Search

Electric tree is a phenomenon of local dielectric breakdown with partial discharge (PD). It is also the final stage of insulation deterioration. The electrical treeing has been discussed by using conventional PD parameters such as the generation phase, charge amount and so on. Since the obtained PD waveforms were processed by an analog filter designed according to the frequency characteristics of the measurement instrument such as an oscilloscope, its waveform is a convolution of the original PD waveform and the transfer function of the measurement system. However, the feature values such as a rise time, fall time and frequency spectrum in the original PD waveform should show the behavior of electrons and ions in the discharge space. In this paper, the original PD waveforms generated in micro-silica/epoxy composite with the electrical tree were acquired, and the feature values of the original PD waveform were analyzed. In addition, the behavior of the electric charges in the discharge space was assessed from the change of the feature values.

Mica epoxy insulation as used for generator stator windings is required to be improved as regards withstand electrical stress and heat dissipation. Replacement of epoxy resins by epoxy nanocomposites is a useful method to fulfil a purpose of such improvement. This paper is aimed to evaluate PD-initiated breakdown (BD) performances as one of the most crucial characteristics using simulated model specimens. Two constituent materials, i.e. mica tapes and resin layers were set as about 40 and 10 µm in thickness, respectively. EDX analysis shows that the model specimens include silica nanofillers properly dispersed in narrow nanocomposite regions and are suitable for PD initiated BD experiments. It was found that PD initiated BD lifetime was 2.8 times in nanocomposites than in neat epoxy under a rather high electric stress condition.

Polymer nanocomposites have attracted attention for various superior insulation performances. However, the effect of nanocomposites on the insulation lifetime with an enclosed void defect is unclear. Also, the insulation lifetime extension mechanism has rarely been reported. This paper experimentally investigates the lifetime extension mechanism of epoxy/TiO 2 nanocomposites with the void defect. Centrifugation and dilution procedures are used to control the dispersion state of TiO 2 nanofillers. Time evolutions of partial discharge (PD) current are also recorded to investigate the insulation degradation procedure. Consequently, it is clarified that the lifetime becomes longer with increasing the maximum agglomerate size of the TiO 2 nanofillers. Results of the PD current monitoring suggest that the lifetime extension of the nanocomposite is achieved due to delaying the electrical tree propagation in the sample.

Epoxy nanocomposites (NCs) are known to have advantages when used in the electrical insulation of power equipment. Agglomerates of nanoparticles, which inevitably form during the fabrication process, can affect the insulating properties of the resultant nanocomposites. Our previous research has clarified that the impulse breakdown strength of the solid part, rather than the surface, of epoxy–TiO 2 NCs with agglomerates removed was higher than that of unfilled epoxy resin. Here, we investigated the dependence of the TiO 2 filler volume fraction on the AC breakdown strength of NCs whose agglomerate size was controlled. The volume fraction and the maximum agglomerate size were controlled using a simple dilution procedure and a centrifugation technique. The AC breakdown strength of the NCs was found to increase with decreasing volume fraction of TiO 2 nanoparticles when the maximum agglomerate size was controlled at the same level. The variation of the AC breakdown strength of the NCs decreased with decreasing maximum agglomerate size, suggesting that the existence of agglomerates larger than a critical size could affect the breakdown.

Agglomerates of nanoparticles which inevitably occur in the fabrication process of epoxy nanocomposite can affect the insulation properties of epoxy nanocomposite. This paper presents an attempt to clarify the effect of agglomerate size on two important insulation properties of epoxy/TiO 2 nanocomposite (NC). One is intrinsic AC breakdown strength and the other is long-term partial discharge lifetime. We used a centrifugation technique to control the maximum size of the agglomerates in the range from 5 μm down to 0.05 μm in each sample. As the results, AC intrinsic breakdown strength was increased with decreasing the maximum agglomerate size and became even higher than the neat epoxy resin when the maximum size is less than around 0.05um. On the other hand, partial discharge lifetime decreased with decreasing the maximum agglomerate size. It should be noted that the PD lifetime distinctively decreased when the agglomerate size is less than around 0.2 um indicating the existence of a critical agglomerate size which contribute to the suppression of partial discharge development resulting in the extension of the partial discharge lifetime.

SiC-based nonlinear conductive stress grading tape (SGT) has been used at an end-turn stress grading system of a form-wound rotating machine. Although previous researches reported that high-frequency repetitive pulses cause electric field enhancement and local heating at the SG system, few paper deals with the detailed electrical and thermal behaviors under pulse width modulation (PWM) waveforms, which contain not only the repetitive high-frequency, i.e., the carrier frequency, component but also the fundamental frequency component. In this study, the surface potential, the electric field, the time-averaged power densities as well as the resulting temperature rise under the 3-level PWM waveforms were investigated by the transient electric field and thermal coupled FEM simulation. Furthermore, the behavior of the accumulated charges within the SGT was analyzed, based on the simulation results. Consequently, it has been revealed that the charge behavior and the power densities by the carrier-frequency component and those by the fundamental frequency component could be separated individually.

Electric field grading system (SG system) is introduced to prevent partial discharge at the stator coil end of a rotating machine. The SG system consists of corona armor tape (CAT) and stress grading tape/paint (SGT/SGP). The CAT is a carbon-loaded highly-conductive tape and is wound over the main insulation. A SGT/SGP is a silicon carbide powder-loaded tape/paint with field-dependent conductivity and is wound over partially the end of CAT. In general, the conductivity of SGT/SGP also has temperature dependence, but has little field-dependence under higher temperature condition. The coil temperature under operating condition exceeds 100°C and this means the electric field on the coil end may increase due to the less non-linearity of SGT/SGP. When the temperature is high, the density of air decreases and surface discharge may occur easily even under the same field distribution. To examine this issue, light emission of partial discharge (PD) on a model bar system under 1-kHz AC voltage was observed by a photomultiplier tube and a digital camera equipped with an image intensifier. The temperature on SG system was set to 30, 70, and 100°C. Unexpectedly, light emission from PD became weaker and the light emission area became smaller as the temperature increased. Some reasonable explanations for this are as follows: the change in the occurrence phase of PD: and the amount of water contained in the vicinity of SG system surface. Although further investigation is required, but the unexpected increase in the PD pulses has not been observed so far within the measured range.

In this study, transient analysis of Stress Grading(SG) system under PWM waveform is conducted to reveal the effects of fundamental frequency on potential distribution and power density. Risetime, level and jump voltage of PWM waveform used in an analysis is 1μs, 3 and 5 kV, respectively. Fundamental and carrier frequencies are changed in the range from 50–500 Hz and 0.5-6 kHz, respectively. For comparison, analyses under sinusoidal and square voltages which correspond to fundamental and carrier frequencies are conducted. Analytical results show that maximum surface field on SG system under PWM waveform is 1.1 times higher than that under square voltage regardless of fundamental and carrier frequencies. Power density under PWM waveform increases than that under square voltage. The value of power density under PWM waveform is estimated by the sum of power density under sinusoidal voltage which corresponds to fundamental frequency and square voltage which corresponds to carrier frequency. The error of this estimated value is in the range from 8–15 %. This shows that estimated value is accurate enough to estimate temperature rise under PWM waveform.

Polymer nanocomposite material, i.e., polymer insulating material with nano-sized inorganic additives, has recently received much attention for their superior dielectric performance as insulating media of high voltage power apparatus. The long-term insulation lifetime is one of the important factors, which determines the insulation reliability of the apparatus. In this research, the insulation lifetimes of epoxy/TiO2 nanocomposite and neat epoxy resin were experimentally investigated. The long-term experiments were carried out with a test electrode system with an artificial enclosed-void, which simulated air cavities contained within the insulation of the actual-scale apparatus. The time evolutions of partial discharge (PD) intensities were also recorded during the long-term experiment. Consequently, the epoxy/TiO2 nanocomposite showed more than 16 times longer lifetime compared to the neat epoxy resin. Finally, the superiority of the epoxy/TiO2 nanocomposite was discussed based on the PD energies.

Additives and fillers are traditionally employed in polymer insulating materials like an epoxy resin (EP) to improve the mechanical, thermal, and sometimes even the electrical properties. To understand the influence of filler on the electrical properties of EP/nano-sized titanium oxide (nTiO 2 ) nanocomposites, treeing breakdown tests were performed. The breakdown voltage of the EP/nTiO 2 nanocomposite with a 1-vol% nTiO 2 content was higher than that of the EP at the same electrode distance. The nTiO 2 might capture electrons, leading to an electron avalanche. The improvement effect by adding the nTiO 2 was also observed in the nanocomposite–mica sheet multi-insulation system. The addition of an appropriate amount of nTiO 2 to EP may suppress tree initiation and/or tree propagation.

Partial discharge (PD) measurement has been used as a test to evaluate the insulation systems and as a quality control test for the new power equipment. A continuous generation of PDs will decrease the insulation performance and lead to the dielectric breakdown. Many researches related to the PDs detection and diagnosis of the degradation by PDs were conducted. The occurrence time (phase) or the charge amount has been investigated as the parameters representing PD characteristics. However, the PD waveforms itself represents the processes such as the formation of electron avalanches and sweep out of ions. In this paper, a device which can acquire all PD waveforms was constructed and we also demonstrated the utility of the measurement system by analyzing the changes in characteristics of PD waveforms. In addition, the difference between waveforms with PD generated in air and inside electrical tree tubes was presented.

Agglomerates of nanoparticles, which inevitably occur during the fabrication process of epoxy nanocomposites, can affect the insulation properties of the epoxy nanocomposites. This study presents an attempt to clarify the effect of the agglomerate size on the impulse breakdown strength of epoxy/TiO 2 nanocomposites. In particular, we changed the process conditions of the particle dispersion method using centrifugation to control the agglomerate size of several epoxy/TiO 2 nanocomposite samples. The experimental results clarified that centrifugation can be effective for controlling the agglomerate size of epoxy/TiO 2 nanocomposites. In addition, we examined the impulse breakdown strength of the epoxy/TiO 2 nanocomposites whose agglomerate sizes were 0.5–50 μm. The impulse breakdown strength of the epoxy/TiO 2 nanocomposites increased with a decrease in the agglomerate size. Finally, we determined the critical size of the agglomerates required for obtaining the epoxy/TiO 2 nanocomposites with a higher breakdown strength when compared with that of an unfilled epoxy resin.

This paper investigates the electroluminescence (EL) and the dissipation current properties of the Epoxy/Ti02 nanocomposites under AC ramp electric field application. The volume fractions of the samples are 1, 5, 10 and 20 vol%. As a result, the inception and the extinction electric field of the EL, 2nd and 3rd harmonics of the dissipation current increased with decreasing the volume fractions. Furthermore, the nanocomposite sample with the volume fraction of 1 vol% showed the superior results compared to the neat epoxy sample. It is considered that the small amount of the nanofiller additives and the resulting homogeneous nanofiller dispersion within the epoxy is important to improve the performances.

Ultra-high frequency (UHF) partial discharge (PD) measurement, i.e., detecting electromagnetic (EM) waves in the UHF range radiated from PD, is an attractive method for diagnoses of power transformers and has recently received much attention. However, the EM waves tend to suffer attenuation while propagating within a transformer to UHF sensors, which in some cases results in decreasing detection sensitivity of PD signals. In order to understand the propagation and attenuation characteristics of the EM waves within transformers, simulation technique of the EM wave propagation is essential. Although many previous researches on the EM wave simulation have been published, validations of those simulated results by comparing with the experimental ones are insufficient. In this research, at first, the attenuation characteristics of signal amplitudes, cumulative energies as well as frequency components of the EM waves while propagating through transformer windings were investigated both by time- and frequency-domain measurement. The EM wave propagation was simulated based on time-domain finite integration technique (FIT), then the simulated results were compared with the experimental ones. Consequently, the simulated EM waveforms, their signal attenuation level and S-parameter showed good agreement with the experimental results. Therefore, the validation of this EM wave simulation was successfully confirmed.

SiC-based nonlinear resistive material has been used for end-turn stress grading (SG) of turbogenerators over decades. In order to reduce power dissipations and avoid thermal breakdown of the SG material, high-voltage class generators adopt multiple-layer SG system, at which two SG materials with different nonlinear resistivity are used. With increasing capacity and efficiency of the apparatus, electrical, and especially thermal stresses at the system may become problematic, hence higher reliable SG system is required. For suppressing the local heating effectively, an optimization method of the multiple-layer system was investigated, where a new combination of the two SG materials was proposed and length of the SG layer in the longitudinal direction along a coil was optimized by using analytical approximations of the power dissipation. The optimized SG system showed 20% lower power dissipation and also 15% higher flashover voltage than those of the conventional one. Consequently, the effectiveness of the optimization method proposed was successfully confirmed.

In order to clarify breakdown mechanisms in insulation oil / pressboard composite dielectric system, it is effective to measure the electric field distributions. We have investigated the electric field distribution in transformer oil using Kerr electrooptic effect. At first, Kerr constant of the transformer oil was measured using the optical system. The constant was determined to be 2.0×10−15[m/V2]. Next, we measured the electric field in transformer oil in AC electric field in the order of milli-seconds. In order to reduce the error due to the fluctuation of light intensity of the laser, we used the improved optical system. As a result, we have been able to be confirmed that the change of light intensity when AC voltage application was followed to the principle of Kerr effect. In addition, as compared with the result measured by the conventional system, the result could be improved by the new system. Thus, we have been able to confirm that time series electric field measurement on the order of milli-seconds in transformer oil under AC voltage by using optical system based on Kerr effect. In the future, electric field measurement will be performed when partial discharge occurs by applying AC high voltage or pulse high voltage in insulation oil / pressboard composite dielectric system. The electric field distribution before and after the partial discharge will be measured and the breakdown mechanism will be investigated by based on the result.

Stator coil end of high-voltage rotating machines is equipped with stress grading system (SGS), which consists of two dielectric materials (conductive slot coating and stress grading tape/paint (SGT/P)). SGS grades surface potential curve on coil end and prevents surface partial discharge. Improving SGS and raising voltage limit of rotating machines are key to produce more efficient and durable products. Because SGS consists of non-linear materials, actual measurement of surface potential profile along SGS is essential for discussing advanced insulation design. However, there has been no suitable method to measure transient surface potential of as high as 100 kV maximum voltage. The authors have been developing Pockels potential sensor and have been measuring surface potential of coil bar. However, Pockels potential sensor has many advantages such as wide-band frequency response, small delay time and small disturbance to subject, Pockels potential sensor can measure only as high as 20 kV peak due to the close distance between high potential subject and grounded electrode. In this study, surface potential measuring method, which uses Pockels field sensor and includes calculation of surface potential from measured field data, is presented. Measurement system consists of Pockels crystal, laser, and photodetector. The relationship between surface potential and field is numerically calculated in advance and used to estimate surface potential by deconvolution process. Pockels crystal scans 1 mm above subject surface with 1mm measuring pitch. The potential on a sample bar coil under 10kV and 30 kV 50 Hz voltage is successfully measured with this system.

In order to improve the insulation performance of oil-immersed power transformers, the characteristics of the creeping discharge under impulse voltage have been investigated. In this work, by using streak camera, investigations of the influence of the white discharge trace on the creeping discharge characteristics were implemented. The results showed that the velocity of the discharge developing along the white trace was about a thousand times larger than that of the discharge progressing on the pressboard surface. The results also showed that the length of the discharge progressing on the pressboard surface after the white trace formation was shorter than that before the white trace formation. On the other hand, the discharge time delay became larger. These results are thought to be explained by the residual charge generated by the discharge developing along the white trace.

SiC-based nonlinear resistive grading materials have been used for end turn stress grading of form-wound rotating electrical machines, such as high voltage motors and turbine generators, over a long period. Although these materials work well under power frequency, the electric field and local heating become problematic under PWM repetitive pulses for the stress grading systems of converter-fed rotating machines. In order to select the appropriate stress grading materials and to optimize the insulation design of end turn structure for PWM waveforms, accurate knowledge of the nonlinear conduction characteristics of stress grading materials as a function of temperature and electric field is required. In this paper, the authors analyze sophisticated sample configurations for accurate measurement of nonlinear conductivity characteristics of the materials based on transient nonlinear FEM computations.

This paper deals with the discharge characteristics in the oil/Pressboard composite insulation system under the application of a standard lightning impulse voltage. Discharge images were observed using a digital camera. We investigated the barrier effect on partial discharge inception voltage (PDIV) and breakdown voltage (BDV). Furthermore, we observed the discharge generation across the insulation barrier and also evaluated the voltage required for the phenomenon by the electric field analysis.