Han-Goo Cho

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.

Tracking resistance by the RWDT and flashover characteristics of polymeric materials of outdoor HV insulator

This paper describes the results of a study on the tracking performance of high voltage polymeric insulating materials based on the rotating wheel dip test (RWDT). The influence of surface degradation was evaluated through measurement of the flashover performance after and before tracking test. It is concluded that the surface degradations of polymeric insulating materials are directly related to the partial discharge and weight loss. Leakage current on the surface of specimens is useful in evaluating degradation aspects. In the RWDT, the breakdown specimen is not affected by the dry flashover voltage, despite the fact the that the surface degradation of tracking test has different state on each specimen.

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.

Effects of alumina trihydrate on the electrical insulation properties of HTV silicone rubber

This paper reports on a study of the influence of the level of alumina trihydrate (ATH) on the electrical insulation properties and the tracking and erosion resistance of high temperature vulcanized silicone rubber (HTV-SIR). The electrical resistance of silicone rubber was measured using a high resistance meter. It decreased with increasing temperature and decreased greatly more than 373 K (273 C). Also it decreased greatly with increasing level of ATH. The tracking and erosion resistances of HTV-SIR with various ATH level were evaluated by employing IEC 587 inclined plane test. It was observed that the tracking and erosion resistance improved greatly more than 130 phr as increasing level of ATH. It was found that the dielectric breakdown voltage of silicone rubber decreased a little with increasing level of ATH.

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 temperature distribution and thermal stress analysis of pole cast resin transformer for power distribution

In this paper, the temperature distribution and thermal stress analysis of 50 kVA pole cast resin transformer for power distribution are investigated by FEM program. The temperature change according to load rates of transformer and thermal stress have also been investigated.

The effect of acid rain on the tracking resistance of polymeric insulating materials

We attempted to investigate the effect of acid rain on the resistance against acid rain and tracking resistance of silicone composite materials. The analytical techniques to study the resistance against acid rain are contact angle, and weight loss measurement, SEM, EDX, and XRD. And, we measured time to tracking breakdown, leakage current, and weight loss caused by erosion to study tracking resistance.

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

Simulation on interface mechanical stress in porcelain insulators for transmission line by cement growth using ANSYS/NASTRAN program

The main ageing cause of many porcelain suspension insulators for transmission and distribution line with dead-end stings is a mechanical stress in interface between porcelain and cement materials. It was known that the principal mechanical stresses which give electrical failure is the results of the displacement from cement growth. We studied the effect of cement displacement resulting environmental ageing parameters on porcelain insulator mechanical properties used by analytical simulation program (ANSYS/NASTRAN) and test methods. For alumina porcelain samples, an expansion above 0.2% at 5,000 kg load is predicted to cause fracture. These simulation methods using ANSYS/NASTRAN program is very correct to experiment results. These simulation analysis and experimental results show that cement volume growth affects severely to be mechanical failure ageing.

Influence of chromium oxide additive on electrical characteristics of ZnO varistor

The effect of chromium oxide (Cr/sub 2/O/sub 3/) additive on the electrical characteristics of ZnO-Bi/sub 2/O/sub 3/-Sb/sub 2/O/sub 3/-based ZnO varistors in the prebreakdown and breakdown regions was studied by comparing the electrical properties of three different system samples. When Cr/sub 2/O/sub 3/ is added into ZnO varistor, the breakdown voltage and the nonlinearity coefficient decrease. Cr/sub 2/O/sub 3/ increases the leakage current in the breakdown region and reduces the voltage ratio in the breakdown region, which leads to bad electrical characteristics. C-V characteristics show that this deterioration is related to lower barrier height, donor concentration, and interface defect density of the intergranular layer of the O varistor.