Conference Agenda

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Session Overview
Session
Dielectrics I
Time:
Monday, 20/June/2022:
3:30pm - 5:30pm

Session Chair: Jennifer Zirnheld, University at Buffalo
Location: Ballroom C

Oral Session

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Presentations
3:30pm - 3:50pm
Dielectrics I: 1

Effect of Humidity on Electrical Characteristics of an Arc in Air at Normal Flight Altitudes

D. Grosjean1, D. Schweickart2

1Innovative Scientific Solutions, Inc., United States of America; 2Air Force Research Laboratory/RQQE, United States of America

Although prevention of unwanted electrical arcing is an important goal in the design of aircraft electrical power systems, insulation aging and unexpected damage lead to a need for monitoring of a system for arcing faults in order to maintain reliability. One popular technique for arc detection is current signature analysis which relies upon predictable current/voltage characteristics of electrical breakdown of a gaseous gap. Once initiated, the voltage and current characteristics of an arc are a function of a complicated interaction of numerous collisional processes, creating many challenges in modeling and simulation. A more practical approach is to design an arc-fault detection system based upon consistent laboratory and field measurements.

Because gas constituents are major determinants of arc electrical characteristics, potential errors in design and checkout of an arc-fault-detection scheme can arise if variations in gas pressure and specie concentrations are not considered. Experience has shown that partial-discharge characteristics in air are affected by humidity (water vapor) but it’s not clear if there is a similar effect in arcing in an air atmosphere.

Measurements were made of electrical characteristics of arcs in air at various humidity levels at pressures corresponding to sea level and 60,000-Ft (18.3-km) flight altitude. Results show that there is no significant effect resulting from variations in humidity that may be introduced by uncontrolled laboratory air, even when water-vapor levels are well above those expected at temperatures < 0 C.



3:50pm - 4:10pm
Dielectrics I: 2

Simulation and Analysis of Initial Stages of Negative Discharges in Air for Needle-Plane Electrode Configuration

M. Hamidieh, M. Ghassemi

Virginia Tech, United States of America

This paper presents a model for the initial stages of negative discharge in the atmospheric air under an extremely nonuniform field condition caused by a needle-plane electrode configuration. To model the dynamics of charged species, the drift-diffusion model is employed while taking into account the secondary electron emissions and addressing the mathematical challenges to solve the problem. Simulations are carried out in the COMSOL Multiphysics software. Analyses are performed mainly on the variations of the generation and loss rates of the charged species, their concentration evolution, and corresponding electric field distribution alterations.



4:10pm - 4:30pm
Dielectrics I: 3

Study of anode-initiated surface flashover in vacuum with spatiotemporally resolved optical emission spectroscopy

R. M. Clark1, M. P. Mounho1, W. C. Brooks1, M. M. Hopkins2, J. J. Mankowski1, J. C. Dickens1, J. C. Stephens1, A. A. Neuber1

1Texas Tech University, Lubbock, TX, United States of America; 2Sandia National Laboratories, Albuquerque, NM, United States of America

Improving the flashover holdoff of electrode-insulator junctions in vacuum systems is a critical step in developing future large-scale pulsed power devices. The insulator stacks in such environments are typically positive 45 degree systems. This configuration exhibits improved voltage holdoff over 90 degree systems due to the suppressed field at the cathode triple junction at the cost of an enhanced field at the anode triple junction. One finds a roughly three times higher field amplitude at the anode, and it is thus argued that the flashover mechanism for these geometries is anode-initiated. To investigate the underlying physical mechanisms of anode-initiated surface flashover, a positive 45-degree flashover fixture has been developed using a hemispherical anode.

The specific geometry localizes the flashover path for improved diagnostic accessibility while maintaining comparable fields to the traditional ring-link insulator stack structure. The source is an eight-stage, 240 kV Marx generator with rise times of a few tens of nanoseconds, resulting in anode triple junction fields in excess of 500 kV/cm for a cross-linked polystyrene (Rexolite) insulator. In addition to electrical diagnostics and imaging, spatially-resolved optical diagnostics are implemented to interrogate the developing plasma light from the UV through the visible. Light emitted from insulator regions near the anode and the cathode is focused into fiber optics by two pairs of rod lenses. The fibers are fed into nanosecond fast photomultiplier tubes for spectrally-integrated light intensity waveforms, which are compared against voltage waveforms to determine initiation fields. Results have indicated that early anode light precedes early cathode light by several nanoseconds during the early stages of the flashover. Time-resolved anode and cathode spectra are obtained with an Oriel MS-257 spectrograph and accompanying intensified CCD camera. The spatiotemporal development of desorbed gas species, electrode involvement, and bulk insulator involvement in the flashover process are examined.

SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525



4:30pm - 4:50pm
Dielectrics I: 4

Electrical diagnostics and nanosecond imaging of vacuum surface flashover

M. Mounho1, J. Young1, R. Clark1, W. Brooks1, M. Hopkins2, A. Neuber1, J. Stephens1

1Texas Tech University, Lubbock, TX; 2Sandia National Laboratories, Albuquerque, NM

When an insulator in vacuum between an anode and cathode becomes electrically stressed due to the application of a fast high voltage pulse on the anode, the surface of the insulator is typically the first location to breakdown. An experimental apparatus and diagnostics have been designed which localize anode-initiated vacuum surface flashover so that the relationship between voltage, current, and temporally resolved images may be derived to characterize this phenomenon. The high voltage source is an 8 stage Marx generator that stores 20 J of energy and can provide a voltage pulse up to 240 kV. The voltage and current are monitored using a capacitive voltage divider (CVD) and current viewing resistor (CVR). The optical diagnostics include an intensified charge-coupled device (ICCD), with nanosecond resolution allowing for temporally resolved imaging. In addition, time-integrated images are captured using a DSLR which provides the full evolution of the flashover path.

The electrode geometry consists of a hemispherical anode with a 2 cm radius and a planar cathode; the electrodes are separated by 0.6 cm. The insulator geometry under investigation is a positive 45-degree wedge made of cross-linked polystyrene (Rexolite). This geometry aims to pull electrons away from the surface of the insulator, preventing electron multiplication, the driving mechanism of cathode-initiated flashover. The dominant mode of breakdown then becomes anode-initiated vacuum surface flashover; however, little is known about the underlying mechanisms initiating this process. The primary model of anode-initiated flashover was outlined by Anderson [1], who postulated that an initial plasma forms around the anode, with dielectric or gaseous inclusions potentially playing a role. This plasma then creates a cascade of localized bulk-dielectric breakdown propagating toward the cathode due to field enhancement around the edge of the plasma formation. This model and other contributing mechanisms are ultimately what is under investigation, and experimental results are compared to the Anderson-model of anode-initiated flashover where appropriate.

References

[1] R. A. Anderson, “Surface flashover measurements on conical insulators suggesting possible design improvements,” Sandia Labs., Tech. Rep., 1976.

SNL is managed and operated by NTESS under DOE NNSA contract DE‐NA0003525



4:50pm - 5:10pm
Dielectrics I: 5

Utilization of Additive Manufacturing to Improve the Voltage Distribution Across Inductor’s Turns

F. K. Alsaif, Y. Zhang, R. Borjas, K. Alkhalid, J. Wang

The Ohio State University, United States of America

As a result of prominent developments in power electronics, inductor’s stray capacitances require special attention. Inductor impedance can significantly change at high frequency range due to its stray capacitances. Hence, inductor design for traditional 50-60 Hz alternating current (AC) systems can neglect the effect of these parasitics. However, designing an inductor for an advanced power electronics application, a closer look at these parasitics is needed. Emerging wide bandgap (SiC, GaN) devices in power electronics applications have introduced higher switching frequencies and faster switching speeds (higher dv/dt). Higher switching frequency means that the inductor impedance is dominated by those stray capacitances. Furthermore, higher dv/dt could cause insulation failure in the inductor due to the increased voltage stress. Thus, an understanding of inductor parasitics and voltage distribution across its turns are needed. The parasitic capacitances and voltage distribution across inductor turns were investigated for insulation requirement purposes under high dv/dt [1]. The simulation showed that the voltage was not uniformly distributed across the turns. As the switching speed increases, a higher dv/dt will be observed. Higher dv/dt will result in a higher voltage stress across all the turns and less uniform voltage distribution. Thus, the insulation requirements will be higher which leads to increased volume, cost and decreased power density and reliability. In this paper, an innovative approach is implemented to improve the voltage distribution across an inductor’s turns. Optimization and additive manufacturing are used to harness the inductor’s parasitic capacitances to achieve that goal. Both simulation and experimental validation of the improved voltage distribution will be presented in the final paper. As a result, higher power density, lower cost and more reliable systems can be achieved. Advancements in 3D printing technology were critical to realize the concept proposed here. Precise printing resolution and cheaper costs have helped the 3D printing to emerge in power electronics applications nowadays.

[1] Zhang, X., Li, H., Brothers, J. A., Fu, L., Perales, M., Wu, J., & Wang, J. (2016). A gate drive with power over fiber-based isolated power supply and comprehensive protection functions for 15-kV SiC MOSFET. IEEE Journal of Emerging and Selected Topics in Power Electronics, 4(3), 946-955.



5:10pm - 5:30pm
Dielectrics I: 6

Breakdown performance of industry grade Kapton under PWM waveforms

A. Y. Mirza, A. Konstantinou, H. Nguyen, A. Bazzi, Y. Cao

University of Connecticut, Storrs, CT, USA

Conventional insulation systems were designed for sinusoidal excitations at power frequency. With the advent of inverter-fed applications, especially with wide bandgap semiconductors, large voltage transients and fast switching speeds have led to a steep rise in premature insulation failures within inverter-fed machines. About 66% of high-voltage machine failures are due to faults in the stator insulation [1]. This is especially true in transportation electrification applications including marine propulsion, and renewable integration of e.g., wind turbines. To avoid this problem, there is a need to understand the impact of power electronic inverters on conventional insulation systems, advance new materials with better electrical performance, evaluate their safety in medium/high-voltage applications, and develop new standardized testing methodologies. In this study, we attempt to address all the points mentioned above.

To understand the impact of PWM high-voltage and high-frequency emerging power electronic inverters on insulation lifetime, a medium-voltage H-bridge inverter test-bed was built [2] to stress industrially available Kapton (Polyimide) – Kapton HN, Kapton CR, and Kapton MT. Breakdown time of these materials have been compared and the testing methodology has been developed.

Additionally, a novel polymer-based coating with self-assembling, inorganic 2D nano-filler montmorillonite (MMT) was applied on polyimide (Kapton HN film) using a versatile spray coating process. Results of time to breakdown of our nano-coated samples show a 50% increase in the lifetime when compared to the industrially available insulation materials. The most attractive feature about MMT is its anisotropic electrical conductivity which means that it shows insulative character in one direction and conductive character in the other [3]. Our material can find its applications in innumerous industries, like transportation electrification, electric ships, to improve the lifetime under harsh conditions.

This material is based upon work supported by the Office of Naval Research under award numbers N00014-15-1-2413 and N00014-19-1-2306.

[1] K. Younsi, P. Neti, M. Shah, J.Y. Zhou, J. Krahn, K. Weeber, C.D. Whitefield, “On-line capacitance and dissipation factor monitoring of AC stator insulation”, IEEE Transactions on Dielectrics and Electrical Insulation, Vol.17, pp.1441-1452, 2010.

[2] A. Mirza, W. Chen, H. Nguyen, Y. Cao and A. M. Bazzi, "High-Voltage High-Frequency Testing for Medium-Voltage Motor Insulation Degradation," 2018 IEEE Energy Conversion Congress and Exposition (ECCE), pp. 2444-2447, 2018,

[3] Y. Wang, Z. Li, C. Wu and Y. Cao, “High-temperature dielectric polymer nanocomposites with interposed montmorillonite nanosheets”, Chemical Engineering Journal, Vol. 401, 126093, 2020.



 
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