W.J. Sarjeant

Investigation of Surface Flashover on Dielectrics Enhanced by Excimer Laser Processing

One of the factors that contributes to surface flashover on dielectrics is the existence of defects on the surface of the material. For our studies, excimer laser processing was utilized on alumina dielectrics in an attempt to substantially increase the threshold voltages for flashover. Through excimer laser processing, the surface material melts and re-crystallizes to form a uniform surface structure. The defects on the dielectric surface can be minimized, reducing charge trapping, leading to higher flashover voltages. Two sets of samples were tested. The first set of samples was processed using the laser and the second set was not. The samples were then stressed under high voltage to induce surface flashover. Analysis of the effects of the excimer laser processing was performed and are shown

Partial Discharge Characteristics of Low Kilovolt Class Capacitors

A set of advanced metallized polymer film capacitors was characterized as to their partial discharge (PD) behavior. The sample set of these capacitors was each at various stages of capacitor lifetime. The stages of lifetime were new/unused, moderate/midlife, and heavy use/end of life. The onset of partial discharges was initiated via the application of dc voltage not exceeding the design rating of 5 kVdc. The onset of PDs in the polymer material was measured by a partial discharge analyzer (PDA). The PDA was capable of displaying the apparent charge of the PDs down to less than 5 pC. Several of the capacitors under test exhibited characteristics of the phenomenon called graceful aging. Graceful aging being a process undertaken by dielectrics when the intrinsic defects already present within the material are cleared away by partial discharges. This process can reduce the original capacitance by up to a few percent. The loss of capacitance however was compensated for by the increased tolerance to a high electric field stress. This artificial aging by inducing partial discharges and characterizing the behavior may be of benefit to the capacitor industry as an accurate quality control to predict lifetime and performance of capacitors.

Preliminary results on the effect of an inverter environment on coated and uncoated MW35 twisted pair wires

Information is continuously being develop and published on how inverter grade wire will extend the life of electric motors that are exposed to high voltage and frequency pulses. Undertaken in this study is the effect inverter pulses have on MW35 type magnet wire coated with filled and unfilled impregnants, and how humidity might effect life. Presented and discussed is the new data generated as part of this research program, and preliminary explanations are provided for the unusually high aging rates observed, and maybe other possible failure mechanisms.

Characterization of Exploding Film Plasmas Using Emission Spectroscopy

Exploding films have a variety of potential applications including current interrupters, ignition of energetic materials, and sources of intense light. However, characterization of these events remains a challenge as the average duration of an event is only on the order of 100 mus in our studies. An effort to obtain a complete understanding of the exploding film and plasma generation phenomena is underway. This paper investigates the spectral and electrical output of exploding film events on metallized polypropylene film samples. Assuming the plasma generated from the exploding film is a blackbody radiator, peak temperature can be estimated using trends of wavelength versus intensity. This data allows for a more accurate characterization of the plasma that results from the exploding films. A holistic understanding of the deterministic mechanisms of the plasma enables future controllability and tunability in exploding film applications.

Energy Balance of Highly Contaminated Surface Flashover on Thin Films

The ability to produce controllable plasmas is usually confined to high energy applications. Many applications, however, would greatly benefit from a reduction of this input energy. This paper examines a new, low energy method to generate a plasma utilizing the mechanism of surface flashover on thin metallized films. This method of plasma generation was unique in that a controllable plasma was generated while requiring less than 5 J of energy. The plasma was produced by applying an impulse voltage of 2.5 kV to a sample of polypropylene film coated with an aluminum metallization on one surface. The resultant flashover liberated only a small percentage of the metallization off of the polypropylene film. The energy required to create the plasma was determined by evaluating the time integral of the voltage and current product then comparing that quantity to the amount of energy required to liberate the already removed aluminum metallization from the polypropylene film. It was found that the energy required to vaporize the aluminum was about an order of magnitude less than the total amount of energy that went into the film. After taking this into account, the amount of energy required to generate and sustain the plasma was determined. This low energy plasma initiation could have interesting applications as a low power light source or be exploited for other avenues of inquiry.

Trend Analysis of Controlled Thin Metallization Flashover

Tests validating the consistency and repeatability of controlled electrical surface flashover on metallized thin film insulators were performed. The study of electrical surface flashover has been mainly confined to its prevention under various contaminated conditions such as ice. The experiments discussed in this paper have gone in the opposite direction. The focus of the experiments was not to prevent surface flashover, but to enhance it, control its path and control its intensity. This was achieved by selecting an insulator with an extremely contaminated surface. The insulator chosen was polypropylene film coated with a conducting surface composed of metallized aluminum. The surface of the polypropylene film was put under a pulse capacitive discharge of 2500 Vdc. Results have shown that a controlled flashover is achievable and repeatable. The trends discussed in the paper show promise for novel low energy light sources.

A new and novel ultra high speed 1 kV MOSFET SPICE model

Improvements in packaging and fabrication for the DE-SERIES Fast Power/sup TM/ MOSFETs has sparked interest in creating an accurate model of these devices. The goal of these new devices is to design new ultra fast pulse generators. As pulsed power components increase in speed, equipment to take advantage of that speed need to keep up. However, the built-in MOSFET models found in SPICE are more attuned to the modeling of low voltage MOSFETs such as those encountered in integrated circuits, rather than for power MOSFETs. In the absence of a suitable built-in model, the power MOSFET is simulated by combining further elements with the built-in model resulting in a more faithful representation of the power MOSFET. This paper describes the models specific characteristics as well as its adaptability to different devices. For example the turn on and turn off time are both independently adjustable, as well as the saturation voltage and feedback capacitance. This and many other options allow the model to be compared to empirical test results of various MOSFET devices and easily match their characteristics to the model.

Transformer fault protection for low impedance faults

Precision measurements of transformer insulation quality demands minimizing damage during test-to-breakdown. A fault tolerant insertion system has been developed for this application. This paper addresses a current limiting model for this system design that protects the high current insertion transformer from sustaining irreparable damage. The autotransformer is a transformer with a single winding that is tapped at intervals rather than having multiple windings.

A high frequency microdischarge detection technique for electrical insulation and capacitors

Todays modern electrical devices are increasingly subject to, and must withstand, high frequency stressing. Such stressing contributes to accelerated aging of electrical insulation and has resulted in premature failure of components. In order to understand the aging mechanisms (i.e., microdischarge induced erosion, thermal runaway, etc.) and make valid conclusions regarding the service lifetime of components used under high frequency conditions, the electrical insulation must be examined under those same high frequency conditions as those in which it will be used. Conventional electrical techniques used to detect partial discharges are limited to below 2 kHz applied ac test voltages. A new technique was developed, in order to stress electrical insulation and examine the microdischarge activity for frequencies up to 100 kHz. This technique uses series resonance to achieve a high frequency sinusoidal steady state ac test voltage and a bridge type test cell to detect the resulting microdischarge activity. Experimental discharge traces are consistent with simulations, showing improved source voltage attenuation compared to conventional techniques. In addition, the calibration technique shows a critically damped single pulse per discharge event with a temporal resolution of <0.5 /spl mu/sec. Experimental results are presented.

Non-destructive examination of impulse generator pulses

To produce a single-shot impulse waveform with a peak voltage of 600 kV, having 15 kJ of energy, a Marx Bank was used which had six 100 kV, 0.5 /spl mu/F capacitors arranged in six stages, charged in parallel and discharged in series. The overall performance and reliability of this Marx Bank was improved by inserting new high power, noninductive resistors in the discharge circuit to dissipate excess energy resulting from switch misfires and load faults. Among the specific technical design issues addressed is the development of a new nondestructive examination technique (NDET) for different modes of Marx Bank operation, which determines if output load faults or switch misfires will be safely tolerated by the resistors used in the charge/discharge circuits. Initial results, including computer simulations and laboratory measurements, using a new fault protecting, energy-absorbing resistor placed in series with the Marx output load are presented. Also, circuit design recommendations for further reduction in fault energies dissipated in the tail resistors of a misfired stage are discussed. As well as the elimination of current ringing from the fault-protecting resistor under fault conditions is described.