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The effect of rotating gliding arc plasma on flame stabilization in a laboratory scale swirl-stabilized plasma-assisted combustor was experimentally investigated. The characteristics of a gliding arc plasma were studied using high-speed camera and simultaneous measurements of current and voltage waveforms. Effects of plasma on the extension of lean blow-off (LBO) limit and the swirl flames structures as well as OH radical distribution have been studied systematically. Results show that the discharge and combustion are coupled together effectively due to the dynamic processes of discharge. When the plasma is activated, the flame structures are drastically changed; oscillating flame and lifted flame convert to stable columnar flame tending to be attached to the plasma column. Besides, the plasma columns can promote OH formation and can produce much more energetic radicals due to the reactions between the discharge, methane and oxygen. The gliding arc plasma can stabilize the flame, provide an additional anchoring mechanism, and significantly extend the LBO limit. The plasma column can provide active radicals and continuous ignition to sustain the flame, and the thermal effect and kinetic effect may occupy the dominant role.

Frequency Domain Spectroscopy (FDS) is a powerful tool to study the dielectric properties of oil-paper insulation, which is widely used under high electric field strength. The motion of charge carriers under high electric field strength will cause new dielectric characteristics in FDS curves, from which more information could be extracted for insulation state assessment. In this paper, FD spectra are measured under high electric field strength. The electric field dependence of the loss peak in middle frequency and the characteristics of dielectric loss in low frequency are studied. With the influence of trapping on the motion of charge carriers under electric field being considered, charge carriers hopping polarization model and charge carriers motion dependent conductance model are proposed. The trap energy level of oil-paper insulation decreases when the moisture content increases, thus, the motion characteristics of charge carriers change, leading to decreasing loss peak in middle frequency and inverse dielectric loss characteristics in low frequency. These two models introduce the influence of traps on carrier motion into the dielectric response and enable one to gain a better understanding of the dielectric properties of oil-paper insulation under high electric field strength.

Effect of DC electric field on lean premixed biogas turbulent flame structure at low and medium turbulence intensity was studied experimentally using OH-PLIF. OH signal distribution, turbulent burn...

In recent years, frequency domain spectroscopy (FDS) is often used to evaluate oil paper insulation state in power transformer bushing. But it is still very difficult to evaluate the moisture content accurately and quickly. In order to solve this problem, this paper proposes an intelligent algorithm based on random forest regression (RFR) to construct an efficient evaluation method through segmented FDS curves. Furthermore, the characteristics of FDS curves were studied and the intelligent method was compared with support vector regression (SVR) and deep neural networks (DNN). The results show that the dielectric loss, the real part and imaginary part of complex capacitance all move upward with the moisture increasing, so they can be used as the input feature of the evaluation model; The moisture content evaluation accuracy of the RFR model in the whole frequency band is higher than that of SVR and DNN models; With the increase of lower cut off frequency (FDS test stop frequency), the FDS test time is greatly shortened, and the accuracy of the RFR model can still meet the evaluation requirements. Therefore, the data in a compromise frequency band can be used to evaluate the moisture content of oil paper insulation accurately and quickly.

Insulation plays an important role in power system. During the production and operation of power equipment, there will inevitably be insulation defects occurring. Realizing the detection and localization of these defects is of great significance to ensure the safe and stable operation of power system. In this paper, different defect localization algorithms are proposed for transformer, cable and whole substation equipment, respectively. Experimental results show that these algorithms can reach a high localization accuracy.

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.

Epoxy resin and Al 2 O 3 /epoxy resin composite materials are widely used in electrical equipment insulation. In order to evaluate the effectiveness of frequency domain spectroscopy (FDS) for epoxy resin and alumina/epoxy resin composite insulation damp defects, in this paper, two kinds of insulation samples are prepared to study the influence of voltage and moisture on FDS curves of two insulation materials by using dielectric response test platform. The results show that the applied voltage has little effect on the high frequency range (1Hz-5kHz for pure epoxy resin , 20mHz-5kHz for Al 2 O 3 /epoxy resin) of FDS curves, while the tanδ of low frequency band decreases with the increase of voltage. With the decrease of water content, the low frequency band of FDS curve decreases gradually, while the Al 2 O 3 /epoxy resin curve decreases over the entire frequency band (1mHz-5kHz). Since moisture has the greatest influence on tanδ of the two insulations at 0.01 Hz, this frequency point can be used as the characteristic frequency for diagnosing damping defects of the two insulations.

High-voltage bushing is an important part of the operation of transformer equipment. In order to accurately and effectively evaluate the moisture condition of the bushing, this paper uses the polarization and depolarization current (PDC) non-destructive measurement method to do it. In the current research, the characteristics of loss peaks and curve fluctuations of tanδ-f curve can judge whether the bushing is evenly damp, but it lacks the judgment of the high moisture position of the internal damp. This paper uses oil-paper samples of different moisture content to make a preliminary qualitative exploration of PDC measurement methods. Then the polarity reversal method is used to judge the bushings with different moisture conditions, Combined with the migration characteristics of hydrolyzed ions in the oil-paper insulation, it is analyzed to verify the effectiveness of the method.

Power cables in the urban power supply system are laid in cable trenches or directly buried underground, and the insulation is susceptible to corrosion and penetration, causing local insulation aging or abrasion. Early detection of soft faults helps prevent power system faults from occurring. This paper proposes a method that can detect cable multi-point soft faults and distinguish fault types based on Frequency Domain Reflectometry (FDR). Firstly, we establish a numerical calculation model of distributed parameters to analyze the impedance spectroscopy characteristics of different types of defects and the positioning methods and errors. Next, a simulation study was carried out for different characteristics of aging and damage, which means different types of soft faults were successfully distinguished. Then, based on the results of the simulation, experiments are carried out to verify the effectiveness of the positioning algorithm and the feasibility of distinguishing defect types. Simulation and experiment show that the positioning error is within 0.5% and 1% simultaneously. Compared with traditional location methods, the defect location method proposed in this paper can determine the type and severity of the defect while locating multiple defects. The experimental results show that the defect identification method proposed in this paper has great potential for field application.

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.

Compared with high-voltage transmission cables, 10kV distributed cables are currently receiving less attention. Contrary to the continuously improving national overall power supply reliability, the failure rate of 10kV distribution cables is not optimistic. Thermal aging is an important cause of defects in distributed cables, which further develop into failures. This paper conducts research on defects of different severity, uses FDR to determine the severity of defects, and analyzes the impact of defect development on positioning accuracy. Firstly, we establish a simulation model of the aging cable, and use the windowed Fourier transform to locate the multi-point aging. Then, we carry out relevant experiments based on the simulation results to further verify the accuracy of the algorithm. The simulation and test results show that for cables of the same length, the aging time has no effect on the defect location accuracy. The positioning accuracy of simulation and test algorithms are within 0.46%.

Stereolithography (SL) based 3D printing is promising to produce of next-generation dielectric materials because of its high geometric accuracy, less internal defect and high flexibility. Polymer matrix composites (PMCs) are widely used in the 3D printing of dielectric materials. However, the loading of high-refractive-index ceramic filler in PMCs significantly reduces their process compatibility, making it difficult to successfully fabricate the 3D printing materials. In this study, silane-coupling-agent (SCA) treated barium titanate microspheres (BT-S) were prepared using radio-frequency thermal plasma spheroidization (RF-TPS) and KH570 silane treatment. Experimental results showed that the prepared BT-S particles have spherical shape and impact structure. This enhances the 3D printability of UV curable PMCs by the improvement of UV curing performance and fluidity. Finally, 3D printing of UV/BT-S composites were successfully printed using stereolithography (SLA) 3D printing technique.

Electric assisted combustion for hydrogen enriched hydrocarbons may even extend the lean burn limit and provide the further improvement on combustion stability. This study investigates the effect of hydrogen enrichment and DC electric field on lean CH4/air flame propagation. Electric field inside the chamber was generated by mesh and needle electrodes. Effect of hydrogen enrichment on the ion mole fraction in the flame was discussed based on reaction mechanism included neutral and ion reactions. The flame propagation images, flame displacement speed were used to evaluate the combined influences of hydrogen enrichment and electric field on propagating flame. Results showed that the hydrogen addition would increase positive ions mole fraction and the peak value is mainly determined by H3O+. This would be due to that CH increases with hydrogen fraction, which is the main species in the initial reaction for the ion reactions. Electric field effect about flame propagation was suppressed with hydrogen addition due to the competition between the increment in ion mole fraction and the decrement in flame time. Electric assisted combustion is more evident at leaner conditions and elevated pressure. The ratio of ionic wind velocity to flow velocity may be the determined factor to predict the electric field effect about propagating flame. The tendency based on this ratio is in accordance with the experimental results for various hydrogen fraction and equivalence ratio at elevated pressure.

Flashover across a solid surface in vacuum greatly restricts the insulation capability of pulsed power devices and impedes its overall performance. For more than half a century of painstaking research, there are still some ill-defined aspects that need to be studied. This paper, organized in chronological order, briefly reviews the milestones of research progress regarding surface flashover in vacuum. The main influential factors on flashover are analyzed, which are generalized into three aspects, i.e., electrode-dielectric interface, dielectric surface and external factors. The mainstream theories on flashover are discussed. In recent decades, the approach to improve surface insulation performance are summarized, including surface modification techniques, new insulating materials and new insulation structures. It is hoped that this paper will provide a guide for future related scientific research, engineering design and application.

The localisation of partial discharge (PD) sources using the acoustic emission (AE) technique has attracted increasing research attention. The complicated propagation routes and wave-type conversion in power transformer can induce considerable localisation error. In this paper, the catadioptric phenomenon of AE wave propagation is explained in detail. With the incident angle varying, the wave type of the direct wave in tank wall could convert and the velocity will change accordingly. A transformer model is established in which each node refers to a suspected PD position and a shortest route search algorithm is proposed to calculate the shortest path between two nodes in this model. However, the fastest route is most significant to PD localisation rather than the shortest route when TDOA method is used. As a result, an improved propagation route search (IPRS) algorithm, which can recreate the propagation process and calculate the fastest AE routes, is proposed to localise the PD origin. To verify the feasibility of the IPRS algorithm, localisation experiments were performed in 35 and 110 kV transformers, respectively. Compared with other present localisation methods, such as the Chan algorithm, the genetic algorithm and the imperial competitive algorithm, the proposed algorithm can effectively reduce localisation error.

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.

Cables play an extremely important role in the transmission of electrical energy and communication signals in modern large-scale electrical systems. Early cable insulation defects often cause cable insulation breakdown, leading to permanent insulation damage. However, it is often very difficult to accurately detect these potential local defects. This paper proposes a novel non-destructive detection method for cable defect based on broadband impedance spectroscopy. Firstly, a simulation model of cable impedance spectrum is established. The correlation between impedance parameters and cable defects is well analyzed, and the influence of different defect locations on the cable impedance spectrum is studied. The effect of frequency spectrum on the accuracy of fault location is also analyzed. Finally, several cables of different lengths are simulated and the results show that this method has great spatial resolution within 0.3% for cables from several meters to hundreds of meters. It indicates that this non-destructive default location method has great potential in field application of cable diagnosis.

Oil-paper insulation high-voltage bushings are one of the most important power equipment in substations, and their insulation status is critical to the safety and the stabilization of the power system operation. During the production and operation, the bushing may be dampened because of the insufficient drying and impregnation or the intrusion of external moisture, which leads to the deterioration of oil paper insulation and threatens the reliability of bushing. Due to the special insulation structure of the bushing capacitor core, the distribution of moisture inside the bushing is usually non-uniform, especially when the moisture is intruded from the external environment. In this paper, a numerical simulation model of dual-unit laminated oil-paper and 110kV capacitor bushing is established. Then the frequency domain dielectric spectrum (FDS) characteristics of oil-paper insulation with different non-uniform moisture distribution are obtained by simulation. Different types of non-uniform moisture distribution include: non-uniform moisture distribution in different directions of the bushing, and different degrees of non-uniform moisture distribution. When the non-uniform moisture distribution of bushing is along the axial direction, different degrees of non-uniformity of moisture distribution have little effect on the FDS results; when the non-uniform moisture distribution of bushing is along the radial direction, the $\text{tan}\delta-f$ curve shows a clear dielectric loss peak, the characteristics of which are closely related to the degree of non-uniformity of the moisture distribution. Partial damping at the outermost side of the bushing or the end screen are more common in practice, and the FDS results of these two different types of damped bushing show different characteristic. This paper carried out a comprehensive FDS simulation study of different types of non-uniform dampened oil-paper insulation, which is of great significance for the evaluation of the insulation status of the oil-paper bushing.

At present, protection devices such as low-voltage circuit breakers and fuses are commonly used in low-voltage distribution networks, which can effectively prevent short circuits, overloads, and ground leakage. However, this method is out of work in detecting series arc faults caused by poor contact, insulation failure, etc. Therefore, how to achieve accurate detection of series arc faults has become a hot issue in current research. Wavelet transform is usually used for series arc fault detection. But it exists the problem of spectral aliasing, the false detection rate is still high. This paper uses detection method based on the current waveform to carry out research. By building an arc fault platform to simulate series arc faults, normal and arc fault data under different loads have been obtained. The structure of deep learning algorithm can be established through these experimental data. The accuracy of the algorithm is improved by using mini-batch gradient descent, exponential decay learning rate and Adam's optimization algorithm. By establishing test data for diagnostic verification, it was found that the algorithm has an excellent recognition rate.

Bushing is the most important part of transformer to connect external component. Bushing fault is one of the prime reasons causing transformer faults. Moisture is an important factor leading to aging of bushing insulation, so it is necessary to evaluate the moisture content of bushing. In the current research, it can confirm whether the oil-paper insulation bushing is uniform damp. But there is no research related to the moisture location of oil-paper insulation bushing. We use Comsol software to carry out a simulation study of the finite element analysis method for nonuniform damp bushing. And the distribution characteristics of hydronium ions and hydroxide ions under electrode polarization can be obtained from the simulation analysis. In this paper, the polarization and depolarization current (PDC) experimental platform was set up and models of nonuniform damp bushings were made. The position information of the high-moisture capacitive screen can be obtained through two different wiring methods. The validity of the method can be verified by the distribution law of hydrolysis ions in this simulation.

In order to realize the diagnosis of water distribution, this paper analyzes the interface polarization and macroscopic space charge polarization mechanism when the water distribution is non-uniform. The experimental results of this paper and bushing show that when the moisture distribution is non-uniform, there is a significant loss peak in the tan&delta, f curve. The loss peak shifts to higher frequencies as the non-uniformity coefficient increases. There are common intersection points between multiple tan&delta, f curves. Further, this paper realizes the diagnosis of the location of moisture distribution through Frequency Domain Spectroscopy (FDS) testing of different voltages and different wiring methods based on the macroscopic space charge polarization. In the single-cycle FDS test, when the positive electrode is first added to the area with higher moisture content, the amplitude of the tan&delta, f curve is smaller. The tan&delta, f curves under different wiring methods constitute a &ldquo, ring-shaped&rdquo, loss peak. As the voltage increases, the peak value of the loss peak shifts to the lower frequency band. As the temperature increases, the peak value of the loss peak shifts to higher frequencies. Based on the above rules and mechanism analysis, this research provides a new solution for the evaluation of moisture content of oil-immersed polymers equipment.

Capacitive high-voltage bushing is one of the typical oil-immersed paper insulation equipment, which is an indispensable external connection component for power transformers. In order to evaluate the effectiveness of frequency domain spectroscopy (FDS) for diagnosing moisture content of oil-immersed paper insulation bushing, this article studied the frequency-domain dielectric characteristics of oil-immersed paper insulation samples and field bushing. At the same time, the basic principle of interfacial polarisation is used to analyse the samples and field bushing with non-uniform moisture content. The results show that the moisture content affects the whole frequency band of the FDS curve. When the distribution of moisture content is non-uniform, the paper with high moisture content has a great influence on the whole FDS curve of oil-immersed paper insulation. In addition, the FDS curves will show a significant loss peak. The frequency of the peak point gradually moves towards the high frequency as the non-uniformity of the moisture content distribution increases. The experimental results show that the variation law of oil-immersed paper insulation can be used to quantitatively evaluate the moisture content and moisture type of oil-paper insulation power equipment.

The effects of DC electric field on laminar premixed CH4/air spherical propagation flames with excess air ratio λ = 1.0, 1.2, 1.4 were investigated in a constant volume combustion chamber at elevat...

In this paper, the effects of aluminum nitride and alumina ceramics on the discharge mode of He/O2 mixed gas are studied and compared. It is found that when the O2 volume fraction is in the range of 0.2%–1%, dielectric barrier discharge (DBD) with Al2O3 is always in the unstable filamentary mode, and homogeneous and filamentary modes are observed in DBD with AlN. Under the homogeneous discharge mode, there is only one current pulse each half-voltage cycle, and the intensified charge-coupled device discharge image shows that it is glow discharge. The relative permittivity and surface charge decay characteristics of AlN and Al2O3 are measured. The experimental results show that the relative permittivity of AlN is lower than that of Al2O3, but its surface charge decay is obviously slower than that of Al2O3. The derived electron trap density from the surface charge decay of AlN ceramic is higher than that of Al2O3, which shows AlN has a strong charge capacity and can emit more electrons in the second half cycle of the discharge. The higher secondary electron emission coefficient can make the discharge ignition voltage lower and the formation of stable homogeneous discharge easier. This paper provides a new control method for expanding homogeneous discharge in He/O2 mixed gas.

Surface charge accumulated on insulators plays an important role in flashover process in vacuum, which has not yet been fully understood. In this paper, charging behaviors on insulator surface were experimentally examined under negative DC and impulse voltage. For the case of DC voltage, two charging stages were observed, i.e., the area near cathode was at first negatively charged under lower voltage (stage I) and then followed by a positively charged cathode-anode surface (stage II). Surface charging at stage I is considered to be caused by Schottky injection of homo-charges from electrode into sample surface layer. While at stage II secondary electron emission (SEE) process is responsible to the positively charged anode-cathode surface. However, charging at stage I is quite sensitive to pre-stress process, as well as the electrode-insulator interface conditions. The ongoing injection of electrons from cathode can lead to gradually decreasing in surface charge accumulation as the voltage duration increases. For the case of impulse voltage, Schottky injection of homo-charges has little influence on surface charging as the time is too short for charge injection and migration in surface layer. The SEE process will govern surface charging due to its extremely fast establishment process. A good linear relationship is obtained between saturated surface charge density and applied voltage, which is generally consistent with the numerical calculation result based on secondary electron emission avalanche theory. At last, it is confirmed that the ionization of desorbed gas from insulator surface and subsequent neutralization of electrons with positive surface charge is the main cause to eliminate surface charge in flashover.

The temporal behaviors of charging on insulator surface at flashover initial stage is studied under dc voltage in vacuum. It suggests that the surface charging process develops synchronously with multipactor phenomena. A saturated region is firstly formed nearby the cathode triple junction (CTJ), and the insulator surface is charged gradually from cathode toward anode as the multipactor develops forward. The surface charge density increases linearly with applied electric field, but a stronger applied electric field will slow down the charging velocity as the results of violent collisions of secondaries with dielectric surface due to the stronger normal field built by surface charges. The charging velocity mainly depends on the secondary electron yield (SEY) for dielectric, i.e., the higher SEY, the greater the charging velocity is. The emitter height from CTJ has great influence on surface charging at areas adjacent to cathode, but little effect on charging behaviors at other areas. Once the multipactor is triggered by adequate initial electrons released from cathode, the surface will be charged spontaneously even if the field emission process at cathode is terminated, implying the generation of secondary electrons turns into a self-sustaining process. This study strongly supports the speculation in our previous publications regarding the mechanisms of stable luminescence preceding flashover, which is attributed to be initialized by Fowler-Nordheim emission of electrons at CTJ and finally generated by the radiative de-excitation of excited atoms/molecules in the self-stabilizing secondary electron emission (SSEE) process.

Partial discharge (PD) is the symptom of insulation breakdown in power equipment. The high rising steepness of PD current radiates electromagnetic waves which can be coupled with antenna. In this study, an antenna array is applied to PD detection and localisation in air-insulated substations, in which the locations of PD sources are found with time difference of arrival localisation algorithm. The influences of antenna deployment including the size, aspect ratio and shape of antenna array on localisation accuracy are systematically investigated. Three approaches including Cramer–Rao lower bound, numerical simulation of root of mean square error and experiments are adopted to evaluate the localisation accuracy. Three kinds of antenna arrays with diamond, rectangle and Y shapes are designed for locating the coordinates of PD sources. Their localisation performances are compared by these evaluation approaches, and the optimum antenna configurations for coordinate estimation are derived. The results indicate that the optimum array is dependent on aspect ratio of array size. If the aspect ratio is higher than 0.5, the rectangle array gives minimum localisation error and wide bearing ranges with low error. For array with aspect ratio smaller than 0.25, the diamond array is optimum among the three shapes.

Time-resolved evolution of secondary electron emission and trapped charge accumulation in polyimide film is investigated during two interval electrons bombardment, derived from the measurement of displacement current and secondary current via a hemispherical detector with the shielded grid. Under various irradiation current, secondary electron yield (SEY σ ) at a certain injected energy decreases exponentially from initial amplitude σ 0 to self-consistent steady value σ ∞ close to 0.93. The time constant τ of charging process is characterized as a function of incident current I p , and the results indicate that the formula I p × τ is fitted by a hyperbolical law. The influence of I p on the amount of trapped charge is studied and no significant change in its saturation value is observed. The evolution of SEY σ and trapped charge is dependent on incident dose Q p but not the incident rate I p . Furthermore, the trap density and capture cross section are discussed.

The oil-paper insulation in a transformer gradually ages due to the combined effects of thermal and electrical stress that occur during its operation. In this paper, for investigating the changes that occur within the oil-paper during the aging process with different aging stresses, partial discharge (PD) characteristics and trap parameters are measured. An internal discharge cavity model is designed for the electrical stress, and the thermal stress is generated by a baking oven simultaneously. The experiments are designed such that the dominant factor on the aging process is either thermal, electrical, or a unity of both stress factors. The phase resolved partial discharge (PRPD) spectrograms are recorded at different aging time, and the trap parameters are extracted by thermally stimulated depolarization current (TSDC) method. Additionally, the surface morphologies of these samples are observed by using a scanning electron microscope (SEM). The results show that the trap energy presents a decreasing tendency when the electrical stress plays the dominant role, and the discharge energy becomes larger over the aging time until pre-breakdown. When the thermal stress dominates the aging process, the trap energy shows an increasing tendency over the aging time, and there exists a process of ‘material conditioning’ that occurs at the initial stage of the aging process. The change of trap parameters in aged oil-paper will further affect PD characteristics. Correlation analysis of PD and trap characteristics can promote more accurate condition estimation of aged oil-paper.

The discharge mode transitions and the characteristics of atmospheric pressure dielectric barrier discharge in pure helium (He-DBD) are studied. Different discharge modes of He-DBD are clarified, including filament-diffuse mixed mode, filamentary mode, diffuse mode, and multichannels coupled mode. The transitions of different modes are explored by changing the gap width and applied voltage. According to the luminous structure of discharge [short exposure (100~300 ns) image], it is found that there should be only Townsend-like and glow-like discharges that appear in the above modes. In Townsend-like discharge, the secondary electron emission coefficient (SEEC) of dielectric is estimated. Based on which, a fluid model coupled with an external circuit is built to explain the He-DBD mode transitions mechanism and the applied voltage drop’s influence on He-DBD. Through the temporal evolution of a luminous structure, a discharge filament is believed to be a contractive state of discharge channel, which should be an unstable glow-like discharge. The radius of discharge channel, changing with gap width and applied voltage, is measured. The discharge contraction is attributed to the nonlinear relationship between Townsend ionization coefficient and electric field. The enhancing interaction between different discharge channels inhibits the appearing of diffuse discharge around discharge filament, leading to the transition from mixed mode to filamentary mode. In the end, the multichannels coupled mode is proven to be glow-like discharge, and its formation mechanism is explained by the overlap of discharge channels.

Due to its excellent performance, oil-paper insulation has been widely used in many power apparatuses including converter transformer of high voltage direct current (HVDC) system. Space charge accumulation phenomena may significantly distort local electric field inside transformer and cause oil-paper insulation overstressing. As a result of long term operation, oil-paper gradually degrades. In most severe cases, the breakdown of insulation system may occur. In this work, the effect of space charge on breakdown characteristics of aged oil-paper insulation is investigated. Oil-paper samples with different aging states are obtained by thermally accelerated aging for various periods of time, categorized by the degree of polymerization (DP) of 1107 (new), 725, 504, 376 and 241, respectively. Experimental results show that the breakdown voltage of oil-paper samples without pre-stressing remains almost constant with thermal degradation. After the homo-polarity pre-stress, when the DP value is higher than ∼300 (from 1107 to 376), the breakdown voltage has an increasing tendency with an increase in thermal degradation. On the contrary, the breakdown voltage presents a decreasing tendency after the hetero-polarity pre-stress. When the DP is lower than ∼300 (DP241), the changes of breakdown voltage does not remain the same trend as previous samples after pre-stressing. Meanwhile, space charge distributions in different aged samples are measured by the pulsed electro-acoustic (PEA) method, and the trap parameters are correspondingly evaluated. It has been experimentally demonstrated that special attention should be paid to space charge accumulation and aging state of oil-paper insulation, which may lead to severe insulation accidents.

this paper studies the dielectric property of oilpaper in high electric field, especially the effect of moisture on the dielectric loss under variable voltage at power frequency in oil-immersed paper with different moisture content. Oil-immersed paper insulation model samples with different moisture content are prepared in laboratory. Afterwards, increasing voltage and decreasing voltage tanδ-U curves of samples with different moisture content were tested under different experiment condition and a non-linear phenomenon was observed. Test results show that the tanδ-U curve is influenced by the moisture content of oil-immersed paper and test temperature. When the moisture content in the oil-immersed paper is high, the tanδ-U value is gradually increasing with the increase of the voltage. And in the process of decreasing the voltage, the tanδ-U value gradually decreases but is greater than the value at the same voltage in the process of increasing the voltage. The decreasing voltage tan tanδ-U curve is above the increasing voltage tan tanδ-U curve while these two curves overlap at high voltage and have great separation at low voltage. Moisture content changes the overall shape of the curve and the area between the two curves by influencing the transfer of charge carriers in oil-immersed paper. As water is polar, energy level and density of the trap in the oil-immersed paper with different moisture content are different. Water provides more paths for free moving charge carriers so that the accumulation and dissipation velocity of the charge carriers in Oil-immersed paper vary. The temperature doesn’t change the overall shape of the tanδ-U curve. Based on the dielectric response, the tanδ-U curve of samples with different moisture content could be expounded and the moisture Content in oil-paper insulation equipment could be evaluated by the tanδ-U curve.

Ultra-high-frequency (UHF) method is regarded as an effective approach for partial discharge (PD) detection in gas insulated substation (GIS). The feature parameters representing UHF signal characteristics can be extracted from its waveform, and applied to PD source classification and mixed signals separation. In this study, an UHF signal feature extraction algorithm based on cumulative energy (CE) technique is proposed. The CE functions in time domain (TCE) and frequency domain (FCE) are calculated from UHF waveforms and their fast Fourier transform spectrums, respectively. The mathematical morphology gradient (MMG) operation is applied to TCE and FCE to characterise their rising behavior. The feature parameters including width, area and sharpness are extracted from CEs and MMGs in both time and frequency domain. Moreover, the extracted features are applied to UHF signals classification and mixed signals separation. First, experiments on four kinds of typical defects in a 252 kV GIS are performed, and the classification performance of the extracted features is examined with the acquired signals. Second, by clustering the extracted features with fuzzy maximum-likelihood algorithm, mixed UHF signals originated from multiple PD sources are successfully separated. The results indicate that the proposed features are effective for representing UHF signal characteristics.

Partial discharge (PD) measurement and interpretation have become a powerful tool for condition monitoring and failure risk assessment of high voltage power equipment insulation. The occurrence of multiple discharge sources affects interpretation accuracy. This paper presents a PD signal separation algorithm using cumulative energy (CE) function parameters clustering technique. The waveform of PD signals are acquired by digital detection instruments with high sampling rate. Cumulative energy functions in time domain (TCE) and frequency domain (FCE) are calculated from PD waveforms and their FFT spectrums, respectively. Mathematical morphology gradient (MMG) operation is applied to the TCE and FCE to describe their variation characteristics. The feature parameters including width, sharpness and gravity are extracted from CEs and MMGs in both time and frequency domain, and compose a six-dimension feature space. The improved density-based spatial clustering of applications with noise (IDBSCAN) clustering algorithm is adopted to discover clusters in the feature space. The proposed separation algorithm is examined with mixed current impulse signals acquired from PD experiments on artificial multi-defect models and an on-site transformer. The separation results indicate that the proposed algorithm is effective for separating mixed PD signals initiated from multiple sources.

The degradation of oil-impregnated paper induced by partial discharge (PD) seriously affects the life of power equipment. In order to investigate the relationship between partial discharge and the degradation of oil-impregnated paper, PD characteristics and trap parameters of the impregnated paper are measured. In this work, a cavity discharge model is adopted and samples with different degrees of polymerization (DP) are fabricated using thermally accelerated aging to simulate transformers with different lifetimes. For each sample, phase resolved partial discharge (PRPD) spectrograms are recorded for various partial discharge times. The characteristics of partial discharge are analyzed, and the trap parameters of specimens are obtained by the thermally stimulated depolarization current (TSDC) method. Following this, the microstructures of the samples are observed using a scanning electron microscope (SEM). It is found that the damaging breakdown process of oil-immersed paper can be divided into three stages: initial stage, developing stage and pre-breakdown stage. There also exist two levels of trap on the surface of oil-impregnated paper: shallow (0.10-0.59 eV) and relatively deep (0.60-0.90 eV) energy. Importantly, the thermal aging makes the trap energy deeper while electrical aging makes the trap energy shallower.

Oil-polypropylene (PP) composite insulation has been widely used in power capacitors, and also being extended to other equipment such as specific transformers, etc. As discharge can readily creep along PP surface, the interface between oil and PP is a weak point in the insulation structure. The oil-PP insulation degrades under the impact of thermal and electrical stress during normal operation. In this study, the influence of thermal degradation on surface discharge characteristics for oil-PP insulation is investigated. Degraded samples are obtained by thermally accelerated aging of virgin oil-PP samples. The microscopic morphology of PP samples are observed with SEM, and the physical and electrical properties of degraded oil are measured. In addition, the isothermal surface potential decay (ISPD) measurement is employed to investigate the charge keeping capability of PP samples. After that, surface discharge detection experiments under AC voltage are performed for the virgin and degraded samples. The ISPD results indicate that charge decay rate of degraded PP sample is faster than virgin sample, which imply that space charge memory effect of virgin PP samples is more remarkable. For this reason, the PD inception phase of virgin sample is smaller than those of degraded samples. It is found that thermal degradation of oil-PP insulation enhances the surface discharge activity. It is considered that rough surface profile and higher bubble conservation capability of degraded PP sample, and impurities in degraded oil are responsible for the enhancement effect of thermal degradation on surface discharge.