Repetitive pulse application of self-healing high voltage capacitors

Abstract

In the last fifteen years, self-healing high voltage capacitors have become standard technology for singleshot and low repetition rate (< 1 shotlminute) applications in R&D environments, such as inertial confinement fusion, electromagnetic launchers, electrochemical guns, high field magnet facilities, etc. Such capacitors offer higher energy density andor longer life and higher reliability in many applications.Standard self-healing capacitors, built with vapordeposited metallized electrodes, have limited ability to carry both peak pulse and continuous RMS (root mean square) currents, generate more heat than discrete foil capacitors. and have lower thermal conductivity for heat dissipation.For these reasons, many pulse power applications have been unable to utilize self-healing technology.For example, moderate to high repetition rate (?IO Hz), high voltage capacitors built today are generally not of the self-healing type due to the higher energy losses and poorer thermal conductivity of metallized electrode capacitors. This results in large thermal gradients and overheating.Instead, such capacitors are still manufactured using discrete foil electrodes, which provide excellent electrical and thermal conductivity.The economic and technical benefits of self-healing capacitors continue to drive research and development to expand their operational envelope. Areas of research include "hybrid" electrode systems, integrated cooling, segmented electrodes, and higher temperature dielectrics. This paper will explore the application of self-healing capacitor technologies to repetitive pulse power systems. Present status of the technology will be described and future performance improvements will be projected. LDISCRETE FOIL ELECTRODES A. Film Capacitor StructuresThis paper primarily focuses on wound film capacitor structures used for large energy storage, pulse discharge applications. Other manufacturing techniques, such as flat-stack or monolithic constructions generally utilize the same dielectric and electrode materials, and are therefore subject to the same capabilities and limitations.A typical extended foil capacitor dielectriclelectrode structure is shown in schematic cross-section in Figure 1. Such a stack of layers is wound onto a permanent core or removable arbor to form a capacitor winding. The foil 0-7803-7540-8/02/$17.000(LOM IEEE 634 electrodes are typically 4-12 um-thick aluminum foils. The dielectric layers are typically 5-20 um-thick Kraft cellulose paper, polypropylene, or polyester film. Termination of the foil electrodes is by soldering, "schooping" (e.g. arc spray or flame spray), or other techniques on the right and left edges of the structure.

Repetitive high peak current pulsed discharge film-capacitor reliability testing

“…Consideration of different chargeup voltages is justified by the fact that the different energy levels can be sufficient for good operation, while degradation of the capacitor can be strongly accelerated by higher bias voltages [4]. The temperature range À55 to 25°C is dictated by a requirement for functionality; exploration of higher temperature levels aims to assess the impact of temperature on the degradation rate of the component [6].…”

“…One such feature is self-healing, which eliminates single point of failures due to dielectric breakdown [3,4]. In the case of a dielectric breakdown, a thin electrode around the defected region is rapidly evaporated due to the large fault current densities.…”

“…Table 2 summarises the main features of the most common capacitor technologies. For the case at hand, film capacitor technology is the most suitable candidate, satisfying the requirements in terms of capacitance and voltage range as well as low equivalent series resistance (ESR), together with self-healing or graceful aging characteristics [3][4][5]. Although the repetition frequency is in the order of 0.05 Hz the high peak current levels associated with the fast pulsed discharge represent a severe stress for the capacitor and require bespoke reliability testing, taking into account different bias and environmental conditions.…”

Temperature Rise of Metallized Film Capacitors in Repetitive Pulse Applications

“…In moderate repetition rate (≥10 Hz), high-voltage capacitors are generally not based on metallized films due to higher energy losses and poorer thermal conductivity of the MFC [4]. So far, such capacitors are still in discrete foil form, which provides excellent thermal conductivity but lower energy density [3], [4]. With expanding of pulse applications, high repetition pulsed power technology has been an essential topic.…”

Custom design of components and power supplies for pulsed power systems

“…Only the lowest loss materials are used, usually an all-polypropylene dielectric. Thermal path length is kept to a minimum, and aluminum foil electrodes serve as both electrical and thermal conductors [3]. Very high power densities can be achieved in film capacitors.…”