Hirotaka Muto

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.

Detecting characteristics of various type antennas on partial discharge electromagnetic wave radiating through insulating spacer in gas insulated switchgear

A widely used, effective method for diagnosing the insulation of a gas-insulated switchgear (GIS) is to detect partial discharges using signals in the ultrahigh frequency (UHF) band. To detect partial discharge signals that propagate inside a GIS, it is necessary to install a sensor inside or set on the outside of a GIS tank. For GISs that have already been constructed, it is effective to use external diagnosis using electromagnetic wave (EM-wave) signals that radiate (leak) from the insulation spacer of the GIS tank, and there is strong demand for such a diagnosis technology. Externally leaking EM-waves are often detected using an antenna, and multiple antennas supporting different frequency bands are commercially available. The accuracy of external diagnosis could be improved by comparing and evaluating the characteristics of various antennas. The insulation spacer of a GIS tank is usually installed on the flange and fastened with metal bolts. GIS tanks are made electrically continuous by these metal bolts and have the same grounding potential. EM-waves leak from the part of the solid insulator surrounded by these metal bolts and the GIS tanks. This leaking part forms a slot antenna and has a resonant frequency that depends on the spacing between the bolts in the circumferential direction of the flange. At this resonant frequency, the output of externally leaking EM-waves is higher and enables more sensitive measurement. In this study, the detection characteristics with different antenna positions and directions were compared in consideration of the number and positions of the bolts on the spacer, and the frequency characteristics of various antennas were obtained. To improve the detection sensitivity, a prototype of an improved dipole antenna was fabricated and its characteristics were evaluated. Based on the experimental results, an ideal diagnostic system based on the externally leaking EM-waves is proposed.

Propagation characteristics of electromagnetic waves in three-phase-type tank from viewpoint of partial discharge diagnosis on gas insulated switchgear

The authors have been examining the diagnosis method for partial discharge (PD) occurring inside gas insulated switchgear (GIS) using the UHF method with a focus on electromagnetic (EM) wave mode conversions at L-shaped, T-shaped pipes, and components existing in actual GIS. Based on these conversion characteristics, an on-site, high-accuracy PD diagnosis algorithm was constructed for use on high-order mode EM-waves. All the researches so far were targeted at an isolated-phase-type (singlephase- type) tank. The present study was targeted at a straight-pipe three-phase structure, the most basic form, in order to apply the evaluation method to a three-phase-common-type tank (three-phase-type tank). Through the implementation of EM-wave propagation experiments and the EM-field analysis, the presence of TEM, TE11, and TE21 modes was verified also in the three-phase-type tank. Next, the EM-field analysis was conducted on models equivalent to actual apparatuses of 72 to 550 kV classes. Then, formulas for simplified calculation of cutoff frequencies in TE11 and TE21 in a three-phase-type tank were proposed. Furthermore, the basic concept of the PD diagnosis method was examined and the work flow was presented.

Propagation properties of electromagnetic wave through T-branch in GIS

To investigate the transmission of electromagnetic waves excited by partial discharge through T branch section and L-shaped sections of a gas-insulated switchgear (GIS) bus structure, electromagnetic fields were numerically simulated. The modes of the simulated electromagnetic wave were found to transform to different modes, e.g., from TE11 to TE21, after transmission and reflection through T branch or L-shaped sections of a GIS. These characteristics found in the simulation, mode transformation and dependence of resonance frequency on excitation direction of partial discharge, were also confirmed experimentally. In the case of 90-degree bent direction, the characteristics of mode transformation in a T branch and those in an L shaped section are similar. In an obliquely angled tank although electromagnetic-wave propagation behavior becomes similar to that for the L shaped section, the ratio of mode transformation becomes smaller, and the transmitted component of the input mode tends to increase, as the tank shape becomes closer to straight

Electromagnetic wave radiated from an insulating spacer in gas insulated switchgear with partial discharge detection

As a means of diagnosing partial discharge (PD) signals propagate inside a gas insulated switchgear (GIS), a study for the leakage of electromagnetic waves (EM-waves) emitted from the insulating spacer was implemented. The EM-waves leaking out from the solid insulator have the resonance frequencies depend on the spacing between adjacent bolts in the direction of the flange circumference, because the leakage portion is the equivalent of a slot antenna. In this paper, using an electromagnetic analysis model which has a simulated spacer on a concentrically-shaped GIS tank, the output characteristics of the EM-waves that leaked out from the slit were analyzed under various conditions such as the spacing between adjacent bolts, the width of the spacer, the dielectric constant of the spacer and the form of the flange. Also the actual measurement by the experimental equipment used to simulate the model was implemented for comparison with the analytical results. Consequently, the optimal specifications of the sensor and the measurement method used to achieve highly-sensitive detection for practical use were summarized and proposed as well as evaluating the effectiveness of the electromagnetic analysis model adopted in this paper.

A mechanism of gate oxide deterioration during As/sup +/ ion implantation

The deterioration or catastrophic breakdown of thin gate oxides during ion implantation is studied. The effect of ion beam density, the distribution of the gate oxide deterioration over a wafer, and the effect of photoresist coverage are shown quantitatively by measuring the number of interface states generated in MOS capacitors. It is shown that the four charge sources contribute to the deterioration of gate oxide: the irradiated ion beam, the secondary electrons emitted from the gate electrode, the charges accumulated on the photoresist surface around the gate electrode, and the secondary electrons emitted from a wafer holder. The first three charges accelerate the deterioration of the gate oxide and the last one reduces it. A model of the gate oxide deterioration in ion implantation that is very useful for finding methods of reducing the charging damage is presented. >

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.

Partial discharge inception voltage for two insulating materials (PVC and PE) under inverter surge voltage

Rise time of pulse voltages in inverter systems has decreased due to improvements in efficiency, ranging from several microseconds to several tens of nanoseconds, depending on the type of switching device (MOSFET, IGBT, GTO) used in motor drive systems. Although partial discharge (PD) can occur at any parts of the whole inverter drive systems (motor, load cable, switching devices, inverter), the behavior of PD under such steep pulses has not been clearly understood. We report PD inception voltage for two different insulating materials (PVC and PE) using an optical method to avoid electrical noise caused by applied surge voltage itself. We measured the effect of frequency on PD inception voltage (AC, mono-polar pulse), finding that the PD inception voltage showed a tendency to increase with an increase in the frequency, and we present the effect of pulse polarity. These experimental results are discussed with respect to the dielectric properties (dielectric constant, charge decay characteristics) of the materials.

Diagnosis for oil-immersed insulation using partial discharge due to bubbles in oil

This paper deals with the partial discharge (PD) phenomenon and its use in the detection of small bubbles in oil and application to insulation diagnosis for oil-immersed power transformers. Bubbles in the oil strongly affect the reliability of insulation of oil-immersed power transformers as well as foreign particles. As the first step of the diagnosis of discharges in oil-immersed transformers, we conducted experiments to study the PD phenomena by measuring the PD characteristics and by observing the behavior of bubbles in oil. The experimental results, show that the /spl Phi/-q patterns strongly depends on the size of bubble.

Frequency spectrum of various partial discharges in GIS

Before the breakdown of electrical apparatus partial discharges occur. Measuring partial discharges is very important for predictive maintenance for GIS. In a GIS, several types of defect are probable, such as a metallic particle fixed on conductors, a particle on the spacer and a moving particle. For predictive maintenance it is necessary to understand the characteristics of partial discharges for these defects. We measured frequency spectra and phase distributions for these defects using a practical bus model and made the characteristics of partial discharges clear.