Yoshikazu Shibuya

Surface Charging on Epoxy Spacer at DC Stress in Compressed SF 6 Gas

The charge accumulation and its mechanisms are studied based on the experiments of post and conical epoxy spacers subjected to high dc voltages in compressed SF6 gas. The charge measurement is performed using electrostatic probe and dust figure techniques. The charge accumulation is strongly influenced by the surface condition of epoxy spacer. Heterocharge deposits locally on the surface of untreated spacer. The flashover voltage of highly charged spacer decreases considerably at the time of polarity reversal. In the case of epoxy surface treated with horning, only a small quantity of charge is deposited uniformly on the surface of spacer. The mechanism of inhomogeneous surface conduction is suggested by the local surface charging on the untreated spacer. Field emission is also found as the other charging mechanism on the conical spacer set in rough finish electrode system.

Factors Controlling Surface Flashover in SF6 Gas Insulated Systems

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.

Stability and Long Term Degradation of Metal Oxide Surge Arresters

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.

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

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.

Evaluation of Surge Degradation of Metal Oxide Surge Arrester

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.

CHARGE ACCUMULATION ON SPACER SURFACE AT DC STRESS IN COMPRESSED SF6 GAS

ABSTRACT The charge accumulation on the surface of an epoxy spacer is studied at dc stress in compressed SF 6 gas, using cylindrical model spacers with metal inserts at both ends. The accumulated charge is measured in compressed SF 6 gas by the probe which is placed in a test vessel. Dust figures are also used to observe the pattern of accumulated charge. Heterocharge accumulates on the circumference of a spacer and saturates itself in 5 hours at 100 kV dc. The charge distribution hardly decays for a long time after the removal of dc voltage and remains unchanged even after the exposure to the air. Three patterns of dust figures are observed on spacers. One of them appears even at relatively low voltages and is insensitive to applied voltages. In other two patterns, charge accumulates in proportion to the applied voltage. Charge accumulates rather uniformly on a silica-filled spacer, while it deposits on local area of a spacer filled with alumina. Charge accumulation phenomena are assumed to be determined by the electrical conduction on the surface of a spacer. The charge distribution analyzed by the surface electrical conduction agrees with that in the experiment.

Voltage-time characteristic of electrical breakdown in SF 6

V-t characteristic of impulse and switching surge breakdown in SF6is studied on various gaps. The characteristic is categorized into three patterns depending on the configuration of the gap and gas pressure. The properties of the V-t characteristic in these patterns are generalized as a semi-empirical formula which will be useful in the quantitative evaluation of the insulation coordination and the abnormal voltage protection of SF6gas insulated power equipments. The gap conditions in which those patterns of V-t characteristic are observed are also discussed in the Appendices.

Influence of VFT on Shell-Type Transformer

When a disconnecting switch in a gas insulated switchgear (GIS) operates, a high frequency oscillating surge called very fast transient (VFT) is generated. The VFT may set off overvoltage in a transformer winding. In the paper the authors evaluate the influence of VFT on a shell-type transformer. To simulate waveforms of VFT invading a transformer, a simple equivalent circuit for a shell-type transformer in a high frequency range is derived experimentally. Using this model the overvoltage waveforms are analyzed for various configurations of GIS. Then, the influence of VFT on a 500 kV shell-type transformer winding is investigated. The results lead to a conclusion that VFT does not threaten the reliability of insulation of the transformer winding

High frequency voltage oscillation in transformer windings and electrical breakdowns properties of interturn insulation immersed in oil at VFT voltage

Switching operations in gas insulated substations (GIS) can generate very fast transient overvoltages (VFTO) which propagate in the GIS with little attenuation. When a transformer is directly connected to the GIS through a gas/oil bushing, the VFTO can enter the transformer and excite the voltage oscillation in the windings. In the paper, we firstly show a resonance phenomenon due to the superposition of the traveling waves which was found in the experiments using the coil windings of a 500 kV transformer. The fundamental resonance frequency ranged at about 2 MHz and its voltage at the interturn (turn-to-turn) of the coil could reach 0.25 times of the applied voltage in the worst case. Secondly, the electrical breakdown properties of interturn insulation in oil were studied, applying the unipolar and bipolar voltages with 2 MHz oscillatory frequency. The breakdown voltages at the unipolar and bipolar VFT oscillatory stresses were about 1.15 and 1.25 times higher than that at the standard lightning impulse, respectively. We discuss the experimental breakdown stresses with the ones estimated from the analysis of VFTO in the transformer windings and also show the methods to cope with the VFTO.

Vapour flow patterns observed in free-burning arcs

The vapour flow pattern of a free-burning arc of 20 mm length is studied varying the cathode and anode diameters independently from 5 to 22 mm. Roughly speaking, there are two flow patterns which depend on the electrode arrangement. One is a bidirectional flow pattern and the other a unidirectional flow pattern. In both cases, outward flow are always observed without any indication of the air entrainment. A rough estimation shows that the radiation loss is more than 84% of the total energy loss in the vapour dominant arc generated in the present condition.