Abstract: Short circuit current limitation in distribution utilities can be an effective way to improve power quality, since the expected voltage sag amplitude during faults can be dramatically reduced A simple series LC circuit tuned at the net frequency, with the capacitor shunted by a metal oxide varistor (MOV), proves to be a well suited limiter to reach the goal In the paper, the properties of such a circuit are analyzed and its operation is investigated through computer simulation The relevant pro and con aspects are outlined and discussed

Abstract: The SnO 2 based varistor systems recently presented in the literature appear to have a promising potential in commercial applications. Experimental evidence shows that there is a dependence of nonlinear constant values with thermal treatment under different atmospheres. Thermal treatments in oxygen and nitrogen rich atmospheres at 900°C prove this dependence, indicating that the nonlinear constant values are significantly lower when the material is submitted to a nitrogen atmosphere. Moreover, electrical properties can be restored when the varistor is subjected to thermal treatment at the same temperature in an oxygen atmosphere, indicating that the mechanism seems to be reversible. This paper discusses this behavior focusing in the grain boundary region. Ta 2 O 5 mol% concentrations are also analyzed and the results indicate an optimum Ta 2 O 5 concentration of 0.05 mol% for the electrical properties ( α =44 and E B =6150 V cm −1 ).

Abstract: The nonlinear electrical properties of TiO2–Y2O3–Nb2O5 ceramics were investigated as a new varistor material It was found that an optimal doping composition of 9975%TiO2–060%Y2O5–010% Nb2O5 was obtained with low breakdown voltage of 88 V mm−1, high nonlinear constant of 70 and ultrahigh relative dielectric constant of 76×104, which is consistent with the highest and narrowest grain boundary barriers in the composition Samples doped with 010 mol% Nb2O5 exhibit the highest permittivitty and resistivity at low frequencies and comparatively lower values at high frequencies in comparison with other samples studied In view of these electrical characteristics, the ceramics of 9975%TiO2–060%Y2O3–010%Nb2O5 is a viable candidate for capacitor-varistor functional devices The performance of the ceramics as a function of Nb-doping depends primarily on the extent of substitution of Ti4+ with Nb5+ In order to illustrate the role of grain boundary barriers for high Nb-doping co-concentrations in TiO2–Y2O3–Nb2O5 varistors, a grain-boundary defect barrier model was introduced

Abstract: The Sb 2 O 3 in a ZnO-based varistor composition was systematically substituted with SnO 2 in the range from 0 to 100%. The microstructural characteristics, average grain size and the general phase composition of the samples were not influenced by the substitutions. In all the samples we observed ZnO, Bi 2 O 3 -rich and spinel phase, where the composition of the spinel depends on the Sb 2 O 3 to SnO 2 ratio in the starting composition. The non-linear coefficient α of the samples was not influenced by the SnO 2 substitution, while the threshold voltage decreases and the leakage current increased with larger additions of SnO 2 . The capacitance–voltage measurements indicated strong increase in the donor density and the density of interface states at the grain boundaries while the depletion-layer width decreased and the barrier height was comparable in all the investigated samples. According to the electrical properties, SnO 2 had a strong donor effect in the ZnO varistor ceramics.

Abstract: Microwave (millimeter wave) sintering technology used for enhancing the densification behavior and electrical properties of ZnO electronics ceramic materials was described. Successfulness in application of such a novel technique to sinter ceramics relies heavily on the microwave (millimeter-wave) absorption efficiency of the materials. Semiconducting oxides such as Bi2O3-doped ZnO materials perform satisfactorily in microwave (millimeter-wave) sintering. Using susceptors to facilitate absorption efficiency of the samples greatly improves the sinterability of the materials. The advantage of the microwave (millimeter-wave) sintering process over the conventional sintering technique is best demonstrated by fast firing of Bi2O3-based ZnO varistor materials, in which, the whole sintering process required only 18 min. The varistor characteristics obtainable are: α=38, JL=55.5×10−6 A/cm2 and Vbk=600 V/mm.

Abstract: The effect of Sb2O3 on the electrical and physical properties of SnO2-based ceramics were investigated by measuring the densities, permittivities, the properties of V–I and boundary barriers. SnO2·Co2O3 ceramics cannot exhibit electrical nonlinearity. A little amount of Sb2O3 can improve the nonlinear properties of the samples greatly. The height and width of the defect barriers were calculated. It was found that the sample doped with 0.01 mol% Sb2O3 exhibits the highest density (ρ=6.90 g/cm3), the highest reference electrical field and the best electrical nonlinearity (α=12.9), which is consistent to its highest and narrowest defect barriers. The effect of Sb2O3 dopants can be explained by the substitution and segregation of antimony ions. The effect of cooling rate was also investigated. The samples cooled at 2°C/min exhibit better nonlinear electrical properties. A grain-boundary defect barrier model of SnO2·Co2O3·Sb2O3 varistor was also introduced.

Abstract: A varistor has a thermal fuse between a lead and an electrode. The fuse includes a link extending between the surface of an insulator and the fused electrode. The electrical connection of the link and the electrode is maintained by a low temperature solder fillet. That part of the link between the electrode and the insulator is surrounded by hot melt electrically insulating material. Upon sustained over-voltage conditions, the link and the solder fillet melt, and an insulating gap is rapidly created by molten hot melt material.

Abstract: ZnO-based varistor samples with a relatively high Sb2O3 to Bi2O3 ratio of 5 were fired at 1200 °C and found to have a high threshold voltage (VT) of 280 V/mm and a low energy-absorption capacity of 50 J/cm3. The introduction of rare-earth oxides (REO) increased the energy-absorption capacity of Pr6O11- and Nd2O3-doped samples to 110 J/cm3 while their threshold voltage (VT) remained slightly above 300 V/mm. Doping with Pr6O11 and Nd2O3 altered the formation of the spinel phase and significantly changed its particle size and distribution which, as a result, had a positive effect on the energy-absorption capacity of the REO-doped samples. Doping with small amounts of Pr6O11 and Nd2O3 appears to be promising for the preparation of ZnO-based varistors with a high breakdown voltage and a high energy absorption capacity.

Abstract: A simple thermo-mechanical model is applied to evaluate the influence of the nonuniformity of ZnO varistor disks used in surge arresters on their energy handling capability. Puncture is the dominating failure mode for slightly nonuniform disks, but cracking becomes more likely as the degree of nonuniformities increases. It is shown that minimization of the chance of a failure of varistor disks at high-current pulses can be achieved by adjusting their resistivity in the upturn region of the I-V characteristic. Simulation of the behavior of varistor disks under high-current 4/10 /spl mu/s pulses required by the ANSI standard tests shows that these tests provide very little information about the actual energy handling capability of the disks. This conclusion suggests that alternate test methods should be developed and included in the relevant standards.

Abstract: NO x sensing properties of SnO2-xCr2O3 as a varistor-type gas sensor have been investigated in the temperature range of 200–600°C. The breakdown voltage of SnO2 shifted to a higher electric field upon exposure to NO2 at 300–500°C, and the largest breakdown voltage shift, i.e. the highest NO2 sensitivity was observed at 400°C. In contrast, the direction of the breakdown voltage shift in NO varied with temperature: the breakdown voltage shifted to a lower electric field at 300°C, but to a higher electric field at 500°C, and remained almost unchanged at 400°C. The NO2 sensitivity of SnO2 was superior to the NO sensitivity at every temperature, and then the SnO2 exhibited good selectivity to NO2 at 400°C. The breakdown voltage of Cr2O3 shifted in the reverse direction upon exposure to NO and NO2, in comparison with those observed with SnO2, owing to its p-type semiconductivity. Thus, Cr2O3 also exhibited certain sensitivity to both NO and NO2 at 200°C, being more sensitive to NO2, though the sensitivities decreased drastically at temperatures higher than 300°C. The addition of 5.0 wt% Cr2O3 to SnO2 resulted in a significant improvement of NO and NO2 sensitivities at 600°C, being accompanied by an increase in the breakdown voltage in air. Especially, the NO sensitivity was superior to the NO2 sensitivity in the concentration range of 20–100 ppm, and then SnO2 mixed with 5.0 wt% Cr2O3 was found to be the most suitable candidate for a NO sensor among the sensors tested. The increase in the breakdown voltage in air induced by the Cr2O3 addition was confirmed to arise from both the decrease in the particle size of SnO2 and the formation of micro p-n junctions at grain boundaries. The decrease in the particle size was also responsible for the increased NO and NO2 sensitivities, but the p-n junctions were suggested to play a more important role in promoting and stabilizing the chemisorption of NO at higher temperatures.

Abstract: In this paper, the findings from systematic experiments with the purpose to determine the degradation effect of single and multiple current pulses on the microstructure of the metal oxide (MO) varistor are described. Six distribution class varistors from one manufacturer were used in these experiments. The first part of the paper describes the electrical condition after application of single and multiple lightning current pulses. The results (before and after each elevated current impulse test) of 1 mA AC reference voltage and residual voltage are presented in this section. We have also investigated a new technique called "return voltage measurement" for monitoring of the degradation in MO varistors. This is described in detail. The second part deals with the microstructure observations of MO varistors. The results of microstructural examination of impulse current on MO varistors are examined in detail. This section also explains the relationship between the microstructural changes and electrical degradation of MO varistors.

Abstract: The effect of La2O3 addition on the densification and electrical properties of the (0.9895-xSnO2+0.01CoO+0.0005Nb2O5+x La2O5 system, where x=0.0005 or 0.00075, was considered in this study. The samples were sintered at 1300 °C for 2 and 4 h and a single SnO2 phase was identified by X-ray diffraction. Microstructure analysis by scanning electron microscopy showed that the affect of La2O3 addition is to decrease the SnO2 grain size. J versus E curves indicated that the system exhibits a varistor behavior and the effect of La2O3 is to increase both the non-linear coefficient (α) and the breakdown voltage (E2). Considering the Schottky thermionic emission model the potential height and the width were estimated. The addition of small amounts of La2O3 to the basic system increases the potential barrier height and decreases both grain size and potential barrier width. © 2001 Kluwer Academic Publishers

Abstract: Overvoltage-protection device, applicable in particular to the low-voltage mains, comprising, between the two lines ( 1, 2 ) of the mains, a gas-discharge arrestor ( 3 ), a varistor ( 4 ) and a thermal-fuse element ( 5 ) tasked with ensuring the thermal disconnection of the device It includes, in parallel with the varistor ( 4 ), a resistor ( 7 ) causing, after the short-circuiting of the gas-discharge arrestor ( 3 ), the heating of the thermal-fuse element ( 5 ) so as to trigger the thermal disconnection of the device

Abstract: A low profile mount for a disc varistor. A thermally sensitive switch is provided both for single and multiple electrode embodiments. The switch may be placed in a shorting circuit and include a spring biased conductor prevented from closure by a heat sensitive element which softens in responsive to excessive heat. The varistor may be fused to prevent excessive current from a short circuited but not open circuited varistor. Methods are also provided.

Abstract: System for surge protection of an object comprising a supply unit, which is connected to the secondary output of an external transformer. The secondary windings (1) of the external transformer are coupled in a star configuration, the star point of the transformer being connected to a downstream neutral conductor (7) and an earth electrode provided near to the transformer. In the supply unit, at least one phase conductor (6) is connected to the neutral conductor (7) by means of a surge protective device of a first type (4), and the neutral conductor (7) is connected to an earth electrode (5) provided near to the supply unit by means of a surge protective device of a second type (9). The surge protective device of the first type (4) comprises a voltage dependent resistor or varistor and the surge protective device of the second type (9) comprises a lightning current arrester or spark gap element.

Abstract: PROBLEM TO BE SOLVED: To solve a problem in which it is hard to manufacture varistors that are excellent in loading service life characteristics and kept high in uniformity. SOLUTION: This varistor is formed of ZnO as a main component, and Pr, Co, Cr, Al or Ga or In, Si, and (Ca+Sr) are added as auxiliary components to the main component, where Si and (Ca+Sr) amount to 0.001 to 0.5 atom % and 0.01 to 0.5 atom %, respectively, and the ratio Ca/Sr is set at 0 to 50.

Abstract: The solid solutions of Sn1−x Ti x O2 (0.0