Performance and Safety of Large Energy Discharge Capacitors

Large capacitors used in the pulsed power industry offer a unique challenge from both a design and application standpoint. The capacitors are getting larger, storing more energy, and delivering that energy in short pulses resulting in very high peak power output. This paper examines the design and application considerations associated with large capacitors focusing on the capacitors built for two recent pulsed power machines, ATLAS and NIF. These capacitors represent large capacitors in the fast (<10uSec) and slow (<1mSec) pulsed discharge domain. The performance of the capacitors under normal conditions, fault conditions and failure conditions are discussed. Also, information concerning the safe operation. The ongoing effort on the part of the capacitor industry to provide smaller, more cost effective components has resulted in evolutionary rather than revolutionary improvements in the use of dielectric materials to store energy effectively. In many applications, the total energy required is fixed and the capacitors have been getting ever smaller. In other applications where banks of capacitors are used, such as high energy pulsed power applications, the maximum weight of an individual capacitor has remained about the same but the stored energy in the capacitors continues to increase. Reliability is a major concern when more energy is being stored in a single capacitor. In the past, this has been a limiting factor. As the capacitors got larger, the probability of a dielectric breakdown somewhere in the dielectric system that would cause the entire capacitor to fail increased. In modern capacitors, this has been addressed by the manufactures in a number of ways. The most significant is the advent of self clearing electrodes being used in large energy discharge capacitors. This paper examines the design and application considerations associated with large capacitors focusing on the capacitors built for two recent pulsed power machines, ATLAS and NIF. These capacitors represent large capacitors in the fast (<10uSec) and slow (<1mSec) pulsed discharge domain. The performance of the capacitors under normal conditions, fault conditions and failure conditions are discussed. Also, information concerning the safe operation.