Yasuo Fujiwara

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

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). >

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.

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.

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.

Advanced Metal Oxide Surge Arrester For Gas Insulated Switchgear (GIS)

SF6 gas has outstanding dielectric strength and heat transfer property. This suggests that zinc oxide (ZnO) elements whose value of E in E-J characteristic is higher than the value of the elements used in a porcelain type arrester is suitable to metal enclosed metal oxide surge arresters (MOA) for gas insulated switchgears (GIS). Several kinds of newly developed ZnO elements with the high values of E and their practical performances are discussed in this paper. The application of these elements realizes more compact and simpler arresters which have outstanding protection performance for GIS.

Nb/sub 3/Sn superconducting coil for AC use

Nb/sub 3/Sn superconductors were developed for AC use, and a coil was fabricated. The Nb/sub 3/Sn superconductors were manufactured using the internal diffusion process. To reduce AC losses, the spacing between Nb filaments was designed to be 0.5 mu m; consequently, the space factor of Nb filaments was 6%. The diameter of a strand was 0.2 mm, and the diameter of a Nb filament was 0.4 mu m. AC losses in the strand were 180 kW/m/sup 3/ at 0.5 T (60 Hz, peak value). A coil was made using the wind-and-react method using conductors composed of 7*7 strands. The Specifications of the coil were an inner diameter of 156 mm, an outer diameter of 188 mm, a height of 34 mm, and a number of turns of 17 turns*4 layers. To reduce wire motion, the coil was impregnated with epoxy resin. The quench current for DC operation was 1280 A, and the maximum magnetic field of the conductors was 1.6 T. Coil degradations were not observed. The magnet was tested under AC 60-Hz operation. The quench current was 340 A (r.m.s.). The cause of quenching is thought to be the temperature rise of the conductors due to coupling losses among the strands.

A shell type superconducting transformer for experiments using Nb3Sn superconducting cables

A shell-type superconducting transformer was developed for experiments using Nb3Sn superconducting cables. The designed capacity is 667 kVA (single phase), the voltage is 440/220 V, the current is 1515/3030 A and the percent impedance is 16 percent. Main features of the transformer are as follows: (1) Magnetic field in superconducting coils is decreased by increasing the number of high and low coil groups. (2) Large-scale superconducting cables are not needed when the number of high and low coil groups is increased. (3) Epoxy impregnated coils are used to withstand an electromagnetic force at 120 Hz. The Nb3Sn basic strand was manufactured by the internal tin diffusion process. The cable consists of seven insulated subcables, and the subcable consists of seven strands. The primary (HV) coil of the transformer was excited, in which the secondary (LV) coil was shortened. The primary current reached 1618 Arms without quenching, and the reached capacity corresponds to 712 KA.