Hiroshi Morii

Fundamental insulation characteristics of high-pressure CO 2 gas for gas-insulated power equipment - effect of coating conductor on insulation performance and effect of decomposition products on creeping insulation of spacer -

Currently, environmental problems such as global warming are important issues, and SF6 has been identified as a greenhouse gas with a long atmospheric lifetime. Therefore, in the long term, it is preferable to reduce the amount of SF6 used as an insulating gas. It is thus important to discuss the possibility of using more environmentally friendly gases as alternative insulation for gas-insulated apparatus. In this paper, we describe the fundamental insulation breakdown characteristics of high-pressure CO2 gas at gas pressures of up to 1.0 MPa under simulated practical conditions, including the insulation breakdown characteristics of a high-voltage conductor with an insulating coating and the effect of decomposition products on the creeping insulation of a spacer. With the aim of enhancing insulation performance, we discuss the effect of the type of insulating coating on insulation performance. The breakdown electric field was increased by 20% by coating the conductor. It was verified that the application of an insulating coating is a practical method for enhancing the insulation performance of high-pressure CO2 gas. It was also verified that the decomposition products have only a slight effect on the creeping insulation of the spacer except for when there is heavy pollution on the insulating spacer. However, if a large amount of decomposition products is expected to be deposited on the insulating spacer during operation, which may cause a severe interruption to the current, it will be necessary to consider this factor in the insulation design.

A study on the accuracy of surface charge measurement

The measurement of accumulated surface charge for thick specimens requires multipoint probe outputs to establish the inverse calculation for the determination of an unknown charge distribution. Until now, studies on the various errors associated with the measurement have been conducted only for simplified arrangements mainly in axisymmetric geometry where the charged surface is parallel to the ground. We have numerically analyzed a model measurement set-up more comparable to practical conditions by a highly efficient surface charge method. We have studied the effect of probe position, the induction from charge existing not directly beneath the (probe) sensor and the difference in matrix components computed by two numerical methods. In particular, we have studied the accuracy of the reconstructed charge distributions by numerical simulations of the inverse calculation. It has been shown that the assumed measurement errors make much larger differences in the reconstructed charge distributions, although the influence depends considerably on the assumed charge distribution. Reducing the condition number of the matrix improves the accuracy of the inverse calculation for uniform and linearly changing charge distributions.

Study on surface charge distribution of an insulating hollow cylinder in vacuum

This paper deals with the surface charge on the inner surface of an insulating hollow cylinder. We measured the charge by employing an electrostatic probe before and after the spark conditioning procedure. The probe output after the conditioning showed much uniform pattern with respect to the circumferential direction. The computer simulation was also conducted to analyze the charge distribution. The simulation results showed that the probe output was sensitive to the density of electric flux on the outer surface resulting from the SEEA charge.

Depression of insulator charging in vacuum by partial mechanical processing

It has been clarified by researchers that insulators having contact angles, e.g. plusmn45 deg with regard to the plane electrodes, is hard to acquire charge when they are exposed to high voltages in vacuum. For example, an insulator in the shape of a truncated frustum is free from charging. However, since such insulator has inevitably large volume, it obstructs the compactness. On the other hand, roughening insulator surface is also effective to prevent the surface from charging. In this case, the cost for roughening the entire surface may be high when the insulator is large. In this paper we review our previous investigation results and describe new experimental results aimed at developing feasible and compact insulators used in vacuum. Surface charging characteristics of partially and mechanically processed insulators have been studied for this purpose. That is, the charging phenomena when such insulators are exposed to a ramped DC or AC voltage have been observed by using a capacitive probe embedded in the cathode. Firstly, we describe of a partially roughened cylindrical insulator, where the height of the roughened surface measured from the cathode is varied keeping the total length constant. Secondly, we describe a cylindrical insulator having a truncated conical frustum or a chamfer at its cathode-side end. Based on these experimental and simulation results we prove that the partial mechanical processing is effective to control and suppress the insulator charging in vacuum.

Control of surface charge on insulating hollow cylinder by using shield ring in vacuum

In vacuum, insulators acquire charge due to well-known Secondary Electron Emission Avalanche (SEEA), and the charge is considered harmful for the insulation of high voltage vacuum devices. This paper deals with the charge depression effect of a shieldring, a ring type electrode, which is often applied to high voltage devices in order to relax the electric field near the triple junction on a cathode. In the present study a shield-ring is located in an insulating hollow cylinder made of Al2O3. The charge distribution on the inner surface is measured by employing an electrostatic probe located outside of the cylinder. The probe has scanned the whole surface of the cylinder and produced a probe output pattern that varies in shape depending on the ring height. On the other hand, we have calculated the charge distribution on the inner surface of the cylinder by employing a rotationally symmetric i.e. two dimensional simulation method relying on the SEEA mechanism. Giving the calculated distribution, we have further calculated the probe output by using a three dimensional simulation method. The calculated and measured probe output patterns show a reasonable agreement for several ring heights tested in this study. Based on these experimental and analytical results, we have revealed the effectiveness of using a shield-ring to depress the SEEA charging.

An approach to diagnose pressure hazard of vacuum bulbs

Vacuum bulbs may encounter the pressure rise hazard that results in failure in interrupting over currents in power systems. The pressure may unexpectedly increase due to a slow leak through cracks after a long period of operation in the power system. Several methods for monitoring the pressure onsite have been proposed so far, but most of them are poor in the ability to checkup the hazardous pressure level of 1 Pa or less. The present paper reports the results of a primitive attempt conducted for diagnostic purposes. In the study, by monitoring the electrification on the insulator surface of a real vacuum bulb, we successfully provide a method to detect the pressure rise around 10−1 Pa.

Depression of Insulator Charging in Vacuum by Partial Mechanical Processing

This paper describes experimental results on surface charging of partially mechanically processed insulators in vacuum. The charging phenomena were observed when insulators were exposed to a ramped dc voltage, by using a capacitive probe embedded in the cathode. First, we examined a partially roughened cylindrical insulator. The height of the roughened surface measured from the cathode, was varied while keeping the total length of the insulator constant. Second, in the same way, we examined a cylindrical insulator having a truncated conical frustum or a chamfer at its cathode-side end. Based on these experiments we have proved that the partial mechanical processing is effective for suppressing insulator charging in vacuum

Factors affecting the charging characteristics of solid insulators in vacuum — influence of voltage shape and pressure —

This paper deals with factors affecting the charging characteristics of insulators exposed to high voltages in vacuum. We examined the influence of voltage shape on charging. A ramped dc and impulse voltages were tested in a low pressure and found that the amount of charge was larger for the impulse voltage than that for dc voltage. We also examined the influence of pressure that varies from 10-5 Pa to 1 Pa and found that an insulator acquired more charge in a high pressure range than that acquired in a low pressure range.