentally proven high-voltage switching solution that enables low-voltage wide-bandgap devices to be used in medium-voltage systems. The proposed high-voltage switch has the potential to significantly reduce the complexity of MMC-based arbitrary waveform generator, enabling compact, cost-effective high voltage testing system capable of emulating realistic electrical stresses of renewables-rich hybrid power grid.
Within this context, this PhD thesis addresses the fundamental challenge of realizing high-voltage and high-speed switching, using commercially available low-voltage wide-bandgap semiconductor devices. The work investigates series connection of SiC MOSFETs and GaN HEMTs as a cost-effective and scalable approach to enhance voltage-blocking capability.
The thesis establishes a comprehensive understanding of voltage imbalance mechanisms in series-connected devices, identifying gate-drive signal mismatch as the dominant contributor to dynamic voltage imbalance, while also revealing the critical and often overlooked influence of measurement-probe-induced parasitics on voltage distribution across series connected devices. Based on these insights, a transformer-coupled, gate-current-synchronized driving approach is identified as the most effective voltage-balancing technique. To overcome the inherent frequency and duty-cycle limitations of conventional transformer-based drivers, a novel programmable dual-transformer gate driving architecture is developed. This approach decouples switching control from transformer constraints, enabling flexible, microcontroller-compatible arbitrary waveform generation while maintaining nearly uniform voltage sharing across series-connected SiC MOSFETs. Experimental validation demonstrates stable operation at kilovolt levels with nearly even voltage balance.
The work further extends and modifies the proposed new series-connection and gate-current synchronization concepts to ultrafast GaN HEMTs, addressing the challenges posed by nanosecond-scale switching. The developed open-loop, dual transformer gate driving strategy is shown to be well suited for GaN devices, and systematic optimization of transformer and excitation-stage parameters enables balanced voltage sharing at kilovolt levels while preserving the intrinsic speed advantages of GaN technology, confirmed by experimental validation on a hardware prototype under high-voltage, high-dv/dt switching conditions.
Overall, this thesis provides a simple, scalable, and experimentally proven high-voltage switching solution that enables low-voltage wide-bandgap devices to be used in medium-voltage systems. The proposed high-voltage switch has the potential to significantly reduce the complexity of MMC-based arbitrary waveform generator, enabling compact, cost-effective high voltage testing system capable of emulating realistic electrical stresses of renewables-rich hybrid power grid.
This thesis focuses on the design and development of a medium-voltage, medium-frequency solid-state transformer (SST) for large-scale green hydrogen production. The work is motivated by the need to improve the efficiency, compactness, and controllability of power conversion systems that connect renewable energy sources, such as offshore wind, to electrolyzers. Conventional 50/60 Hz transformers and rectifiers are well established but are often heavy, bulky, and limited in performance at high power levels. Solid-state transformers, operating at higher frequencies, offer the potential to reduce system size and weight while improving functionality and efficiency.
The research is conducted within the FlexH2 project, a sponsored program that investigates new concepts for integrating offshore wind energy with onshore hydrogen production. The work presented in this thesis contributes to Work Package 2, which focuses on developing an SST based interface between the medium-voltage AC network and the DC supply of large electrolyzers. Several SST topologies, including the Modular Multilevel Converter (MMC), Resonant Modular Multilevel Converter (MMR), and Input-Series Output-Parallel (ISOP) structures, are analyzed and compared in terms of efficiency, weight, losses, and system complexity.
The main focus of the thesis is on the medium-frequency transformer (MFT), which provides galvanic isolation and voltage conversion within the SST. The study addresses key design challenges, including insulation coordination under non-sinusoidal stress, high-current busbar design, and thermal management in compact, high-power systems. Practical design procedures are proposed for both full-scale and down-scaled transformers. Experimental work on a down-scaled prototype is carried out to verify the analytical and simulation results.
The novel approach using semiconductive coatings is introduced to control electric field distribution and mitigate partial discharges within the transformer. The work also includes guidelines for applying and validating such coatings in dry-type MFT designs.
...
The research is conducted within the FlexH2 project, a sponsored program that investigates new concepts for integrating offshore wind energy with onshore hydrogen production. The work presented in this thesis contributes to Work Package 2, which focuses on developing an SST based interface between the medium-voltage AC network and the DC supply of large electrolyzers. Several SST topologies, including the Modular Multilevel Converter (MMC), Resonant Modular Multilevel Converter (MMR), and Input-Series Output-Parallel (ISOP) structures, are analyzed and compared in terms of efficiency, weight, losses, and system complexity.
The main focus of the thesis is on the medium-frequency transformer (MFT), which provides galvanic isolation and voltage conversion within the SST. The study addresses key design challenges, including insulation coordination under non-sinusoidal stress, high-current busbar design, and thermal management in compact, high-power systems. Practical design procedures are proposed for both full-scale and down-scaled transformers. Experimental work on a down-scaled prototype is carried out to verify the analytical and simulation results.
The novel approach using semiconductive coatings is introduced to control electric field distribution and mitigate partial discharges within the transformer. The work also includes guidelines for applying and validating such coatings in dry-type MFT designs.
...
This thesis focuses on the design and development of a medium-voltage, medium-frequency solid-state transformer (SST) for large-scale green hydrogen production. The work is motivated by the need to improve the efficiency, compactness, and controllability of power conversion systems that connect renewable energy sources, such as offshore wind, to electrolyzers. Conventional 50/60 Hz transformers and rectifiers are well established but are often heavy, bulky, and limited in performance at high power levels. Solid-state transformers, operating at higher frequencies, offer the potential to reduce system size and weight while improving functionality and efficiency.
The research is conducted within the FlexH2 project, a sponsored program that investigates new concepts for integrating offshore wind energy with onshore hydrogen production. The work presented in this thesis contributes to Work Package 2, which focuses on developing an SST based interface between the medium-voltage AC network and the DC supply of large electrolyzers. Several SST topologies, including the Modular Multilevel Converter (MMC), Resonant Modular Multilevel Converter (MMR), and Input-Series Output-Parallel (ISOP) structures, are analyzed and compared in terms of efficiency, weight, losses, and system complexity.
The main focus of the thesis is on the medium-frequency transformer (MFT), which provides galvanic isolation and voltage conversion within the SST. The study addresses key design challenges, including insulation coordination under non-sinusoidal stress, high-current busbar design, and thermal management in compact, high-power systems. Practical design procedures are proposed for both full-scale and down-scaled transformers. Experimental work on a down-scaled prototype is carried out to verify the analytical and simulation results.
The novel approach using semiconductive coatings is introduced to control electric field distribution and mitigate partial discharges within the transformer. The work also includes guidelines for applying and validating such coatings in dry-type MFT designs.
The research is conducted within the FlexH2 project, a sponsored program that investigates new concepts for integrating offshore wind energy with onshore hydrogen production. The work presented in this thesis contributes to Work Package 2, which focuses on developing an SST based interface between the medium-voltage AC network and the DC supply of large electrolyzers. Several SST topologies, including the Modular Multilevel Converter (MMC), Resonant Modular Multilevel Converter (MMR), and Input-Series Output-Parallel (ISOP) structures, are analyzed and compared in terms of efficiency, weight, losses, and system complexity.
The main focus of the thesis is on the medium-frequency transformer (MFT), which provides galvanic isolation and voltage conversion within the SST. The study addresses key design challenges, including insulation coordination under non-sinusoidal stress, high-current busbar design, and thermal management in compact, high-power systems. Practical design procedures are proposed for both full-scale and down-scaled transformers. Experimental work on a down-scaled prototype is carried out to verify the analytical and simulation results.
The novel approach using semiconductive coatings is introduced to control electric field distribution and mitigate partial discharges within the transformer. The work also includes guidelines for applying and validating such coatings in dry-type MFT designs.
Master thesis
(2025)
-
T.B. Plantfeber, P.T.M. Vaessen, D. van der Born, A.G.A. Lathouwers, Sebastian Rivera Iunnissi
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. ...
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. ...
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.
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.
The increasing integration of renewable energy sources and distributed generation necessitates stringent testing of critical power grid components to ensure system reliability and safety. This thesis addresses two principal challenges: the high-current testing of protection relays and the high-frequency waveform testing of energy meters. Protection relays must withstand extreme fault conditions, with standards like IEC 60255-27:2023 mandating tests with currents up to 100 times the rated current for one second (thermal stress) and 250 times for half a power cycle (dynamic stress). Concurrently, the proliferation of power electronic devices introduces high-frequency harmonics (supraharmonics) in the 2 kHz to 150 kHz range, which can severely compromise the accuracy of energy meters. Conventional test equipment struggles to meet these demands, as high-current transformers are typically bulky with high leakage inductance, while generating high-fidelity, high-di/dt waveforms is limited by existing switching technologies. To address the challenge of high-current generation, this research details the design, simulation, and construction of a compact, low-leakage-inductance current transformer. The design process involved a comprehensive methodology, including core material selection (nanocrystalline AMCC1000), winding optimization, and validation using COMSOL Multiphysics and MATLAB/Simulink. A key innovation of this work is the implementation of a parallel-core configuration. By arranging multiple identical C-cores in parallel, the effective leakage inductance and winding resistance are significantly reduced. This novel approach enables the generation of high-magnitude transient currents exceeding 800 A with the rapid rise times required for dynamic withstand tests, a feat not achievable with a conventional single-core design under similar size and input power constraints (230 V, 32 A). For the testing of energy meters, this thesis explores the development of a versatile high-frequency current waveform generator. A proof-of-concept system was designed and implemented using an Hbridge inverter (L298N module) controlled by an Arduino microcontroller with custom firmware. This lowpower prototype successfully generated a variety of complex, high-di/dt current waveforms, including trapezoidal and phase-fired sinusoids, which are essential for evaluating the susceptibility of energy meters to supraharmonic disturbances. The results validate the inverter-based methodology and lay the groundwork for a future high-power implementation using advanced Gallium Nitride (GaN) devices and DSP-based control, which will overcome the limitations of the current prototype. In conclusion, this research contributes novel and practical solutions for the comprehensive testing of modern power system components. The developed parallel-core transformer provides an efficient method for high-current relay testing, while the waveform generator demonstrates a flexible approach for assessing energy meter performance under non-sinusoidal conditions. Together, these advancements support the development of more robust and reliable equipment, enhancing the stability and accuracy of evolving electrical networks.
...
The increasing integration of renewable energy sources and distributed generation necessitates stringent testing of critical power grid components to ensure system reliability and safety. This thesis addresses two principal challenges: the high-current testing of protection relays and the high-frequency waveform testing of energy meters. Protection relays must withstand extreme fault conditions, with standards like IEC 60255-27:2023 mandating tests with currents up to 100 times the rated current for one second (thermal stress) and 250 times for half a power cycle (dynamic stress). Concurrently, the proliferation of power electronic devices introduces high-frequency harmonics (supraharmonics) in the 2 kHz to 150 kHz range, which can severely compromise the accuracy of energy meters. Conventional test equipment struggles to meet these demands, as high-current transformers are typically bulky with high leakage inductance, while generating high-fidelity, high-di/dt waveforms is limited by existing switching technologies. To address the challenge of high-current generation, this research details the design, simulation, and construction of a compact, low-leakage-inductance current transformer. The design process involved a comprehensive methodology, including core material selection (nanocrystalline AMCC1000), winding optimization, and validation using COMSOL Multiphysics and MATLAB/Simulink. A key innovation of this work is the implementation of a parallel-core configuration. By arranging multiple identical C-cores in parallel, the effective leakage inductance and winding resistance are significantly reduced. This novel approach enables the generation of high-magnitude transient currents exceeding 800 A with the rapid rise times required for dynamic withstand tests, a feat not achievable with a conventional single-core design under similar size and input power constraints (230 V, 32 A). For the testing of energy meters, this thesis explores the development of a versatile high-frequency current waveform generator. A proof-of-concept system was designed and implemented using an Hbridge inverter (L298N module) controlled by an Arduino microcontroller with custom firmware. This lowpower prototype successfully generated a variety of complex, high-di/dt current waveforms, including trapezoidal and phase-fired sinusoids, which are essential for evaluating the susceptibility of energy meters to supraharmonic disturbances. The results validate the inverter-based methodology and lay the groundwork for a future high-power implementation using advanced Gallium Nitride (GaN) devices and DSP-based control, which will overcome the limitations of the current prototype. In conclusion, this research contributes novel and practical solutions for the comprehensive testing of modern power system components. The developed parallel-core transformer provides an efficient method for high-current relay testing, while the waveform generator demonstrates a flexible approach for assessing energy meter performance under non-sinusoidal conditions. Together, these advancements support the development of more robust and reliable equipment, enhancing the stability and accuracy of evolving electrical networks.
Master thesis
(2025)
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M. Rom, F.A. Muñoz, Helko (H.E.) van den Brom, P.T.M. Vaessen, D. van der Born, J. Dong
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. ...
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. ...
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.
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.
Master thesis
(2025)
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L. Šćulac, Frank Mauseth, P.T.M. Vaessen, Hans Kristian Hygen Meyer, Hani Vahedi, Mohamad Ghaffarian Niasar
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.
...
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.
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.... ...
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.... ...
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....
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....
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....
...
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....
Evaluating EMF Emissions in Submarine HVAC Cables
Innovative Approaches for Modeling Current Distribution and Environmental Impact
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.
...
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. ...
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. ...
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.
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.
Implementation of a high-voltage MMC submodule
For a multilevel modular converter
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.
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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.
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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.
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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.
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.
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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.
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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.
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.
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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.
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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.
Master thesis
(2020)
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Avinash Nayak, A. Rodrigo Mor, F.A. Muñoz Muñoz, L.C. Castro Heredia, P.T.M. Vaessen
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. ...
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.