Tatsuro Kato

Experience of GIS condition diagnosis using partial discharge monitoring by UHF method

Partial discharge (PD) monitoring system by UHF method is suitable for internal condition diagnosis of gas-insulated switchgear (GIS) due to its high sensitivity.We have developed the UHF method diagnostic system, which uses the neural network. Through the experience of using PDM system, we have built confidence in its performance. The GIS condition diagnosis using PDM by UHF method is quite useful on site. This paper describes outline of our PDM system and also introduces some examples of our experiences during both factory test and on-site test with which we have ensured advance diagnoses for insulation reliability of GIS.

Impulse breakdown mechanism based on discharge propagation process under non-uniform electric field in air

In this paper, we aim to clarify the discharge mechanism leading to breakdown under non-uniform electric field in air, especially on the propagation process of discharge current channel. The discharge measurement was performed in a needle (φ1 mm)-plane electrode system with gap length g=15-100 mm in 0.1 MPa dry air under applying a positive impulse voltage. We observed discharge current waveform and fast-framing images of light emission with image intensifier controlled by a nanosecond pulse. We found out two types of breakdown mechanisms from streamer initiation to breakdown (BD); secondary streamer progress and leader progress. In addition, we investigated the transition region between both types of BD mechanism, focusing on the case of gap length (g=30 mm). Finally, we found that the secondary streamer propagation would dominate the formation process of the discharge channel and the mechanism leading to BD. Also, we found the criterion of impulse breakdown mechanism in air.

Positive streamer propagation and breakdown characteristics in non-uniform air gap

Discharge phenomena in a needle-plane electrode in air were studied with nano-second time resolution by means of synchronous measurement of streak image, light intensity, discharge current and fast-framing image. When positive impulse voltage was applied, a primary streamer initiated from the needle tip and propagated to the grounded plane electrode. A secondary streamer initiated after the primary streamer arrived at the plane electrode. There are close relations between the applied voltage, the primary and the secondary streamer current and the initiation time of the secondary streamer. The breakdown can start when the secondary streamer have arrived at the plane electrode. Moreover, the time to breakdown becomes shorter with the increase in the secondary streamer current.

Streamer development mechanism under non-uniform electric field in air

We discussed the development mechanism of the streamer channel leading to breakdown in a short air gap, by using an ultra-high speed measuring system. We clarified that the initiation of the secondary streamer leading to breakdown is attributed to an electron injection from the grounded plane electrode to the residual channel made by the primary streamer. In addition, by selective measurement of the discharge current waveform of single streamer channel leading to breakdown, the breakdown was verified to be dominated by the development of the secondary streamer and the subsequent Joule heating process focusing on the single streamer channel.

Influence of space charge by primary and secondary streamers on breakdown mechanism under non-uniform electric field in air

We have clarified that the primary and secondary streamers have a significant impact on breakdown mechanism in air. In this paper, we focused on the highly-conductive channel progress breakdown, and considered the initiation condition of highly-conductive channel from the viewpoint of the secondary streamer initiation. We revealed that the highly-conductive channel initiation voltage in the case with the grounded electrode covered by a dielectric barrier was higher than that without the barrier. We suggested that the electric field relaxation effect of the residual positive space charges formed by the primary streamer is greater than that by the secondary streamer. The highly-conductive channel is initiated from the region where the electric field relaxation is weak.

Calculation model for predicting partial-discharge inception voltage in a non-uniform air gap while considering the effect of humidity

A calculation model for predicting the inception of partial discharge (PD) in a non-uniform air gap on the basis of theories of electron avalanche and streamer phenomena is proposed. “Partial discharge inception voltages” (PDIVs), under an assumed absolute humidity of less than 20g/cm3, calculated using the proposed model were compared with experimentally measured ones and found to be almost equivalent. Furthermore, the relationship between negative ions and humidity was incorporated into the calculation model, and it was confirmed that the calculated and measured PDIVs tend to be the roughly the same under a wide humidity range under various gap length between electrodes. The proposed partial-discharge inception model based on theories mentioned above was therefore judged to be effective for predicting changes in PDIV when humidity changes.

Observation of electrical treeing in epoxy resin by X-ray phase contrast imaging

Epoxy resins are widely used as insulating material in electrical apparatus. As such epoxy resins are non-transparent due to the effect of fillers, it is hard to optically observe electrical treeing in them. In this paper, we observe electrical trees in unfilled and filled epoxy resins by X-ray phase contrast imaging. When X-ray passes through very small tubular material such as electrical trees, edge-enhancement images are obtained. It makes easier to detect trees, but, on the other hand, wave-optical analysis is necessary to obtain a tree diameter accurately from an apparent tree diameter in an X-ray image. Through the computational wave-optical analysis, it turns out that the apparent tree diameter converges to a certain value with decreasing the diameter of measured tree. In addition, optimal arrangement of the equipment and minimum detective value are investigated based on this calculation. In case of unfilled epoxy, minimum detective value in the condition of optimal arrangement is found to be 3.5μm.

Classification of impulse breakdown mechanisms under non-uniform electric field in air

To clarify the breakdown mechanism and the conditions leading to the breakdown under non-uniform electric field in air, we studied the details of breakdown characteristics and mechanism in air by using an ultra-high speed measurement system. We clarified that there were two types of breakdown mechanism in air. The one is the channel heating breakdown, and the other is the leader-type channel development breakdown. These breakdown mechanisms are dominated by three conditions: arrival of secondary streamer from high-voltage anode to opposite grounded cathode, channel heating, and leader-type channel initiation. We studied these conditions quantitatively, and revealed the conditions leading to the breakdown, and the breakdown mechanisms in air were classified. Finally, we clarified the factors, e.g. critical instantaneous power for the secondary streamer arrival, dominating these conditions based on the breakdown mechanism under non-uniform electric field in air.

Development of compact gas-filled bushings with new inner-grounded electrode

In Europe and the USA, the use of gas-filled bushings with hollow composite insulators has increased. Hollow composite insulators are superior in that they are light, explosion proof, and pollution resistant, and their economic efficiency is advantageous at larger classes especially. However, developing compact gas-filled bushings by improving electric field distribution is important. The main issue for making design more compact is finding a way to reduce electric field strength on the outside hollow composite insulator around the inside grounded electrode tip. A new-type inner grounded electrode structure which consists of ring-shaped electrode and cylindrical electrode is proposed. This paper describes the effect of reduction of maximum value of electric field strength on the outside hollow insulator by the new-type one. Finally, the insulation performance of the prototype of 800-kV gas-filled bushing is described with a discussion of the results of testing fundamental insulation and overvoltage.

Development of discharge inception simulator considering environmental factors using advection-diffusion model

Insulation design with discharge simulation based on physical phenomena is used in designing electrical instruments to facilitate their miniaturization and obtain high insulation reliability. We developed a model to calculate partial discharge inception in a non-uniform air gap by considering how environmental factors, especially humidity, affect insulation performance. We upgraded the calculation model that built in the distribution of negative ion density using the amount of moisture on the electrodes. As a result, we can estimate the discharge inception voltage with high accuracy. Additionally, we attempted to extend the simulator for three-dimensional creep analysis.