Conference Agenda

The transition toward ultra-wide-bandgap ((U)WBG) power devices has accelerated the demand for compact, electrically robust packaging capable of withstanding extreme field stresses. This study presents a comprehensive three-dimensional (3D) electrostatic analysis of the electric field distribution within a compact package developed for vertical β-Ga2O3 Schottky barrier diodes (SBDs). The device structure integrates field management using a high-permittivity dielectric (ZrO2) field plate deposited on a bilayer of ZrO2 and interfacial Al₂O₃. To evaluate the influence of package-level geometry on electric field stress, two distinct comparative studies are conducted using COMSOL Multiphysics: (i) the effect of substrate thickness variation (400 μm vs. 650 μm) on the electric field profile, and (ii) the impact of the field plate itself by comparing devices with and without the termination structure. The 3D simulations capture complex field interactions arising from dielectric interfaces, and from the full package geometry. The results confirm that the presence of the high-k ZrO2 field plate significantly reduces field crowding at the Schottky contact periphery, demonstrating its critical role in achieving higher breakdown voltages. These findings provide a validated framework for optimizing insulation design in β-Ga2O3 power modules, decoupling the electrostatic performance from substrate parameters to enable reliable high-voltage operation in next-generation energy conversion systems.

As the core component of electric vehicle (EV) powertrains, the insulation performance of traction motors is critical for vehicle reliability. With the widespread adoption of 800V fast-charging platforms, motor systems face more severe electrical stress environments. While higher voltages reduce copper losses and improve efficiency, conventional insulation systems rated below 540V encounter intensified partial discharge (PD) risks. Under high-frequency pulse width modulation (PWM) excitation from inverters, Class I insulation systems composed of organic materials - whether employing hairpin or round wires - must effectively suppress PD throughout their service life. This requirement calls for the adoption, during motor design, of PD inception voltage (PDIV) testing methods capable of simulating inverter pulse conditions. By comparing operating voltages and incorporating environmental factors (aging, temperature, humidity), sufficient insulation margins can be ensured. This study systematically evaluates stator winding PDIV characteristics under simulated electrical stress using a full-bridge pulse generator producing adjustable square waves (amplitude/rise time/pulse width). The research investigates coupling effects between pulse parameters and environmental conditions, providing methodological support for insulation design and testing in high-voltage traction motors.

Inverter-fed motors have become increasingly prevalent in both industrial and mobility applications due to the growing demand for environmentally friendly technologies. As their adoption expands, enhancing motor performance and reliability has become increasingly important. Concurrently, advances in power electronics have resulted in higher switching frequencies and faster voltage rise times (dv/dt) in inverter output pulses. Consequently, the surge voltages induced in inverter-fed motors have increased, raising concerns regarding their impact on motor insulation systems. These surge voltages can initiate partial discharges (PD) within the insulation of motor windings, which gradually degrade the insulation and may ultimately lead to dielectric breakdown. Therefore, understanding PD characteristics under repetitive surge voltages typical of inverter-fed operation is essential for improving motor reliability.

In practical applications, inverter-fed motors are often exposed to humid environments. It has been reported that variations in humidity can influence both the partial discharge inception voltage (PDIV) and the repetitive partial discharge inception voltage (RPDIV). The present study aims to elucidate the factors affecting PDIV and RPDIV under humid conditions relevant to inverter-fed motors. Experiments were conducted in a humidity chamber using twisted-pair insulation test samples and a pulse power supply based on a SiC-MOSFET module to measure PDIV and RPDIV under various humidity levels. In parallel, analytical evaluations based on the volume–time theory were performed to investigate the mechanisms underlying the observed variations.

The volume-time theory is an estimation method for PD probability that takes into account the spatial and temporal variations of the electric field strength and the initial electron generation probability in a gas. In the analysis, two humidity-dependent parameters were considered as potential influencing factors: the relative permittivity of the insulation coating and the density of negative ions in air. Incorporating these factors into the computational model allowed a quantitative assessment of humidity effects on PDIV and RPDIV. Experimental results indicated that both PDIV and RPDIV decrease with increasing humidity. Analytical results suggested that this decrease is primarily attributed to the increase in the relative permittivity of the insulation coating, whereas the effect of negative ion density in air is negligible.

These findings indicate that variations in PDIV and RPDIV of inverter-fed motors under humid conditions are predominantly governed by changes in the dielectric properties of the insulation coating. This understanding provides valuable insights for designing more robust motor insulation systems capable of withstanding diverse environmental conditions and the high dv/dt stresses associated with inverter operation.

Safe operation of superconducting magnetic levitation (SC Maglev) systems requires accurate and reliable insulation monitoring for the large number of propulsion and levitation-guidance coils installed along the U-shaped guideways. These coils are critical components of SC Maglev systems. Any insulation degradation in these coils can compromise system safety and performance. Ideally, monitoring the condition of these coils should be performed from a running train, enabling real-time assessment without disrupting operations or requiring manual inspections, which can be time-consuming and impractical given the large number of coils. When precursor phenomena indicative of insulation deterioration occur, it is essential to determine whether the affected coils can continue normal operation or require immediate maintenance, repair, or replacement.

To address this need, we are developing an on-board radio interferometer system equipped with a vector-antenna capable of detecting partial discharges (PDs), which are precursors of insulation degradation. Partial discharges generate high-frequency electromagnetic (EM) waves that propagate from the source and can be detected remotely. The radio interferometer system captures not only the amplitude but also the directional characteristics of the EM waves, providing more detailed information on the location and nature of PD events.

When installed on a SC Maglev train traversing the operational route multiple times per day, the radio interferometer system can capture EM waves emitted by PDs in any degraded ground coils on each pass. Signals are digitized and accumulated over repeated runs, enabling statistical analysis. This allows trend identification, separation of transient noise from persistent PD activity, and extraction of features indicative of insulation defects.

In this study, a PD source was generated in a mock propulsion coil positioned at the center of a three-coil arrangement. The on-board radio interferometer system automatically received the EM waves emitted from the PD source and stored them as digital data. By calculating the average waveform and amplitude spectrum for each channel of the interferometer’s antenna-array, distinctive features of the PD signals were identified. Two antenna-arrays, each equipped with a vector-antenna, were oriented toward the ground coils on both sides, enabling simultaneous monitoring.

By integrating these statistical results with previously reported PD localization methods, the reliability of detecting and identifying propulsion coils exhibiting insulation abnormalities can be enhanced. This approach has the potential to provide continuous, non-invasive, and highly accurate monitoring of SC Maglev systems, supporting the safe and efficient operation of trains on the guideway network.

Wide-bandgap (WBG) inverters impose fast-switching voltage pulses that accelerate turn-to-turn partial discharge (PD) degradation in electric vehicle hairpin windings. This study evaluates magnet wires using an improved back-to-back flat-contact configuration representative of rectangular conductors. Samples were aged for 24 hours under 2.5 kV, 40 ns repetitive pulses with controlled overshoot, frequency, and duty cycle. Insulation degradation was quantified using partial discharge inception voltage (PDIV), dielectric frequency response, temperature monitoring, and atomic force microscopy. Non-corona-resistance, wires exhibited greater PDIV reduction and dielectric loss increase compared to corona-resistance wires. Switching frequency most reduced PDIV, overshoot increased dielectric loss, and combined electro-thermal stress at 100 °C markedly accelerated PD-driven degradation, whereas thermal exposure alone produced minimal change.

This paper presents experimental investigations on inverter voltage endurance testing of enamelled copper wires under conditions relevant to electric vehicle applications. With the transition from 400 V to 800 V drivetrains and the increasing use of fast-switching semiconductors such as SiC-MOSFETs, electrical and thermal stresses on motor windings have intensified, making the durability of enamel insulation a critical factor. To address this, a self-developed inverter pulse generator was employed, capable of producing bipolar square-wave signals up to 4.0 kV peak-to-peak, rise times between 50–400 ns, and switching frequencies up to 20 kHz, thus enabling systematic studies of parameter influences.

The test methodology primarily relied on twisted-pair samples of enamelled copper wires, in accordance with international standards, with insulation degradation tracked via partial discharge (PD) activity, breakdown voltage measurements, and lifetime testing under controlled oven temperatures. To reduce overshoot and measurement artifacts, a low-inductive arrangement and calibrated high-voltage probes were implemented.

Results indicate that partial discharge inception voltage (PDIV) differs significantly between power frequency (50 Hz) and inverter-like voltage operation, though PDIV itself is largely insensitive to switching frequency. No statistical correlation was observed between 50 Hz breakdown strength and PDIV. However, electric lifetime shows a clear dependence on several stress factors: it decreases with increasing switching frequency, higher peak-to-peak voltages, faster rise times, and elevated temperatures. Regression analyses confirm strong statistical significance between switching frequency, PDIV, and lifetime.

Material selection plays a decisive role in endurance. Among four tested enamel systems, a conventional insulation exhibited the shortest characteristic lifetime (~0.18 h), while nano-particle enhanced wires achieved lifetimes up to 213 times longer. Nonetheless, confidence intervals for nano-filled samples were broader due to non-uniform particle dispersion. Comparisons between twisted-pair and plate-type test geometries revealed that the latter yields lower data scattering, likely due to reduced mechanical stresses during preparation.

In summary, the findings underscore that inverter-induced stresses can drastically shorten wire lifetime and that insulation material design, combined with appropriate testing methodologies, is essential for reliable lifetime assessment. Future work within a government-funded project aims to establish a general lifetime model incorporating electrical, thermal, and mechanical stress interactions.