Litz wires, which are utilized to suppress eddy current, often have complex structures. This paper presents a partial element equivalent circuit (PEEC)-based 3D model for Litz wires with round conductor. The model accounts for both transverse and longitudinal magnetic fields. The discretization of the Litz wire is based on cylindrical elements resulting in a reduced number of elements. Cylindrical element analysis is based on a 2D analytical method. The proposed model is compared with 3D FEM, which shows the model has good accuracy and fast computational speed. It is promising to facilitate Litz wires optimization.

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More electric aircraft (MEA) is an important direction for future aircraft development. The printed circuit boards (PCBs) of power electronics equipment in MEA operate in a complex environment of low air pressure, square wave voltage, compact layout, and strong electromagnetic interference, which makes the PCB more prone to partial discharge (PD). However, the effective detection method and related discharge characteristics for PCB PD under square wave voltage are still unclear. Therefore, this article proposes a PCB PD detection method based on fluorescent optical fiber, which has good anti-electromagnetic interference ability. The PD characteristics of the PCB with three typical structures under three air pressures are studied, including PD nonbreakdown characteristics, changes in optical pulse amplitude and pulse repetition rate during PD aging, PD breakdown path analysis, and PCB surface electric field simulation. The analysis shows that air pressure, space charge, and PCB surface flatness all have important influences on the PD of PCB under square wave voltage. It provides important theoretical support for the insulation protection design and fault diagnosis of PCB for MEA in the future.

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The evolution of electrical power systems demands an increasing reliance on unpredictable renewable energy resources (RES). However, integrating these resources poses challenges, as their intermittent nature introduces transient events that can significantly impact power transformers. These transient phenomena may initiate energy oscillations in the form of weakly-damped resonances between system elements, i.e., transmission lines and cables, transformers, and the grounding system. Such conditions may impose stresses beyond the tolerance of insulating materials, leading to fast lifetime degradation and, eventually, the failure of critical components in the network, such as the costly power transformers. The impedance of the grounding system can limit the dissipation of surges, hence, causing severe overvoltages upon transient phenomena. By employing detailed transient models of crucial system components, this research puts forward a comprehensive analytical study of the transient interactions. In this regard, an analytical high-frequency transformer winding model based on lumped elements, and wideband frequency-dependent models for cables and the grounding system derived by applying electromagnetic theory are presented. These models, integrated into electromagnetic transient software, enable the identification of vulnerabilities and examination of case studies involving lightning strikes and switching events. Furthermore, the details of a novel protection method applied to safeguard the transformer are discussed in this paper. The presented protection method consists of a ring toroid core and a resistive suppressor on the secondary side of the core. This protection component is connected in series with the transformer to decrease the harmonic content and magnitude of the transient signals. The design procedure of the series protection device against voltage transient signals is presented and elaborated. @en

This article proposes a new configuration of a modular multilevel converter (MMC) and a Marx generator to generate fast-rising impulse waveforms. This new configuration improves the capabilities of the MMC-based high voltage arbitrary wave shape generator to generate fast-rising impulse since the MMC topology faces many inherent limitations. Similar to the conventional superimposed circuit of the ac transformer or dc rectifier circuit with the Marx generator, three hybrid circuits of MMC and the Marx generator are introduced, where the most optimal choice is made considering the practical aspects of testing, such as the size, cost, and preparation time. Then, a detailed analytical study is performed on the Marx generator circuit and the MMC circuit, and both circuits are coupled together to deliver a complete guideline on choosing various system parameters when the impulse wave shape and the load capacitor are given. The concept of this new hybrid configuration is demonstrated with a scaled-down prototype where the impulse with a rise time of 1 μs is superimposed on different arbitrary wave shapes. Similarly, the MATLAB-Simulink simulation model validates the proposed configuration for a 200, k V dc link voltage and 67 submodules with the desired impulse performance.

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The rapid increase of integrated distributed generators results in higher fault currents in the future modern grids. A remedy for the concern is employing series reactors as fault current limiters. This paper elaborates on a ferromagnetic core series reactor, which, when saturated, adversely affects the operation of the series reactor during faults. The main goal of the paper is to calculate grid and series reactor coefficients by applying a simplified power line model during a fault condition. These coefficients are the primary considerations of a series reactor design to avoid its saturation. Moreover, the study of the relationship between the reactor inductance and obtained coefficients will be carried out. The obtained results are validated by simulations performed in MATLAB Simulink.@en

Dealing with the fast-rising current of high voltage direct current (HVdc) systems during fault conditions, is one of the most challenging aspects of HVdc system protection. Fast dc circuit breakers (DCCB) have recently been employed as a promising technology and are the subject of many research studies. HVdc circuit breakers (CBs) must meet various requirements to satisfy practical and functional needs, among which fast operation, low voltage stress, and economic issues are the key factors. This article presents the procedure for designing a superconductive reactor-based DCCB (SSR-DCCB) for HVdc applications. In the proposed structure, a full-bridge power electronic configuration controls the superconducting reactor to limit the dc fault current and create a dc zero-crossing; it is connected to the HVdc line by a series transformer. After successfully suppressing the line fault current (current zero current), an ultrafast disconnector isolates the faulty line. The main advantage of the proposed HVdc CB is its ability to interrupt the dc fault current without using the solid-state main breaker and limit the magnitude of the fault current and voltage stress. The proposed SSR-DCCB is investigated in MATLAB/Simulink, and an experimental prototype setup validates the results.

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Power transformer energization involves a significant electromagnetic energy exchange among system components, which periodically oscillates with the natural frequencies of the system. As a result, weakly damped resonating overvoltages (OVs) may prevail, overstressing the system and thus leading to potential insulation failure. This phenomenon is particularly notable in cable-transformer systems, where the coinciding of natural frequencies is more likely to occur. The prestriking phenomenon during circuit breaker (CB) closing plays an important role in the excitation of the resonance frequencies. Specifically, repeated prestrikes can create high-frequency resonances during switching-on operations. This paper deals primarily with the mutual interactions between cable and transformer during transformer energization. Furthermore, by using a suitable model, the impact of CB prestrikes on the resultant resonance excitation is analyzed. Results demonstrate that cable-transformer interactions can lead to high-frequency oscillatory OVs with extreme magnitudes.@en

The energy transition involves integrating numerous pieces of equipment that undergo frequent switching operations and face the risk of lightning strikes. Consequently, power systems are exposed to fast transient switching and lightning surges, necessitating enhanced protection solutions for power equipment. Over the past decade, viable solutions have emerged to mitigate fast transients and safeguard transformers in the form of a ring-core parallel inductor and resistor circuit (R-PIR). Although this device effectively protects medium-voltage transformers from fast transients, a precise and comprehensive model for this component is still lacking, especially considering the importance of refining the R-PIR for broader applications. This paper introduces a detailed model of the R-PIR as a protective device, validated by electromagnetic transient simulations and finite element methods, which are also confirmed by experiments. The main goals of the research work are to investigate the performance and design features of the R-PIR comprehensively and demonstrate how the designed R-PIR protects transformers against fast transients. The research work is validated by experiments conducted in a lab environment. It is concluded that designing the R-PIR within an appropriate frequency range can considerably suppress transient overvoltages to which the transformer is exposed.

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More and more printed circuit boards (PCBs) will be used in electric aircraft to achieve higher power density of the on-board electric system, while PCBs in aircraft are more likely to generate partial discharges (PDs) due to external factors such as compact structure and low air pressure. Therefore, this article proposes a fluorescent fiber-based method for detecting and evaluating PDs on PCBs. The detection method is effectively immune against the presence of the electromagnetic, acoustic, and vibration interference. Based on the optical detection, the evolution regularities of PCB surface appearance changes, optical PRPD patterns, and optical pulses during the aging process of PCB under different air pressure are analyzed in this article. Then, 12 assessment features are extracted for the PD aging process, and the contribution of these 12 features to the severity assessment at different air pressures is obtained using the minimal-redundancy-maximal-relevance (mRMR) algorithm. Finally, different numbers of PD features are tested for PD severity assessment by the support vector machine (SVM) algorithm. The evaluation results show that the severity assessment method proposed in this article can achieve an assessment accuracy of at least 91.1% and up to 94.4% under all three air pressures, which has good application and guidance value.

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SF6 is being phased out of the electrical grid as it is a strong greenhouse gas. A 'green' alternative to SF6 is fluoronitrile (C4-FN). The dutch transmission system operator (TSO) TenneT wanted to investigate the electrical behavior of this 'green' alternative as a pilot-gas insulated line (GIL) filled with C4-FN/CO2 (5%/95%) experienced multiple electrical breakdowns during the site acceptance tests (SATs). TenneT hypothesized that the breakdown was caused by the effect of (too much) humidity in the gas. Therefore, the ac breakdown behavior of the gas mixture has been researched under different amounts of humidity and operating pressures. This article also makes a small introduction on how humidity affects partial discharge (PD) behavior of corona, which is often a breakdown indicator. The results of this research conclude that humidity affects the ac breakdown strength. An increase in humidity in the gas results in a decrease in the ac breakdown strength. Moreover, the field configuration determines the amplitude of the impact. The impact of humidity on the ac breakdown strength in a homogenous field is substantially more compared to an inhomogeneous field, where the impact can almost be ignored. Yet, the effect of humidity decreases as the operating pressure increases. The phase-resolved PD pattern and PD characteristics of corona in a C4-FN gas mixture also differ with humidity. On the other hand, the PD inception voltage did not change with the humidity content. As a result of the findings in this research, further research is proposed toward the affected breakdown mechanism and more various PD behavior.

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The aviation industry, responsible for over 2% of energy-related CO₂ emissions in 2022, aims for Net Zero Emissions by 2050. Despite electric aircraft's environmental benefits and improved operational efficiency, the current battery technology limits their range and size. Based on the optimal system voltage and power profile of the reference all-electric aircraft, this paper presents the design of a hybrid reconfigurable battery pack. The design incorporates a combination of high-specific-energy (263 W h kg^-1 at cell level) and high-specific-power (1800 W kg^-1 at cell level) battery types and its performance is compared with that of a fixed configuration battery pack comprising a single battery type. Simulation results suggest a potential 900 kg (18% lighter than fixed configuration) weight savings with reconfigurable pack, translating into enhanced payload, energy savings, or range extension for 9 PAX Eviation Alice electric aircraft at just 0.4% more energy capacity loss in 500 cycles.

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This paper discusses the prototype development and testing of a load invariant, high voltage gain DC power supply using inductive power transfer. The developed supply is intended to power the submodules of the CHB based arbitrary waveshape generator for testing high voltage components and materials. Using the Series Parallel topology, a load invariant voltage gain of 15 is obtained from a 350 V input DC Bus, with an efficiency of 88% and a load regulation of 18%. The obtained 5 kV output can conduct DC breakdown of dielectrics with a damping resistor and can test insulation aging, breakdown under high frequency AC.@en

This paper investigates the high-frequency transformer losses attributed to eddy currents in both conductors and the core. A comprehensive model of a transformer winding is presented, meticulously incorporating skin and proximity effects. The winding resistance increase due to these effects is determined by applying analytical and finite element methods. The investigation highlights that eddy current losses in the magnetic core notably increase the resistance of each winding section in low frequencies. However, the impact of these losses diminishes as the winding length approaches the voltage wavelength at high frequencies. Consequently, the net magnetomotive force and flux in the core become negligible. As a result, the high-frequency impedance characteristics are remarkably similar for windings with and without a magnetic core. These findings are substantiated through rigorous simulations and empirical measurements.@en

To enhance the voltage-handling capability of a switch, the series connection of switching devices is a cost-effective method that preserves many advantages of mature low-voltage devices. Dynamic voltage imbalance and electrical isolation for the devices at the high voltage (HV) side are two important challenges associated with series connection topology. Transformer-coupled gate drivers are excellent for providing both dynamic voltage balance and high galvanic isolation. However, they can only provide the switching function at the transformer pulse frequency. To generate complex waveforms of future power-electronics-dominated grids, a switch with user-defined turn-on/off timing is required for testing grid assets under high-voltage conditions. This article presents a simple, cost-effective open-loop gate driver that overcomes this limitation by introducing two sets of complementary pulse transformers to initialize programmable frequency and duty cycle. Successful experimental verification of the series-connected SiC mosfets prototype is performed at 3.2 kV at various frequencies and duty cycles. The article also demonstrates that the measurement probes placed across series-connected mosfets significantly affect the voltage distribution and validate a compensation mechanism.

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Traditional methods such as Steinmetz's equation (SE) and its improved variant (iGSE) have demonstrated limited precision in estimating power loss for magnetic materials. The introduction of Neural Network technology for assessing magnetic component power loss has significantly enhanced accuracy. Yet, an efficient method to incorporate detailed flux density information—which critically impacts accuracy—remains elusive. Our study introduces an innovative approach that merges Fast Fourier Transform (FFT) with a Feedforward Neural Network (FNN), aiming to overcome this challenge. To optimize the model further and strike a refined balance between complexity and accuracy, Multi-Objective Optimization (MOO) is employed to identify the ideal combination of hyperparameters, such as layer count, neuron number, activation functions, optimizers, and batch size. This optimized Neural Network outperforms traditionally intuitive models in both accuracy and size. Leveraging the optimized base model for known materials, transfer learning is applied to new materials with limited data, effectively addressing data scarcity. The proposed approach substantially enhances model training efficiency, achieves remarkable accuracy, and sets an example for Artificial Intelligence applications in loss and electrical characteristic predictions with challenges of model size, accuracy goals, and limited data.@en

This paper introduces an innovative Magnetic Switch (MFS) designed to control and alter magnetic flux within energy system components, offering an alternative to conventional power electronic devices. The MFS comprises a low-current control coil, a control core, and a high-density magnetic flux-carrying main core combined with a main coil energy system. In this novel magnetic configuration, when a low-power current excites the control coil, the magnetic flux in the main core (supplied by the main coil) decreases to nearly zero. Conversely, when the control coil disconnects from the power source, the magnetic flux within the main core attains its maximum value. This operation positions the MFS as a groundbreaking concept within magnetic-based energy systems, akin to transistors in power electronics. The main outcomes of the MFS concept are that it can vary the magnetic flux of the main core in the large range, and it is a fast magnetic switch with a simple and low power loss control circuit and an independent control coil from the main coil. Analytical studies thoroughly elucidate the performance and advantages of this proposed magnetic switch, substantiated by Finite Element Method simulations and experimental prototype outcomes.@en

This article presents a 3-D numerical impedance calculation method based on cylindrical elements. It can be used to model the Litz wire and further air-core coil wound by the Litz wire. The discretization is based on cylindrical elements, resulting in a small amount of elements. Cylindrical element analysis is based on a 2-D analysis and its analog to 3-D. The analysis considers both transverse and longitudinal magnetic fields applied to elements. The proposed method is applied to several Litz wires and compared with 3-D finite element method (FEM), which validates that the method has good accuracy and fast computational speed. The effectiveness of the method for the air-core coil is validated by measurements. The proposed method is promising in facilitating coil optimization.

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As the integration of renewable energy sources into the grid increases, the insulation systems of grid components such as transformers and switchgear encounter significant challenges due to the transients and harmonics generated by power-electronic-based converters. A test generator capable of replicating these component stresses is essential to accurately evaluate these insulation systems under real-grid conditions. This paper proposes a modular cascaded H-bridge-based high-voltage arbitrary waveform generator, prototyped with three stages to generate customized waveforms (triangular, sawtooth, pulse, and complex) up to 8 kV. The H-bridge modules are designed using Si MOSFETs with a maximum blocking voltage of 4.5 kV. The input to the HV H-bridge module is provided by a 10 kV medium-frequency transformer, whose design is described with a focus on the insulation system and winding configuration. This transformer is driven by a zero-voltage switching driver. This arbitrary waveform generator excels in several aspects, including a straightforward design procedure, compact size, high voltage capability, ease of integration, and cost.@en

Due to climate change more power converters are installed in the grid resulting in the injection of (supra-) harmonic currents and voltages in the grid. As part of a larger research, the impact of harmonic frequencies on insulation materials is studied. This paper discusses the results of breakdown voltage tests of synthetic ester liquid with varying frequency and electrode distance. A test cell according to IEC 60156 is used with electrode distance varying from 0.5 mm to 0.1 mm and the frequency is varied from 50 Hz to 2000 Hz. The results indicate a dependency on both electrode distance and fundamental frequency.@en

Innovative test methods are required to keep up with the increased demand for insulation materials which can withstand the high-frequency square-wave voltages generated by power-electronic equipment. Test systems using high-voltage, high-frequency transformers have proven versatile and easy to realise. The authors investigate the application of amorphous cut cores in such transformers. Analytical and numerical models for the transformer and its frequency response are developed, aiding the design process. An amorphous core-based transformer is designed for 8 kV_pk output at 100 kHz and compared to two previous designs based on ferrite cores. The frequency and pulse response, as well as the high-voltage and thermal performance, are evaluated. The comparison shows that while the low parasitics of the amorphous-based transformer allow for superior frequency response, they are unsuitable for long-duration tests with high pulse repetition frequencies (25–100 kHz) due to increased core losses. The partial discharge inception and flashover voltage are comparable to the ferrite-based transformers.

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