Research On Performance Of High Energy Density Capacitors In Repetitive Pulse Applications

The metallized film capacitor is one of the key components in pulsed power systems due to its high energy density, high reliability and long lifetime. It is needed to investigate the temperature rise and performance changes as the application of metallized film capacitor expanded from single-shot to repetitive pulsed power systems. This dissertation mainly focuses on the application performances of metallized biaxially oriented polypropylene (BOPP) film capacitors, and makes specific researches on the breakdown and self-healing characteristic of metallized films, the fracture of metallized electrodes, the heat transfer characteristic and lifetime performance.This dissertation firstly discusses the capacitor structure and heat distribution in the metallization, and deduces the calculation formula of capacitor equivalent series resistance (ESR). The repetition modes of pulsed power systems, the discharge circuit and parameters are presented. This dissertation analyzes the key issues such as the self-healing characteristic of metallized films, the fracture of metallized electrodes, and lifetime performance, etc.Secondly, this dissertation researches the breakdown characteristic and the self-healing characteristic of the metallized film. The increasing of conductivity under high temperature and high electric field, and the thermal effect is the reason for the breakdown of the film. Based on the Poole-Frenkel effect and enhanced carrier mobility, electrical conductivity is modeled and the film breakdown electric field is theoretically calculated based on electro-thermal breakdown. Studies indicates that the conductivity increases with the temperature and electrical field; and theoretically calculated breakdown electrical field decreases from740V/μm to543V/μm as the temperature increases from0℃to100℃. Self-healing test shows that the statistical proportion of single self-healings appearing in test decreases, while the proportion of multiple self-healings increases as the temperature increases. The temperature has limited effect on the film breakdown, but has significant effect on the self-healing degradation. This may be the main reason for the lifetime decreasing and catastrophic failure of the capacitor.This dissertation researches the fracture phenomenon of the metallized electrodes. The test platform of the current flowing capability of metallized electrode is established to research the formation process of the electrical explosion. The fracture phenomenon and current density threshold of metallized electrodes are studied under the action of repetitive current. The metallized electrode fracture can be divided into the fracture development and serious electrical explosion. The test results shows that the current density threshold for serious electrical explosion under the action of a single pulsed current is about1.07×1011A/m2, and threshold for fracture development under the action of1000shots pulsed current is1.39×l010A/m2. A current density threshold calculation model is proposed based on current density distribution and the Joule heating in metallization. The calculated value matches well with the test result.This dissertation studies the heat transfer characteristic of the metallized film capacitor, and introduces the heat power distribution, heat conduction and thermal dissipation. Research is carried out to investigate the heat volume power density distribution and heat conduction from the metallization to the dielectric film. The temperature rise simulation model is established based on the heat volume power density distribution, the heat conduction equation and boundaries. The simulation results of the temperature rise match well with the test results by the infrared imaging device. The influences of operational and structural parameters on the temperature rise are investigated.Finally, this dissertation investigates the lifetime performance of metallized film capacitors in repetitive pulsed application based on an established lifetime test platform. The test results show that the higher repetition rate, the longer the lifetime is. At the same repetition rate, the charging time, the holding time or time interval change have no significant influence on the lifetime. The analysis of the capacitance loss in lifetime test show that most of the capacitance loss happens in the pulse discharge process. In the end, this dissertation puts forward method to determine the reliable operation range and lifetime of the metallized film capacitor. The capacitor reliable operation ranges such as "voltage-temperature-lifetime" and "current-repetition rate-lifetime" are presented based on the lifetime test results.