Xiao Hui Yang

High energy density capacitors for pulsed power applications

The improvement in the performance of high energy density capacitors used in pulsed power has accelerated over the past few years. This has resulted from increased research sponsored by the US Army Research Laboratory, in support of the US Militarys needs. The capacitor development effort will be discussed as well as the results of both short term and long term testing of a new generation of high energy density capacitors.

High energy density capacitors

Metallized film capacitors with energy densities as high as 3 J/cc and stored energy as high as 260 kJ per unit are now commercially available. These capacitors can be custom-designed for specific applications so as to minimize the size and weight of the capacitance for the lifetime and duty required. Applications requiring pulsed energy discharge times of 10 microseconds to 10 milliseconds are typical, but DC filtering and other types of duty can also be addressed. Packaging options include industry-standard drawn steel cans, proprietary molded plastic cases, and large welded steel cases. The performance of these capacitors as a function of voltage and size will be described.

Pulsed Power Capacitors

The U.S. Army Research Laboratory has sponsored a capacitor development program for film-dielectric capacitors. The program has evaluated dielectric materials for high energy density capacitors from industrial and academic research programs. High-performance capacitors have been developed that meet the needs of todays military applications. The performance of recently developed capacitors will be discussed.

High-Specific-Power Capacitors

Energy density is not the only metric for capacitors. In high pulse repetition rate modulators and other types of power conditioning systems, the reactive power (kVAR) is often the factor driving the physical size of capacitors. The specific power (kVAR/cm3) capability of a capacitor depends on many factors, including frequency, voltage, waveform, duty cycle, ambient temperature, and available cooling. The design of capacitors for high specific power requirements will be described. Designs using high-temperature polymer films as dielectrics and metallized electrodes will be compared to designs using common dielectrics such as polypropylene and both metallized and discrete foil electrodes.

Fast Discharge, High Energy Density Capacitor Performance

Film capacitors with energy densities of greater than 1 J/cc and suitable for millisecond and microsecond discharge applications requiring limited life have been characterized. A variety of polymer films have been tested in model capacitors storing and delivering hundreds of Joules of energy at voltages of 2-10 kV. Self-healing metallized film electrodes have been utilized in these designs to achieve electric fields greater than 500 MV/m. Maximum operating voltage for short-term and long-term use was determined. Life tests were then performed with the goal of achieving at least 1000 charge/discharge cycles at maximum energy density. Degradation and failure of the capacitor samples was determined by measurements of capacitance and dissipation factor. Failure modes in continuous DC, normal charge/discharge pulse service, and short-circuit fault conditions have been determined. Design modifications to increase life and energy density were made based on those analyses. Full-scale capacitors in the 10 kJ to 300 kJ range have been manufactured and tested to end of life

High energy density capacitors for EML applications

Power systems in mobile EML applications require high energy density capacitors as power conversion, energy storage, and power compression devices. Applications include the electromagnetic launch of aircraft, ETC guns and naval launchers for shore bombardment. There is a wide envelope of operating conditions requiring several technology approaches. The ongoing, multifaceted development effort on high energy density pulsed power capacitors at General Atomics Energy Products has yielded capacitors with significantly higher energy densities (>5 J/cc) than were available a few years ago. This paper will describe the performance of this latest generation of capacitors and includes near term projections for products presently under development.

Comparison of film capacitor designs for a high power density application

Three designs for an 8μF 3.1 kV snubber capacitor were built and tested to compare the performance of different technologies for a high RMS current requirement. The designs were (1) extended foil, (2) metalized film, and (3) a hybrid containing both a metalized electrode and extended foil electrodes. All were impregnated with a vegetable oil and epoxy encapsulated in the same size case. The prototype of the hybrid design was only a 6 μF capacitor due to machine limitations on winding dimensions that have since been resolved. A short-term, stepped-voltage test was performed on samples of each design until the units failed or to a maximum of 8.0 kV, more than 2.5 times the rated voltage. Short-circuit discharge, AC partial discharge measurements, and AC stepped-stress tests were also performed and will be reported. The implications of these test data for the selection of specific capacitor technologies for power electronics applications will be discussed.

High acceleration testing of high energy density capacitors

High energy density, energy storage capacitors used in launch applications require capacitors that can withstand extreme levels of acceleration. Some requirements call for capacitors to be capable of being launched at tens of thousands of Gs. The reference point for GA-ESIs high energy density capacitors was 200 Gs at the time the tests were initiated. A series of tests was performed where the capacitors were accelerated in a launch tube to over 30,000 Gs. The testing methods and the results of the tests are discussed.