It is our great honor, on behalf of the local executing committee of the joint conference, to write a preface to this special issue on the joint conference of 10th International Workshop on High Voltage Engineering (IWHV2016) and 2016 Japan-Korea Joint Symposium on Electrical Discharge

The spacecrafts need high-voltage power generation for high power. However, the high voltage can cause severe discharge problem on spacecrafts. This paper describes the mechanism of spacecraft charging and discharging. The mitigation methods are also mentioned.

The objective of this study is to synthesize a distributed-parameter line model of rails, which have sleepers and a ballast bed, for a surge simulation by means of circuit analysis. The model is composed based on simulation results by Finite-Difference Time-Domain (FDTD) method to take into account a complex configuration of the rails. The transient voltages calculated by FDTD method are approximated by analytical functions. It enables an analytical time-to-frequency transformation without numerical error. The line parameters including frequency dependent effect are derived from the results. The characteristic impedance is inductive due to the ballast bed and sleepers. Influences of the permittivity of the sleepers is observed on the wave propagation characteristic as well as the characteristic impedance. The accuracy of the proposed model for the circuit analysis is confirmed by a comparison with FDTD simulation result of a circuit with a simplified impedance bond model.

Special facilities such as data centers, computer centers, and hospitals with operating rooms require a extremely static electromagnetic environment. In this study, we investigated the use of insulated-cable-type down conductor as a measure for protection from lightning for these facilities. The overvoltage observed at the injection point of the lightning impulse in the insulated cable was evaluated by an impulse test and a transient analysis using a distributed constant circuit. Overvoltages were measured under a lightning impulse voltage using a commercially available insulating cable of 11m length. The distributed circuit parameters of the analysis model for the cable were determined based on the results of the impulse experiments. Using the constructed analysis model, the lightning overvoltages in the injection point of the cable were analyzed for different lengths of the cable and steepness of the lightning current. It was found that the overvoltage caused by the lightning current in the cable can be evaluated by the reflection of the lightning current using a common ground system. In addition, the overvoltage values could be estimated from the steepness of the lightning current, surge impedance and propergation velocity of the cable, and the cable length.

One of the common measures against lightning surges is potential equalization by common grounding. However, since the effects of lightning surges due to common grounding is not organized quantitatively in an environment in which high-voltage (power supply) systems and low-voltage (signaling and telecommunication) systems are mixed, as with the case with railways, the study of common grounding of railway systems is not in progress at present. In this paper, we propose a method for calculating optimal grounding systems of railway telecommunication systems in terms of lightning surges by combining the calculation using the analysis model for railway telecommunication systems and mathematical optimization techniques.

Surge arrester with a spark gap has been installed for reducing electrical failures in Japanese distribution system. The top and the bottom of a ceramic spacer for sustaining an electrode gap in a surge arrester contact with electrodes, of which the electric field becomes high. Therefore, high electric field region such as the triple junction point becomes a source to supply electrons to a spark-gap. In this study, we made some electrodes with a small step edge on an inclined plane of an electrode. The electric field around the small step edge became high, and we aimed that the small step edge functioned as if it were a triple junction points for a conventional spark gap. Based on measurements of V-t characteristic and the number of electrons per unit time, we investigated that the artificially formed high electric field region lower impulse spark-over voltage and that a spark-over voltage occurs within a duration of 0.4-1.2µs when a positive impulse voltage of 25-42kV with 1.2/50µs is applied. The results will contribute on a further stabilization of discharge and increasing a degree of freedom for design of a spark gap.

The frequency response analysis (FRA) of transformer has been expected to be a good method for diagnosing the transformer condition. As the transformer impedance is affected by the configuration, the change of the impedance will be a good indicator for the change of the transformer configuration. On FRA measurement, the windings without measuring is set with an open circuit or a short circuit. In this paper, the experimental result about the open circuit test of FRA which can diagnose core defects etc. is described. Specifically, the influence on the impedances measurement by residual magnetic flux (RMF) and injection voltage level are described.