Se-Won Han

Nonuniformity of electrical Characteristics in microstructures of ZnO surge varistors

The nonuniformity of electrical characteristics of grain boundaries in ZnO varistors was systematically analyzed. The high nonuniformity exists in barrier voltages and nonlinearity coefficients in different grain boundaries. The barrier voltages have normal distributions, only a few grain boundaries were electrically active, and the grain boundaries can be simply classified into good, bad, and ohmic ones according to the electrical characteristics of grain boundaries. The average barrier voltage is equal to 3.3 V by the direct method, but it is only 2.3 V by the indirect method. There is a high difference between the barrier voltages by direct and indirect measurement methods. These few good grain boundaries are responsible for the good varistor effect, and control the leakage current of ZnO varistor at low values of applied voltage. The Al/sub 2/O/sub 3/ dopants affect the electrical characteristics of grain boundaries by changing the electron status in grain boundary and intragrain.

Lightning overvoltage protection of AC railroad vehicles by polymeric arresters

In order to improve the operational reliability of AC railroad vehicles, ZnO surge arresters are used to protect them against overvoltages. The most serious threat to arresters installed on the roof of an AC railroad vehicle is the lightning overvoltage. When lightning strikes the mast or conductors, the distribution system can be treated as a system of multiple conductors including feeders, messenger wires, contact wires, protective wires and two tracks. The lightning impulse discharge current through the arrester and the absorbed impulse discharging energy by arrester in the actual field operation and the protective effect of arrester were analyzed by EMTP and discussed when lightning strikes the mast or distribution lines. When the arrester Is installed on the roof of a railroad vehicle, the potential difference on the primary side of high voltage transformer inside the vehicle is suppressed below 150 kV under very serious condition.

Electrical parameter statistic analysis and parallel coordination of ZnO varistors in low-voltage protection devices

Analyzing the nonuniformity of electrical parameters of ZnO varistors is useful to select and coordinate the ZnO varistors in parallel operation and improve the protection capability of surge-protection device (SPD). The nonuniformity of 1-mA dc voltage and impulse residual voltage of commercial low-voltage ZnO varistors were estimated by statistic analysis in this paper. The Minimum Gobble Distribution was used to describe the distribution of 1-mA dc voltage and impulse residual voltage, ZnO varistors were verified conforming to the Minimum Gobble Distribution by the Kolmogorov-Smirnov test. The relationship between the impulse residual voltage and 1-mA dc voltage has a chaos phenomenon, and they have a close relative degree by t-test, the impulse residual voltage can be estimated by voltage ratio and 1-mA dc voltage. The nonuniform phenomena of electrical parameters of ZnO varistors were simulated by Voronoi network. The current overload of ZnO varistor with reduced residual voltage is serious, in order to keep safe operation of SPDs, when we design a SPD with ZnO varistors in parallel, a safe margin coefficient of discharge withstand current should be considered according to the actual relative standard deviation of surge residual voltages of used ZnO varistors.

Aging characteristics and mechanisms of ZnO nonlinear varistors

The aging threatens the operation stability of ZnO nonlinear varistors. Ordinary, the aging characteristics of ZnO varistors is measured by the power loss changing with time. Analyzing a lot of experimental results, we observed the characteristics of power losses of ZnO varistors can be classified into three types according to applied experimental temperature and applied voltage: (1) the power loss steeply rises with the time and leads to thermal breakdown, when ZnO varistor has a bad performance; (2) the power loss first quickly rises with the time, and then slowly increases with the time, and at last sharply rises with the time and leads to thermal breakdown; (3) the power loss first rises with the time, and then decreases and slowly reaches a stable value with the time. The aging mechanisms for three different aging types of ZnO varistors were discussed based on the changing of Schottky barrier and the migration of zinc interstitials inside ZnO grains, and heat treatment effect.

The effect of geometrical nonuniformity on electrical characteristics of ZnO varistors with Al/sub 2/O/sub 3/ dopant

The Al/sub 2/O/sub 3/ dopants affect the grain growth. The ZnO average grain size and standard deviation decrease with increasing Al/sub 2/O/sub 3/ content. The microstructure of ZnO varistor is more uniform with the increase of Al/sub 2/O/sub 3/. Al/sub 2/O/sub 3/ dopant decreases the pore ratio by the spinning effect of of ZnAl/sub 2/O/sub 4/ spinel. The Al/sub 2/O/sub 3/ dopants can effectively improve the voltage ratio and affect the electrical characteristics of ZnO varistors. The sample A2 with 0.005 wt.% Al/sub 2/O/sub 3/ content has a minimum voltage ratio and the best nonlinearity coefficient in high current region. Microstructure property has direct relations to breakdown voltage and nonlinearity coefficient. With the decrease of average grain size and standard deviation, the breakdown voltage and the nonlinearity coefficient increase. The complex analysis technique is used to characterize the AC response in heterogeneous microstructure system. The depressed angle in the complex impedance plane indicates nonideal (non-Debye or Cole-Cole) relaxation and is related to the degree of nonuniformity existing in barrier levels within each junctions.

Effect of Winding Coil Diameter on AC Insulation Breakdown Voltage of Polyamideimide/Nanosilica Wire

Copyright ©2016 KIEEME. All rights reserved. This is an open-access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted noncommercial use, distribution, and reproduction in any medium, provided the original work is properly cited. pISSN: 1229-7607 eISSN: 2092-7592 DOI: http://dx.doi.org/10.4313/TEEM.2016.17.4.231 OAK Central: http://central.oak.go.kr TRANSACTIONS ON ELECTRICAL AND ELECTRONIC MATERIALS Vol. 17, No. 4, pp. 231-234, August 25, 2016

Failure characteristics of suspension-type porcelain insulators on a 154 kV transmission line

The paper reports on the failure characteristics of porcelain suspension insulators removed from a 154 kV transmission line in Korea. It is found that it is very important to choose the test data describing the homogeneity of porcelain, which is related to the micro-crack propagation in the porcelain body, for leveling the ageing state of the porcelain insulators. Tests such as the Rockwell hardness, is found to be effective in calculating the statistical values on the micro-crack fracture of the porcelain body. The combined thermal and mechanical tests have a close relationship to the dielectric strength of the porcelain body. The use of the Weibull distribution function is shown to be a very reasonable way of calculating the failure probability of aged porcelain insulators

Thermal properties of whole-solid-insulated polymeric ZnO surge arrester

A new type of 220 kV whole-solid-insulation polymeric ZnO surge arrester was developed, in which there is no air gap inside the arrester due to silicone rubber filling all the gaps between the ZnO varistor column and the polymeric housing. The electrical, and therefore thermal, overloading of the ZnO varistors could not cause a flashover inside. So the whole-solid-insulation polymeric surge arresters are of a safety-type design. The thermal dispersion capability of the newly developed polymeric arrester was measured and calculated by a special software based on the finite element method (FEM), which is better than that of a traditional ZnO arrester with a porcelain housing. The thermal stability of the developed 220 kV surge arrester was analyzed by FEM analysis software. The thermal balance diagram and the permitted energy absorption capability were discussed. The thermal stability property of the polymeric housing was measured. There are good agreement between the calculation results and the measurement ones.

Accelerating aging of transmission line porcelain suspension insulators by autoclaving

Porcelain suspension insulators aged for 1, 5 and 10 years on Korean transmission lines and new insulators are tested for autoclave expansion. The compressive strength of the insulators aged in an autoclave with conditions of 250degC and at 20 atm for 30 minutes, was about 7.6 kgf/mm2, which is close to that measured on insulators aged for 10 years in the field. From simulation results, the cement displacement changed linearly with temperature. At a temperature of 200degC, the shear stress was approximately 7 kgf/mm2; a stress that is brought about by a 0.07% expansion of the cement. It is evident that the cement would fracture at a 0.07% expansion, because the cement has about 7 to 9 kgf/mm2 flexure strength. A turning point in the shear stress with mechanical load occurred at 0.02% cement expansion. From an analysis of the porcelain body it is shown that there is sufficient margin of strength to guard against fracture of the porcelain body even for a cement expansion more than 0.12%.

Thermal impact characteristics by forest fire on porcelain insulators for transmission lines

In this study the thermal impact characteristics by forest fire are extensively investigated using temperature controlled ovens. The test conditions for thermal impact damage are simulated according to the characteristics of natural forest fire. The test pieces are suspension porcelain insulators made by KRI in 2005 for transmission lines. In the thermal impact cycle tests with 300degC thermal impact gradient (-70 to 230degC), cycling in 10 minute periods, no critical failures occurred in the test samples even with long cycle times. But in tests with thermal impact gradient from room temperature to 200-600degC, cycling in 10 to 30 minute periods, there were critical failures of the porcelain insulators according to the thermal impact gradient and quenching method. In the case of thermal impact by forest fire, it was found of that duration time is more important than the cycling time, and the initiation temperature of porcelain insulator failures is about 300degC, in the case of water quenching, many cracks and fracture of the porcelain occurred. It was found that the thermal impact failure is closely related to the displacement in the cement by thermal stress as confirmed by simulation. It was estimated that the initiation displacement by the thermal impact of 300degC is about 0.1%. Above 1% displacement, it is expected that the most porcelain insulators would fail.