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Ultrasonic method is a non-invasive partial discharge (PD) detection method for power equipment, which plays a more and more important role in partial discharge location. The propagation characteristics of ultrasonic in converter transformer can be effectively analyzed by using numerical method. In this paper, according to the actual size of a 110kV converter transformer, the 3-D finite element simulation model of converter transformer is established to simulate the propagation of ultrasonic signals in the tank. A 2-D model is then established to simulate the influence of windings on the propagation of ultrasonic signals. The simulation results show that when the partial discharge is generated, it will spread around in the form of a spherical wave, which spreads over the entire converter transformer space within 2000μs. The sound pressure amplitude is exponentially decreased with the increase of distance between PD sources and prbes, and the drastic attenuation is caused by the windings and core. When the discharge source is located between the winding and the core or between the windings, the wave arrival time from the partial discharge source to the ultrasonic sensor will be increased. The analysis can provide the basis for the detection and localization of partial discharge in converter transformer.

Deterioration of insulation materials caused by partial discharge is one of the important reasons for the failure of power equipment. The noise caused by the interference of the external electromagnetic environment seriously affects the accuracy of partial discharge detection. Aiming at the problem of partial discharge noise suppression, this paper proposed a partial discharge signal noise suppression method based on adaptive window singular value decomposition. This method first randomly generated a series of windows with different positions and lengths on the noisy partial discharge signal; Then singular value decomposition method was employed to denoise the signals in each window; Finally, the denoising index was chosen as the fitness function, through the particle swarm algorithm to obtain the optimal solution of the window position and length. The noise signals obtained by simulation and experiment were verified, and the method was compared with adaptive singular value decomposition, wavelet denoising, and other methods. The results showed that this method could achieve a better denoising effect.

Partial discharge (PD)-based diagnosis is extensively employed in condition assessment of electrical equipment. In the case of multiple PD sources, discrimination of mixed signals is significant for reliable PD interpretation. To improve the separation performance of three or more PD sources, a multi-step discrimination method is proposed. The cumulative energy functions are exploited to characterise wave shapes of PD signals, and width and sharpness features are extracted and classified to separate mixed patterns. With the objective of maximising a novel evaluation parameter of separation capability, the oblique line and length of structure element are optimised in the feature extraction stage. For the multi-step discrimination method, the feature extraction and clustering procedures are repeatedly applied to the whole dataset, sub-classes, sub-sub-classes etc., until no more clusters are generated. To evaluate the separation performance of the proposed algorithm, a mathematical model for PD pulse is proposed, which is the multiplication of Heidler enveloping function and an oscillating function whose frequency spectrum confirms Gaussian distribution. In the end, the multi-step discrimination method is tested with PD current pulses and ultra-high-frequency signals of three artificial defects in transformer and gas-insulated system, and the results prove the effectiveness of the proposed algorithm.

Traditional methods cannot detect hot spot temperature, mechanical vibration and partial discharge signal and other state parameters of the transformer in real time. Optical fiber sensing technology has a wide range of application prospects due to its high accuracy, strong anti-interference ability and distributed measurement. However, the premise for its application is that the embedded optical fiber does not affect the safe operation of the transformer. This paper explores the interact between embedded fibers and transformers. In this paper, four different types of optical fibers were initially selected to prepare fiber-oil composite insulation. Samples under different aging time were obtained through accelerated thermal aging experiments. The mechanical strength and optical loss characteristics of fibers with different aging degrees were tested separately. At the same time, moisture content, acid value, oil chromatogram, dielectric loss factor and volume resistivity of insulating oil were tested. The results show that the mechanical strength of the PVC tight fiber decreases significantly as the aging time increases. PVC tight fiber significantly improves the dielectric loss factor, CO 2 /CO and total hydrocarbon content in insulating oil. The mechanical properties of the other three optical fibers remain basically unchanged and have little effect on the insulating oil. Thermal stress does not affect the optical transmission characteristics of all optical fibers. The PVC tight fiber is not suitable as the embedded optical fiber in transformers, while the other three optical fibers meet the basic requirements of the embedded optical fiber. The experimental results provide solutions for the safety risk assessment of the embedded optical fiber in transformers.

In this work, samples of oil-immersed paper with different moisture levels and aging condition were studied. Frequency domain spectroscopy (FDS) of the samples was examined at 40 °C under different excitation voltages. A non-linear phenomenon in frequency domain dielectric response with different moisture contents and aging condition was observed. The non-linear phenomenon appears to be related to the hindrance effect of cellulose on the movement of ions and the Poole-Frenkel effect on the concentration of ions. With aging, the nonlinear loss peaks at different frequencies caused by the effect of nonlinear conductance and interface polarization appear in the FDS results. The non-linear phenomenon was modeled and verified experimentally. The characteristic parameters and quantitative method to estimate moisture content and the degree of aging degree of oil-immersed paper can be obtained by the proposed method.

Methods to infer the location of partial discharge (PD) in high-power transformers using acoustic emission (AE) data have been extensively studied. The inner complex structure of the transformers is one of the most critical points in localization with AE method. Windings and cores affect acoustic wave propagation by changing the arrival time because of inhomogeneous propagation. A transformer physical model has been established herein by taking these complex factors into consideration. Each node in the model is a potential PD position, and an acoustic wave route comprise a series of nodes. The velocity and propagation factors are set for each node according to its acoustic wave propagation characteristics. A propagation-time estimation algorithm is proposed to calculate the propagation-time. Based on the transformer physical model, a particle-swarm-optimization route-searching (PSORS) algorithm is employed for searching the position of the PD source. By comparing time differences of measured AE signals and the ones estimated by the PSORS algorithm, the velocities and positions of particles are continually adjusted, which can ensure their convergence to the PD source position. Localization experiments were performed in 35 and 110 kV transformers, respectively, to verify the applicability of the proposed algorithm. A protrusion defect is used to trigger PD pulses, and four AE sensors with two different arrangements are employed. The results confirm that the accuracy of proposed localization method is insensitive to the presence of metal structures blocking acoustic wave routes.

In this study, a case-based diagnostic model of a power transformer for oil chromatography is proposed. A nonlinear normalised mapping method is proposed for achieving ideal distribution characteristics of oil chromatogram gas in the case base. A similarity degree algorithm is proposed to calculate the similarity degree in the target spatial domain between the case to be diagnosed and the cases in the case base. A threshold method is proposed to determine the number of similar cases. Also, a method for calculating the weight of the diagnosis result according to the case distribution in the case base is proposed. The diagnostic method proposed is used to deploy the diagnostic system on the Internet. Defeat diagnosis conclusions of two transformers by the method and system in this paper are consistent with the disintegration results. It can be concluded that the proposed method has important practical application value in engineering.

The insulation diagnosis method based on frequency domain dielectric spectrum is widely used in field test because of its nondestructive testing, rich insulation information and so on. FDS measurement results are easy to be affected by many factors. However, interpretation on the dielectric properties of oil paper insulation with moisture content, impurity ion concentration influence mechanism of the current research is not very ideal, so it is necessary to determine more appropriate and effective dielectric response simulation and experimental analysis of characteristics. In addition, moisture content (m.c.) is an extremely important factor which can determine the insulation state of Oil-immersed paper. Therefore, the evaluation of moisture content of oil-immersed paper has obvious significance. In this paper, based on the experimental results, the extended Debye model parameters of the field transformer are obtained. Then, the influence of the insulation resistance value in the Debye model on the FDS curve is simulated by MATLAB modeling. The simulation and experimental verification show that the regularity of the change of the FDS conductance process is obviously different from the moisture content influenced by the impurity ions. The impurity ions mainly affect the low frequency section of the imaginary part of the complex capacitor. A non-linear phenomenon in dielectric response with different moisture contents under different is observed. We found that the non-linear phenomenon has relationship with the hindrance effect of cellulose (m.c.

Partial discharge (PD) in power transformer degrades the dielectric insulation and results in insulation failure and breakdown after a long period. In fact, a partial discharge detector can gather signals from two or more PD sources which increase the difficulty on pattern recognition and insulation state assessment. How to separate multiple PD signals is meaningful to subsequent data processing. In this paper, a PD separation algorithm based on cumulative energy (CE) function is introduced. The CE functions in time domain (TCE) and frequency domain (FCE) are calculated from PD waveforms and their FFT spectra, respectively. By using an oblique line to cross the CE curves, width features are extracted from the intersection points between them. Through the mathematical morphology gradient (MMG) operation, sharpness features are extracted to characterize the rise steepness of CE. A clustering algorithm is adopted to discover different clusters in feature space and separate PD signals. After that, a transformer experiment platform is introduced and some different defects are placed at different places in this transformer to produce PD signals. The pulse current method is applied to gather their PD waveforms and to record phase resolved partial discharge (PRPD) which can be used to analyse discharge characteristics of different defects. The separation algorithm is examined with mixed PD current pulses acquired from experiments. The results prove the feasibility of separation algorithm with different PD defects.

So as to adapt to the condition-based maintenance of power equipment and to overcome the disadvantages and deficiencies of traditional condition evaluation model in data acquisition approaches, component classifications, performance indicators and scoring model, a comprehensive condition evaluation model (CCEM) was proposed in this paper based on multi-source information. Combined with the offline test, online detection, online monitoring, inspection and operating environment, equipment criteria, undesirable service condition (USC) and family defects were reclassified in this model. Practice example verified that the proposed model can describe the power transformer state more comprehensively and immediately. The evaluation results provide scientific guidance for front-line operation and maintenance staffs to develop equipment maintenance strategy, and also practical basis for the condition evaluation of other power equipment.

An argon dielectric barrier discharge (DBD) atmospheric-pressure plasma jet (APPJ) is designed and employed for surface modification of polytetrafluoroethylene (PTFE). The plasma diagnostics and dielectric surface analysis are coupled together to investigate the mechanisms of plasma modification. The discharge power is obtained by Lissajous figure, and electron excitation temperature (EET) is measured through an optical emission spectrum and calculated by a Boltzmann diagrammatic method. The surface properties of modified PTFE samples are characterized by the static contact angle, surface resistivity, scanning electron microscope (SEM), and X-ray photoelectron spectroscopy. The results show that the discharge power, EET, and surface wettability increase with the Ar flowing rate, and a slight decrease of surface resistivity is revealed after plasma treatment. The surface roughness of PTFE is enhanced, and the oxygen-containing hydrophilic groups are incorporated by the impacts of APPJ radical species. Moreover, the hydrophilicity of PTFE can be improved by a surface presanding process, particularly after APPJ treatment. Finally, the surface modification mechanisms of APPJ on PTFE are discussed.

A particular Cathode-like luminescence (CLL) phenomenon occurring on vacuum-dielectric interface under high DC electrical field before or accompanied with flashover is reported, which is widely existed in vacuum-dielectric system. It is considered that the CLL presented in this paper is initialized by field electron emission at Cathode Triple Junction (CTJ) and finally generated by a process of radiative recombination due to the self-stabilizing secondary electron emission (SSEE) and collision on surface of insulator. The spectrum of CLL is measured and analyzed, and it shows that there are not any characteristic spectrums of C, O, N, which means that the CLL is not produced by discharge of desorption gas. In addition, simulation results also approved that the SSEE process in fact exists. Finally, its relevance to flashover is also discussed. It shows that flashover seldom occurs on the channels of CLL, which seems that the CLL has an inhibition effect on flashover. However, the exact mechanism is still unclear. This work will be researched in near future.

Surface flashover phenomena in vacuum restrict the overall performance of vacuum solid insulation system. Since accidents caused by surface flashover in vacuum occur constantly, it is always the international highlight to study some new insulating materials and insulation structure with high electric withstanding for vacuum insulation. On the basis of the novel low melting temperature machinable glass ceramics for vacuum insulation system, which has excellent machinable performance and good electrical properties, the samples of embedded electrode into machinable ceramics are machined, and then the samples' flashover voltage of different embedded mode and embedded depth are investigated under nanosecond pulse voltage in vacuum. The electric field distributions and electric field of cathode triple junction (CTJ) are simulated to explain the experimental results. The experimental results indicate embedded electrode in cathode can increase the surface flashover voltage of the sample; with increasing of the embedded depth, the surface flashover voltage tends to be increased. The simulation results indicate embedded electrode in cathode can decrease the electric field of CTJ, and with increasing of the embedded depth, the electric field of CTJ tends to be decreased.

Summary form only given. Flashover phenomena in vacuum restrict the performance of insulation system, which has been paid much attention by many researchers. However, due to the rapidity and complexity of flashover development process, the mechanism of surface flashover phenomena is still not fully understood. In this paper, an ICCD camera is adopted to observe the dynamic process of surface flashover on alumina ceramic in vacuum under impulse voltage. A needle as the HV electrode and a round-shaped steel as ground electrode are placed on the alumina ceramic, which forms an asymmetric electrodes configuration. Negative and positive impulse high voltages are applied on needle electrode, respectively. And the development processes of surface flashover are investigated. The results show that when negative voltage is applied on needle electrode, light emission always starts from cathode and then move to the anode, while positive voltage is applied on needle electrode, due to high electric field near needle electrode, light emission can be observed both at cathode and anode. But at the anode, the light emission region is limited in a small scale. It can also be seen that light emission near the cathode is developing to anode with applied voltage increasing, and the light branches are only developed in the direction of anode.

Surface flashover in vacuum is a kind of complicated surface and interface phenomena, which seriously restricts the development of high voltage electro-vacuum devices. Introducing a kind of machinable ceramic with excellent machinable performance and good surface electrical capability into vacuum insulation system, and considering the commonly used truncated cone structure, machinable ceramic samples with different coning angles are machined. Their surface flashover voltage are investigated under pulsed voltage in vacuum, and the electric field distributions and surface charge density of different coning angles are numerically simulated. The experimental results indicate that, the flashover voltage of cone insulators is higher than that of cylindrical insulators, the flashover voltage of truncated cone samples with positive coning angle is much higher than that of with negative coning angles, and the flashover voltage with ∼45°conical frustum is highest. It is considered that, the coning angle plays an important role on the electric field of cathode triple junction (CTJ) and surface charge distribution. When the coning angle is positive, the electric field of CTJ and surface charge distribution is smaller, and when the coning angle is negative, these values are bigger, so its higher CTJ electric field and surface charge result in a lower flashover voltage than that with positive angle, and also the short free path of electrons and decreased energy of electrons colliding with insulator surface lead to a higher flashover voltage than that of cylindrical insulators.

This paper presents experimentally flashover evolution initiated from cathode to anode via an intensified charge-coupled device camera under the condition of adjusting electric field distribution. These results suggest that a flashover process across insulator surface in vacuum is greatly affected by the local electric field at both cathode and anode triple junctions. Flashover can be initiated at either cathode or anode. This paper might be helpful to clarify the dispute about how the flashover is actually initiated.

Three kinds of electrode configurations are employed for studying the spurt length of atmospheric-pressure plasma jets (APPJs). The influences of Ar flow rate, applied voltage, and electrode width on APPJ length are investigated. It is found that, for all the electrode configurations, the maximal length of the APPJ is achieved at 3 L/min of flow rate. With the increase of the applied voltage, the length of the APPJ increases and gradually tends to saturation. Furthermore, the APPJ propagation turns shorter under a wider wrapped electrode. It is considered that the variation of the APPJ length is attributed to the combination effects of Ar flow status, the amount of high-energy particles at the ionization front, and the memory effect of charges on dielectric surfaces.

Recently, the cathode-like luminescence phenomena are observed from the surface of solid dielectrics under steady-state high voltage excitation in vacuum, which is closely related to the self-stabilizing secondary electron emission (SSEE) occurring on insulator surface. In this paper, the theoretical analysis about surface charging based on the SSEE is made, and a one-dimensional Mont Carlo simulation on surface charge accumulation under DC voltage excitation is provided. The results reveal that surface charging process can be divided into three stages including initial accumulation, fast multiplication, and final stable stage. In the final stage, the SSEE phenomena are achieved and surface charges are invariable and reach up to a stable distribution. The simulated stable surface charge distribution is consistent with the theoretical deduction results, which confirms the existence of SSEE process.

A kind of interesting luminescence phenomenon from vacuum-dielectric interface under high electric field is presented in this Letter. It is visible, steady, and continuous, occurring before or accompanied with flashover across dielectric, which is quite different from general electroluminescence from insulator surface reported before. By investigating its optical and electrical characteristics, we consider it as cathode-like luminescence, which is initialized by the field electron emission from cathode triple junction and finally generated by the process of radiative recombination due to the self-stabilizing secondary electron emission and collision on insulator surface.