Xiaoguang Qi

Engineering with nonlinear dielectrics

All dielectrics are nonlinear if subjected to sufficiently high electric fields. A high degree of dielectric nonlinearity can bring simplicity to many problems, especially those involving highly inhomogeneous field geometries. Given the simplifying assumptions that result from the dielectric nonlinearity, the field distribution often can be computed analytically to good accuracy, which is highly desirable given the alternative of a nonlinear numerical computation. This article discusses three examples in which nonlinear conditions affect dielectric performance in a manner that is easily predictable if the fundamentals of nonlinear systems are understood.

Thermal and mechanical properties of EPR and XLPE cable compounds

This paper shows that the mechanical and thermal properties of EPR cable dielectrics are very stable with temperature. Commercially available EPR and TRXLPE cable dielectrics typically have about the same thermal conductivity, although the thermal conductivity of TRXLPE above the melting point of the crystallites drops below that of the EPR dielectrics. The mechanical properties of EPR cable dielectrics are much more stable with temperature than those of the TRXLPE, as EPR polymer is essentially amorphous so that EPR cable dielectrics do not suffer from the very large thermal expansion of TRXLPE as its crystallites melt.

Electrothermal failure of metallized film capacitor end connections–computation of temperature rise at connection spots

Self-clearing metallized film capacitor technology offers the highest energy density among high voltage capacitor technologies. The primary limitation of this technology under pulse discharge conditions is failure of the plasma sprayed end connections. Transient nonlinear finite element analysis with coupled electric and thermal fields is applied to quantify the temperature rise around a contact spot along the end connection as a function of discharge current, contact spot size, time and distance between contact spots along the contact edge. An analytic approximation is developed to compute these parameters which agrees to within a few percent with the finite element analysis.

Computation of electro-thermal breakdown of polymer films

Development of new polymer films is impeded by the need to make large quantities of high quality film for prototype devices. However, DC breakdown should be predictable from material electrothermal properties, i.e., field and temperature dependent electrical conductivity, thermal diffusivity, and boundary conditions. Transient nonlinear finite element analysis readily predicts thermal runaway of a polymer film between metal electrodes. In this contribution, we evaluate analytical approaches to this problem in the context of laboratory testing between metal electrodes. The objective is to predict approximate limits to device performance as a function of electric field and temperature from measurements on prototype materials.

Novel capacitor discharging system

A relatively simple high voltage discharge system has been designed for capacitor pulse discharge with peak current around 1.5kA with a time to peak in the microsecond range. The circuit is novel in that all low voltage electronics are powered from the high voltage supply, and the system operates as a relaxation oscillator. The system can be used to study the degradation of the capacitors as a result of pulse discharge.

Dielectric properties, of metallized paper-film capacitors

A capacitor formed of fluid-impregnated paper metalized on one side and a polymer film results in a relaxation peak caused by the much more rapid polarization of the dielectric fluid impregnated paper relative to the polymer film. This results in the capacitance being a function of frequency, with a substantial increase in capacitance at very low frequencies. A theory for such a capacitor is developed and compared with measurements on actual capacitors with relatively good agreement. The implications of the theory for discharge current and discharge efficiency are discussed.

Transient finite element computation of the temperature rise in metallized film capacitor end connections caused by underdamped discharge

Loss of end connection integrity is the most common failure mode for metallized film capacitors under pulse discharge conditions as a result of electrothermal effects associated with the intermittent nature of the contact between the sprayed metal end connection and the edge of the capacitor film. In this paper, the temperature rise as a result of varying polarity reversal is analyzed through the use of transient finite element analysis which shows that polarity reversal is unlikely to cause greater damage to the end connection than discharge without polarity reversal. This suggests that the adverse effect of polarity reversal is dielectric or electrical, but not electrothermal in nature.

Absorption current vs. charge voltage for industrial quality capacitors

Absorption current as a function of charge voltage has been measured on a population of industrial quality capacitors with the finding that above some charge voltage the absorption current starts to increase rapidly. Based on the nature of the measured absorption current vs. voltage, a mathematical procedure has been developed which provides a systematic method for defining this threshold voltage varies widely over the population. Possible reasons for this variation are discussed based on extensive dissection of capacitors and detailed knowledge of the production process

Test for end connection integrity of metalized film capacitors

The wire arc metal sprayed end connections of metalized film capacitors limit their performance for high current discharge applications. We have developed a solid state discharge circuit with integrated current-induced partial discharge detector to evaluate the quality of end connections with a single high current discharge, and we have demonstrated a strong correlation between this test and winding discharge life. Such a test can be very useful to the industry, as if the quality of individual windings can be assured, a large capacitor made from many such windings will have greatly improved dis-charge performance and reliability.