In electric power systems, switchgear technology is an important fundamental technology. In order to improve the environmental impact of power transmission technology, it is necessary to raise the voltage class of vacuum circuit breakers and to realize compact and reasonable DC circuit breakers. In order to understand the physics of breakdown and arc discharge phenomena in vacuum circuit breakers, we are investigating the mechanism of breakdown caused by small particles, improvement of creepage breakdown voltage, and characteristics of vacuum arc discharge under DC interruption (measurement of plasma temperature, electron density, and neutral particle density).

Electrical tree propagates through the solid insulating material, such as epoxy resin, and eventually leads to total circuit breakdown. In recent years, it is known that tree generation and propagation can be suppressed by using nanocomposite fillers in epoxy resins. Using optical microscopy and X-ray phase imaging techniques, we have investigated the characteristics of treeing phenomena in epoxy resins filled with conventional micro-sized fillers and recent nano-sized fillers to explore the tree-suppression process by nanocomposites. In addition to epoxy resin, silicone gel is used as an encapsulant for power modules, which are the key component of AC/DC converters. We measured the electrical tree propagation characteristics under PWM waveform superimposed with repetitive surges, which are severe to the insulation of the power module during operation, and proposed a method to improve the dielectric strength of the power module for higher operating voltages and higher operating frequencies

There is a strong demand to measure electric fields accompanied by electric discharges or containing nonlinear materials. There are various methods for measuring electric fields, amaong them, the electro-optic sensors (based on Pockels effect, Kerr effect) have advantages for measuring high-voltage and discharge phenomena because of their wide measurement response frequencies, electromagnetic non-inductivity, and the fact that the sensor can be constructed using only insulators. We have been developing electro-optical sensors for many years, and are working to increase their sophistication and expand their range of application, such as higher voltages, higher sensitivity, application as surface potential meters, and use of long/short wavelength electromagnetic waves.