Yoshiharu Kaneda

The role of microscopic defects in multistress aging of micaceous insulation

The aging mechanism of mica/epoxy insulations under multiple stresses is investigated with both breakdown tests and a scanning electron microscope (SEM). The result of simultaneous multistress testing shows that thermal and mechanical stresses have a synergistic effect on electric life. Similar results are also obtained for residual electrical breakdown characteristics after sequential application of thermal aging and mechanical stress. It is confirmed with SEM pictures that thermal stress causes isolated delaminations at the mica/epoxy interface. Using a newly developed SEM apparatus with servomechanisms, the development of delamination and cracks are observed during mechanical fatigue tests in realtime. A multistress aging model is proposed with relation to the generation and development of the microscopic defects. >

Electromagnetic waves from partial discharges and their detection using patch antenna

The electromagnetic (EM) wave generated by partial discharge (PD) is analyzed using electric dipole model. The pulse electric and magnetic fields are described by analytical functions. The interaction of patch antenna or microstrip antenna with the EM wave is investigated on EM theory, which leads to a simple transmission-line model. Then, the response of patch antenna to the PD-initiated EM wave is numerically analyzed as a circuit transient. It is found that the generated EM fields and the response of patch antenna strongly depend on how fast PD develops. The sensitivity of patch antenna to PD is expressed in a simple chart. The analytical results are verified by an experiment in which the EM wave from a short-gap discharge is detected by the patch-antenna PD sensor. The patch-antenna system is expected to be used for detecting PDs occurring inner parts such as windings inter-turn.

New noise rejection techniques on pulse-by-pulse basis for on-line partial discharge measurement of turbine generators

On-line partial discharge tests for turbine generators are useful for recognizing abnormal or deteriorated stator winding insulation without a machine outage. One problem related to such a test is additional installation of sensors. We propose that wires of a resistance temperature detector (RTD) embedded in a stator slot are applied as a partial discharge sensor in the form of an RF coupler. The other problem is electrical noise that is a pulse similar to partial discharge in frequency characteristic and has higher amplitude than partial discharge. The sensors in two steam turbine generators showed enough sensitivity and broad frequency bandwidth to facilitate the elimination of noise from the partial discharge measurement. Two new techniques of noise rejection on a pulse-by-pulse basis are investigated with the multiple sensors. One is founded on the correlation between pulse height in two frequency bands. The other is based on the correlation between pulse height from the two partial discharge sensors. It was found that each noise rejection technique was successful in a trial testing.

On-line PD monitoring system for rotating machines using narrow band detection of EM wave in GHz range

On-line PD monitoring system with higher sensitivity and low noise features was developed for rotating machines such as turbine generator and high voltage motor. By using narrow band detection in GHz region, the features were achieved. The system is composed of an antenna, a specifically designed signal processing circuit and data acquisition equipment. The antenna is so-called patch antenna and its dimension is approximately 120 times 120 mm and is designed for detecting a specific narrow band frequency in GHz range. This type of antenna is very easy to install because the thin plate structure does not interfere other high voltage parts inside the rotating machine if installed on the inner wall of the machine housing. The antenna is connected to the amplifier adjusted for the frequency band of the antenna resulting in superior noise suppression. The sensitivity of the system was measured and confirmed to have a good linear correlation with conventional off-line Qmax value from several hundreds pico Coulomb. This sensitivity is enough for on-line insulation monitoring for rotating machines because the low level PD activity during operation does not infer abnormality of the machines.

Noise rejection techniques for on-line PD monitor of turbine generators

Correlation between frequency bands and correlation between PD sensors are applied to noise rejection of on-line PD measurement for turbine generators. Over 90% of external noises are rejected with either technique.