Jinbo Kuang

High field effects in solid dielectrics

High voltage solid dielectrics are employed in a wide range of electrotechnology, from video flyback transformers to transmission-class solid dielectric cables. The design criteria for the application of such dielectrics are well developed, and failure is primarily the result of unanticipated defects. In this context, a defect is anything that causes an electric field that is substantially greater than the design field. Thus the reliability of high voltage dielectrics is tied closely to high field effects, typically in microscopic volumes around defects such as asperities on a conductor or small cavities.

Critical parameters for electrical tree formation in XLPE

Published data for electrical tree inception field at a defect in XLPE vs. the Laplacian field at the tip of the defect are explained on the basis of a minimum distance which the space charged limited field must extend from the defect tip into the XLPE in order to damage enough dielectric in the field direction to facilitate PD inception and tree initiation.

Mechanism for impulse conversion of water trees to electrical trees in XLPE

A lightning impulse causes substantial capacitive current in a water tree channel which, as a result of its small cross section, has relatively low conductance. Transient, nonlinear finite element computations with coupled thermal and electric fields for the geometry of a 15 kV XLPE dielectric cable indicate that an 80 kV lightning impulse can cause the water within a water tree channel to boil over a range of four orders of magnitude in water conductivity, from 10/sup -2/ S/m to 10/sup 2/ S/m. Boiling of the water in a water tree channel reduces substantially the yield stress of the XLPE, raises the pressure within the water tree channel, and is likely to leave a cavity which can support partial discharge resulting in electrical tree initiation.

Thermal-electric field distribution around a defect in polyethylene

In the presence of a stress enhancement, a lightning surge causes space charge formation and power dissipation which can result in substantial temperature rises in dielectrics. Poissons equation and the thermal diffusion equation have been solved in 2D with field-dependent material parameters to compute temperature rises of over 80 K during a 100 ns risetime surge which produces a background field of 40 kV/mm applied to a 100 /spl mu/m protrusion with a 0.5 /spl mu/m tip radius. Such a temperature rise could play a major role in the lightning-induced conversion of water trees to electrical trees.

Failure mechanisms and recent improvements in ZnO arrester elements

ZnO arrester elements can fail by a number of mechanisms, including electrothermal instability and thermal stress-induced mechanical cracking in this brittle ceramic. Breakdown channels at the edge of the electrode are among the most common failure mechanisms at high currents. Such failures take the form of a melt puncture of the element from the electrode edge to the opposite electrode or to the edge of the disk. This failure mechanism limits the energy absorption capability of ZnO arrester elements.

Evaluation of high field conduction models of polymeric dielectrics

The image charge as measured with a guarded needle apparatus continues to increase for a time as the voltage drops after reaching its peak value, before leveling off to a relatively constant value. The increase of apparent charge beyond the peak voltage is the result of charge spreading in the dielectric even as the voltage decreases. Such spreading of charge is very sensitive to the slope of the conductivity with respect to the field in the region of the space charge limited field. Numerical simulations are employed to match the measured material properties to the simulated material properties for various functional relationships between the conductivity and electric field. Estimation of the field dependence of the conductivity in the region of the space charge limited field based on these computations is presented in this paper.

Pipe-Type Cable Losses for Balanced and Unbalanced Currents

Computations of pipe eddy current losses have been undertaken for balanced and unbalanced current using a program for transient nonlinear finite element analysis developed by one of the authors. Good agreement is obtained with experimental data from the literature. The computations indicate that the eddy current losses reach a minimum around 700 A conductor current. Experimental data are available only to about 800 A, so that this phenomenon has not been reported previously and may have an impact on rated ampacities for high pressure fluid filled (HPFF) cable. Computations for unbalanced currents indicate increased losses with increasing unbalance, as expected.

Electro-thermal-mechanical computations in ZnO arrester elements

Transient, nonlinear finite element analysis with coupled thermal and electric fields is employed to compute the electric and thermal field distributions in ZnO arrester elements, including both nonlinear electrical and nonlinear thermal properties. Mechanical stress in the element is computed during post processing, based on the thermal field. The data indicate that a metallic protrusion from the sprayed electrode into the ZnO can cause substantial temperature rise in a microscopic region around the defect. The effects of a delamination between the electrode and the ZnO surface are less severe. Statistical computations have been undertaken to explore the effect of nonconducting grains on the disk conduction threshold voltage and disk nonlinearity. The computations yield similar results to those in the literature based on nonlinear circuit equations but are much less time consuming.

Surge-induced temperature rise in water-containing defects in XLPE: mechanism for conversion of water trees to electrical trees

Circumstantial evidence suggests that large water trees convert to electrical trees as a result of lightning-induced surges. Computations employing transient nonlinear finite element analysis suggest that such conversion involves boiling of the water which results in creation of a cavity which can support partial discharge.

Critical parameters for electrical tree formation in polyethylene

Published data for electrical tree inception field at a defect in XLPE vs. the Laplacian field at the tip of the defect are explained on the basis of a minimum distance which the space charged limited field must extend from the defect tip into the XLPE in order to damage enough dielectric in the field direction to facilitate PD inception and tree initiation.