This paper deals with the application of ZnO varistors—an area which has not been treated in a systematic way in the literature. The paper starts with a brief description of the fundamental properties comprising the electrical behavior as well as the physics, chemistry, and microstructure of the varistor. These properties then form the basis for defining the application parameters that are directly related to the nonlinear current-voltage characteristics of the varistor. This paper provides a detailed description of these parameters and their relation to microstructure and the processing of the varistor. Finally, a discussion is presented on the reliability of the varistor by considering a grain-boundary defect model which explains both the instability and the stability under use conditions.

Application of Zinc Oxide Varistors

A defect model for the grain-boundary barrier has been proposed to explain the phenomena of voltage instability/stability of the ZnO varistor. The key element of the proposed model is the zinc interstitials which are present in the depletion layer as excess zinc, arising from the non-stoichiometric nature of ZnO. Both instability and stability have been described in terms of diffusion of these interstitials in the depletion layer, followed by chemical interactions with defects at the grain-boundary interface. Finally, a large body of experimental data is presented to indirectly validate the proposed defect model.

A Grain-boundary Defect Model for Instability/stability of a ZnO Varistor

The increasing application of SF6 as an insulating gas has led to many studies on SF6 decomposition in gas-insulated equipment. In the presence-of an electric arc, spark or corona, SF6 decomposes to a wide variety of chemically active products which possess completely different properties from SF6. The accumulation of these decomposition products in the equipment has caused concerns regarding personnel safety and material compatibility problems. This paper reviews previous research in SF6 decomposition relating to the operation of gas-insulated switchgears, gas-insulated transmission lines, and electrostatic accelerators. Results on the qualitative and quantitative determination of the by-products and their formation ion rates in various modes of electrical discharges are summarized. The mechanisms leading to the formation of transient and stable products are described. In particular, the influence of discharge energies and impurities on the formation of SOF2 and SO2F2, the two dominant stable by-products, is discussed. The effects of the by-products on personnel safety and equipment ent dielectric integrity are presented. The application of SF6 gas analysis as a tool for diagnosing the internal condition of gas-insulated equipment is assessed. Based on the results of many phenomenological observations, future research activities are suggested to address the issues of safety, compatibility and equipment aging. More fundamental studies on electron, ion, and neutral reaction rates in an SF6 discharge are required to gain a better understanding of the decompositon mechanisms and the influence of the products on equipment operation.

SF6 Decomposition in Gas-Insulated Equipment

A close association has been identified between the theoretical calculation of geomagnetically induced currents (GICs) in a large network, practical measurements of GICs, the results of dissolved gas analysis (DGA) records, and damage in recently failed transformers in Southern Africa. Together these indicate that GICs may contribute significantly to transformer failures on large transmission systems in mid-latitude regions, where GICs are generally thought not to be significant.

Transformer failures in regions incorrectly considered to have low GIC-risk

Geomagnetically induced currents (GICs) can cause saturation of the magnetic circuit of transformers in a power system. This saturation can increase the MVAr absorption of the transformers, leading to voltage-control problems, generating significant harmonic currents, and cause heating of the internal components of the transformer itself, leading to gas relay alarm/operation as well as possible damage. This paper sets out the methods used to examine these effects using a mathematical model explicitly incorporating the electric and magnetic circuits, including the shunting effect of the tank to predict the current and flux waveforms. The model has been used to predict GIC effects for a variety of winding connections for single-, three-, and five-limb core-type transformers connected to the National Grid Company plc transmission system in England and Wales. The size and form of the return limbs along with the tank shunting effect determine the magnitude and the often complex shape of the waveforms resulting from GIC. Field and factory DC injection tests on various types of transformers have been conducted to validate the model and gain an insight into the magnetic behavior of transformers. With the aid of finite-element analysis (FEA) techniques and a consideration of the various constructional arrangements of the core and coils in the tank, it is possible to evaluate the power losses and accompanying temperature rises of the core, structural components, windings, and tank. Some guidance on the acceptable GIC current levels for various transformer types is given.

Geomagnetically induced current effects on transformers

DC excitation of transformers due to geomagnetically induced currents was studied by model experiments. First, differences in the DC excitation phenomenon due to the iron core structure were studied using three typical small-scale models. The results verified that single-phase three-legged cores were most susceptible and three-phase three-legged cores least susceptible to such excitation effects. Secondly, the local heating due to DC excitation was quantitatively assessed using large-scale core form and shell form models with the most susceptible single-phase three-legged cores. The results demonstrated that the maximum temperature rise was approximately 110/spl deg/C due to the GIC (200 A/3 phases). >

An experimental analysis of DC excitation of transformers by geomagnetically induced currents

Various factors controlling the flashover of solid insulators in pressurized SF6, are reviewed and their influences in gas insulated systems are discussed from a practical point of view. Flashover voltage of clean insulator surface is under the influence of the insulator-metal contact as well as the macroscopic electric field distortion due to the high dielectric permittivity of solid insulator. Conducting particles or even fine metal powder can reduce the flashover voltage. Their effects are strongly dependent on the position they are located, the size of the insulator and gas pressure. Humidity of SF, gas should be strictly governed in SF, gas insulated apparatuses, since the condensation of water can decrease flashover voltage considerably. Decomposition products of SF6 due to the arcing in switchgears are deleterious to epoxy insulators particularly when silica is used as their filler. The decomposition products decrease the leakage resistance on the insulation surface. The field strength near positive electrode is enhanced by the electrolytic effect in the surface conduction layer. In some extreme condition, it initiates tracking on the insulator surface.

Factors Controlling Surface Flashover in SF6 Gas Insulated Systems

The thermal runaway process and the long term degradation of metal oxide blocks for a surge arrester are investigated both experimentally and analytically. The critical condition of a thermal runaway of the blocks is formulated. The dynamic stability after surge absorptions is also studied. The stability limit is given as a function of the temperature rise of the blocks due to the surge absorptions. A kind of degradation of metal oxide proceeds under ac stress even at the level lower than that required for the thermal runaway. The life of a metal oxide surge arrester is evaluated combining the degradation process with the above thermal runaway condition. The Arrhenius relation which has been proposed to evaluate the life of metal oxide blocks is discussed in the light of the analysis.

Stability and Long Term Degradation of Metal Oxide Surge Arresters

Metal oxide surge arrester (MOA) is constructed by a series connection of zinc oxide elements having a highly nonlinear resistance. A simple equivalent circuit is proposed to express the dynamic characteristics of zinc oxide element. Using this dynamic model, the effect of lightning strokes to gas insulated substations (GIS) is analized by a computer simulation under various conditions. The results are summarized in a diagram from which the length of GIS bus to be protected by MOA can be determined. Comparing the overvoltage protection performance of MOA with that of a conventional arrester, MOA is found to be more suitable for GIS application.

Protective Performance of Metal Oxide Surge Arrester Based on the Dynamic V-I Characteristics

The surge degradation of zinc oxide elements under a constant ac stress is investigated experimentally. The results are summed up in a set of formulae and figures from which the degradation of any given condition can be evaluated. The increase ratio of leakage current from the initial value is successfully sed as the indicator of degradation of elements. The two types of elements are investigated concurrently-Formation I being used in production and Formation II a new material. Formation II is found to be remarkably resistant to surges and particularly to ac stress. The influence of degradation on the life of metal oxide surge arrester is discussed on the basis of a dynamic thermal stability. The permissible number of surges during the life is determined for the two types of elements. The performance of arrester is expected to be greatly improved by the use of Formation II.

Y. Fujiwara

Papers

An experimental analysis of DC excitation of transformers by geomagnetically induced currents

Factors Controlling Surface Flashover in SF6 Gas Insulated Systems

Stability and Long Term Degradation of Metal Oxide Surge Arresters

Protective Performance of Metal Oxide Surge Arrester Based on the Dynamic V-I Characteristics

Evaluation of Surge Degradation of Metal Oxide Surge Arrester