Masahiro Hanai

Separation of corona using wavelet packet transform and neural network for detection of partial discharge in gas-insulated substations

It is essential to detect partial discharge (PD) as a symptom of insulation breakdown in gas-insulated substations (GIS). However, the accuracy of such measurement is often degraded due to the existence of noise in the signal. In this paper, a method using wavelet packet transform and neural network is proposed to separate the PD pulses from corona in air, which enables more accurate detection of insulation breakdown of GIS.

Source classification of partial discharge for gas insulated substation using waveshape pattern recognition

Frequency-domain analysis of ultra-high frequency (UHF) signals for source identification of partial discharge (PD) occurring in SF/sub 6/ inside gas-insulated substation (GIS) has been widely covered in literature. In this, Fast Fourier Transform and Discrete Wavelet Transform based techniques have been extensively applied to derive classifying features from transformed patterns. On the other hand, it appears feasible to develop a time-domain classifier, which derives features directly from the original waveshape. The time-domain classifier is conceptually simple, and requires potentially less computing resources and simpler algorithmic interface with other intelligent techniques due to elimination of frequency-domain transformation. A novel classifier to extract features directly from time-domain waveforms is proposed for classifying SF/sub 6/ PD from air corona and among the three types of SF/sub 6/ PD, regardless of changes in PD locations and measurement conditions. Three sets of classifying features are proposed. Encouraging results have been achieved with comprehensive experimental data, which verifies and proves the usefulness and feasibility of the time-domain classifier.

Application of functionally graded material for reducing electric field on electrode and spacer interface

For the size reduction and the enhancing reliability of electric power equipment, the electric field stress around solid insulators should be considered enough. For the relaxation of the field stress, the application of FGM (Functionally Graded Materials) with spatial distribution of dielectric permittivity (¿-FGM) can be an effective solution. In this paper, we investigated the applicability of ¿-FGM for reducing the electric field stress on the electrode surface with contact to the solid dielectrics, which was one of the important factors dominating a long-term reliability of the insulating spacer. Firstly, we carried out numerical simulation of electric field to confirm the reduction of the electric stress by U-shape permittivity distribution. Secondly, we investigated the fabrication feasibility of ¿-FGM with the U-shape distribution. Thirdly, we estimated the longterm electrical insulation performance of the ¿-FGM. Finally, we verified the applicability and the fabrication technique of the ¿-FGM to solid dielectrics for improvement of the electric stress and the long-term insulation performance in electric power equipment.

Comparison of sensitivity between UHF method and IEC 60270 for onsite calibration in various GIS

It is more important to obtain the sensitivity line, which is defined as a correlation between the ultra-high-frequency (UHF) signal (mV) and apparent charge (pC), for onsite calibration regarding partial discharge (PD). Synchronous measurement of the UHF method and IEC 60270 enables us to obtain the sensitivity line and to estimate the apparent charge in on-site substations. The sensitivity line of 3-phase gas-insulated switchgear (GIS) is compared to that of single-phase GIS, and differences in defects are also discussed. The fundamental data in this paper contribute to on-site insulation diagnosis of GIS

Determination of Trapping Parameters from TSC in Polyethylene

The trapping parameters of one of the carrier traps in low-density polyethylene were evaluated by using the X-ray-induced thermally-stimulated current (TSC) technique. The values obtained were Et (trap depth): 0.90 eV, µτ (product of mobility and lifetime): 5×10-13 m2/V and nto (initial density of trapped carriers): 2.9×1020 m-3. Assuming the fast retrapping case, Nt (trap density) was 1.4×1021 m-3 and, assuming the slow retrapping case, St (capture cross-section) and ν (attempt-to-escape frequency) were 1.1×10-17 m2 and 3×1013 sec-1, respectively. Whether the retrapping was fast or slow, was not determined from the TSC results only.

Progress in Development of Superconducting Fault Current Limiting Transformer (SFCLT)

We have been developing Superconducting Fault Current Limiting Transformer (SFCLT), which has multifunction of both a superconducting transformer in a steady state as well as a superconducting fault current limiter in a fault condition. This paper introduces the progress in our SFCLT project since 1998, from Step-1 to the latest Step-5, with the concept, design, fabrication and test results of SFCLT. In the latest Step-5, we developed a 2 MVA, 22/6.6 kV class SFCLT with YBCO coated conductors and verified the fundamental function as a transformer, effective current limiting function as a fault current limiter, and recovery characteristics after the fault clearance with its operational criterion.

Permittivity characteristics of epoxy/alumina nanocomposite with high particle dispersibility by combining ultrasonic wave and centrifugal force

This paper proposes a novel technique to fabricate epoxy/alumina nanocomposites with nanoparticle composite process by combination of ultrasonic wave and centrifugal force. The particle dispersion effect of the nanoparticle composite process and its influence on dielectric permittivity were discussed quantitatively. Experimental results clarified that the combination of ultrasonic wave and centrifugal force was effective to increase dispersed nanoparticles and as well as to separate residual agglomerates. We verified that the improvement of particle dispersibility in the nanoparticle composite process by combination of ultrasonic wave and centrifugal force could bring about lower permittivity of the nanocomposites, especially than that of unfilled epoxy material.

A unique sensitivity verification combined with real-time partial-discharge identification method

The sensitivity verification with the ultrahigh-frequency (UHF) method will be discussed in this paper. The sensitivity line with the UHF method based on the conventional (IEC 60270) one is different in the radial position of the defects inside gas-insulated substation (GIS), because transverse-electromagentic mode can be more weakened while transverse electric (TE)/transverse magnetic (TM) modes can be more enhanced as the defect comes close to the enclosure than the conductor. The result makes it difficult to detect the free particle with IEC 60270. A unique sensitivity verification, which is combined with real-time PD identification method, first enables us to identify the defects and thereafter to calibrate the apparent charge of each specified defect according to the sensitivity line.

Electrostatic charging tendency and correlation analysis of mineral insulation oils under thermal aging

In order to investigate the electrostatic charging tendency (ECT) of transformer oil under thermal aging and understand its correlations to the other insulation parameters, a series of accelerated aging tests were carried out on two types of mineral insulation oils, A and B, both of which were mainly used in 500 kV power transformers in China. Oil samples were pre-processed by degassing and dehydrating, and then aged at 120°C for various periods up till to 675 h, meanwhile, different contents of oxygen and copper/pressboard/silicon steel sheet were also added into the oil samples for sensitive study. The results indicate that the ECT of oil A increases notably with aging time, the ECT of oil B however does not change so much during the whole aging period. Injection of oxygen in the samples enhances the ECT of oil A, such a promotion effect is minor for oil B. The correlations between the ECT and other parameters, such as tanδ, volume resistivity (ρv) and acid number were analyzed as well. There are linear correlations between ECT and tanδ, acid number and log (ρv) of oil A, of which ECT and tanδ has the highest correlation coefficient of 0.881. Regarding oil B, there is only a positive linear correlation between ECT and acid number.

Current Limiting and Recovery Characteristics of 2 MVA Class Superconducting Fault Current Limiting Transformer (SFCLT)

We have been developing Superconducting Fault Current Limiting Transformer (SFCLT) with the function of both superconducting transformer in steady state and superconducting fault current limiter in fault condition. As the Step-5 of SFCLT project, 2 MVA class HTS-SFCLT using YBCO coated conductors with the ratings of 22 kV/6.6 kV has been developed. One of the important concerns for the development of HTS-SFCLT is the current limiting and recovery characteristics as well as the transformer function. Thus, in this paper, current limitation and recovery tests of the HTS-SFCLT are carried out. The current limiting test have revealed that the HTS-SFCLT exhibits the excellent current limiting function as a superconducting fault current limiter, where the prospective short-circuit current IPRO=784 Apeak is reduced to 34% at the first peak and 18% at the 5th cycles. The HTS-SFCLT has been able to recover into superconducting state immediately after the fault clearance. According to the transition of generated resistance of the HTS-SFCLT, the criteria of recovery into superconducting state are quantified for different combinations of load current before the fault and prospective short-circuit current during the fault.