Pressboard Surface Research Articles

Partial Discharge Suppression of the Oil-impregnated Paper by Depositing PTFE Thin Film via Magnetron Sputtering

The rapid development of HVDC (High Voltage Direct Current) transmission technology brings new challenges to insulation materials. Oil-paper insulation needs to be strengthened to prevent possible breakdown failure. In this paper, the PTFE functional film was fabricated by magnetron sputtering on the insulation pressboard surface to suppress partial discharge. The partial discharge test was conducted to compare the withstanding ability of the modified pressboard and the untreated pressboard. The results showed that partial discharge inception voltage increased by 18% for the modified pressboard. Phase-resolved partial discharge results also showed a great enhancement in partial discharge withstanding the ability of the PTFE function layer on the pressboard. The molecular simulation was conducted to give a possible explanation for the improvement. Higher bond levels and a large band gap of PTFE played a vital role.

Effect of Electric-Field Components on the Flashover Characteristics of Oil-Paper Insulation Under Combined AC-DC Voltage

Flashover characteristics are closely related to the electrical field distribution on the surface of oil-paper insulation. In this study, the effects of electrical field and DC voltage components on the creepage flashover characteristics of oil-paper insulation under combined AC-DC voltage (CADV) are investigated using two electrode models. The results demonstrate that when the DC voltage component is large, the flashover voltage under the influence of strong parallel electric field component is approximately 1.2 times higher than that under the influence of strong normal electric field component. When the DC voltage component value increases under the influence of strong normal electric field component, the value of corresponding AC voltage component of the creepage flashover voltage exhibits a decreasing trend. It is believed that due to the strong normal electric field component, the electrons intensify the collision on the surface of the insulating pressboard. Under the influence of the DC voltage component and due to the presence of space charge on the insulating pressboard, the distribution shape of carbonized particles on the surface of the insulating pressboard after creepage flashover is different. Furthermore, the instantaneous energy release during creepage flashover under CADV is higher than that under AC voltage, and the greater the DC voltage component value, more severe is the damage to the fibers on the insulating pressboard surface.

Improvement of breakdown and flashover properties against thermal aging of oil paper insulation through addition of TiO 2 nanoparticles

TiO2 nanoparticles can improve the electrical performance of oil paper insulation. In this study, 10 nm TiO2 nanoparticles were prepared and dispersed into transformer oil, and then immerse pressboards in it to obtain nanofluid impregnated pressboards (NP). Its thermal aging was accelerated together with the reference sample at 130°C for 36 days. Analyzing the acid value in the oil, polymerization degree, surface topography, AC breakdown strength, surface flashover characteristics and surface potential decay characteristics. It was observed that TiO2 nanoparticles can reduce the acid value and improve AC breakdown strength of oil. However, the degree of polymerization and surface appearance of NP and oil impregnated pressboards (OP) during thermal aging did not differ significantly. During the aging process, the energy level of pressboard surface traps shifted first towards the shallow and then gradually towards the deep. During all the aging stages, the surface flashover voltage of NP was always higher or close to that of OP. The mechanism of TiO2 nanoparticles improving the breakdown and surface flashover characteristics of oil–paper insulation during thermal aging is discussed from the perspective of water molecules and aging by-products adsorption and introduction of shallow traps by nanoparticles.

An Investigation of the Electrical Properties of Pressboard Impregnated with Mineral Oil-Based Nanofluids at Different Concentrations of Fe3O4 Magnetic Nanoparticles

The purpose of this study was to investigate the electrical properties of pressboard impregnated with mineral oil-based nanomaterial. The nanomaterial focused on in this study was magnetite (Fe3O4) at ratios of 0.01%, 0.03%, and 0.05% by mineral oil volume. The electrical tests were performed on the AC breakdown strengths and positive-negative lightning impulse breakdown strengths on impregnated pressboard were carried out in compliance with IEC 60641 and IEC 60243, respectively. Scanning electron microscopy (SEM) and X-ray Diffraction (X-RD) were used to study the micro surface and show that Fe3O4 nanoparticles of nanoscale size were adhered to the pressboard surface and exist in an amorphous state. The results found that the AC breakdown strengths of pressboard impregnated with mineral oil-based Fe3O4 nanofluids at 0.03 wt% were increased the most. Moreover, the lightning impulse breakdown strengths of pressboard impregnated with mineral oil-based Fe3O4 nanofluids at 0.03 wt% were increased the most in both positive and negative polarities. The results, thus, showed promising directions for applications of Fe3O4 nanomaterials to improve the electrical properties of pressboard.

Charge Injection Suppression and Breakdown Strength Enhancement of Cellulose Pressboard via Al2O3/PTFE Functional Film Depositing

Under dc voltage, space charge effect becomes a key issue to restrict the insulation performance of the polymer insulation. In this research, novel Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> /poly-tetra-fluoroethylene (PTFE) composite functional film is fabricated by magnetron sputtering on the insulation pressboard surface to suppress space charge accumulation and to improve breakdown strength. The depositing sequence for composite film is determined by molecular simulation. The composition and the morphology of the prepared film are characterized and the thermal stability is evaluated. Pulsed electro-acoustic (PEA) results show that the space charge amount is reduced over 40% for the sputtered pressboard, indicating that the functional film owns an obvious positive effect on charge injection suppression. Less injected charges weaken the electric field distortion and lead to higher breakdown strength. This inhibition effect is mainly attributed to the deep trap and charge capture ability introduced by the functional film. This work provides a novel way to tailor the properties of the dielectric materials.

Experiment and Simulation of Pressboard Barrier Effect on the Streamer Propagation in Natural Ester During Positive Pulsed Voltage

In this study, the effect of the pressboard barrier (PB) on streamer pattern in natural ester under the non-uniform electric field is investigated by experiment and simulation. The propagation patterns and characteristics of the streamer in the oil with/without a pressboard barrier are obtained by the shadowgraph technique. The mechanism of the pressboard barrier effect on the streamer pattern and propagation is revealed using a multi-streamer branch simulation method. Results show that the positional relationship between the needle electrode and the pressboard barrier significantly affects the streamer propagation and branching. The barrier effect of the pressboard, the distribution of space electric field affected by the pressboard barrier, and the image force of charge on the pressboard surface are all critical factors for affecting the propagation and branching of the streamer. The greatest threat to the pressboard failure is the z-axis streamer or those branches close to the z-axis that is severely inhibited by other streamer branches. It raises the risk of insulation failure caused by the breakdown of the insulating pressboard.

Visual inspection for transformer insulation defects by a patrol robot fish based on deep learning

In order to effectively detect the internal insulation defects of large transformers, a miniature patrol robot fish is designed and used to observe the pressboard inside the transformer, which can visually inspect the insulation condition of the pressboard. In the process of visual inspection by a patrol robot fish, insulation defects observed in the transformer (such as discharge carbon marks, pressboard cracks, etc.) are small size, indistinctive colour contrast and different shapes. The key purpose of the patrol robot fish is to identify the defect targets and defect types intelligently and quickly from the images taken by the camera on the fish. Considering that there are abundant insulation defects in transformers and a lack of samples for learning, a vision detection method based on deep learning network is proposed in this study. The proposed method integrates the variable autoencoder into the traditional Faster-RCNN target detection network and constructs an improved Faster-RCNN that enhances feature extraction. This method expands the small-scale training sample set and improves the generalisation performance of the model. In order to verify the effectiveness of the proposed method, the improved network is trained and tested, and the test results show that the improved network training model can accurately identify and mark the carbon marks on the pressboard surface.

Characteristics of Creepage Discharge in Oil/Pressboard Composite Insulation

This paper deals with partial discharge characteristics in oil/pressboard composite insulation system, which is used in oil-immersed power transformer. It is found that the discharge propagates in the same route on the pressboard surface as in the previous discharge every time under repeated impulse voltage application. We also revealed that the white trace remained along the path of discharge and affected the subsequent discharge progress because of being a weak point part on insulation. This study presents an investigation of the effect of the white trace on partial discharge inception voltage (PDIV) of the composite insulation system with high-speed optical observation and electrical measurements for the discharge propagation. The discharge propagation mechanism is also discussed with comparing the discharge progress characteristics before and after the white trace formation.

Study of Oil/Pressboard Creeping Discharges under Divergent AC Voltage—Part 1: Fundamental Phenomena and Influencing Factors

This paper presents an experimental study of the fundamental phenomena of progressive creeping discharge as well as the influences of pressboard ageing and voltage amplitude under divergent AC voltages. A ramp-stress test is used to determine the partial discharge inception voltage (PDIV) and the flashover voltage of oil/pressboard interface versus pressboard ageing. Constant-stress tests are carried out to investigate the partial discharge (PD) behaviors, the pressboard surface temperature, and the oil-dissolved gases of progressive creeping discharges. It is found that the pressboard ageing affects only the interface PDIV. The progressive creeping discharges concentrate at the triple-junction and comprises probably oil corona discharges and surface discharges in pressboard dimples. If no pressboard damage is present, the progressive discharges exhibit in general a constant PD pattern but a diminishing intensity, and nevertheless can be sustained for the severely-aged pressboards under strong voltages. The underlying causes are correlated with the competing effects of pressboard ageing on interface moisture and the triple-junction field intensification. Gas analyses indicate the hydrogen is the principal faulty gas and the Duval's Triangle diagnoses the progressive creeping discharges as either low-energy or high-energy discharge fault. No pressboard overheating is observed during the discharges.

Failure modes of insulating pressboard subjected to high electric fields and durations: Evidence of partial discharge and changes at the microscopic scale

The deterioration of insulation pressboard under needle-plate discharge was tested, and the degradation stage was divided according to observed experimental phenomena. Based on the improved Top-hat watershed image segmentation method, the white marks area of the pressboard were tested. Fibres with different discharge states were extracted from SEM images. The fibre width was calculated, and the porosity of the surface of the insulation pressboard at different degradation stages was calculated. Simultaneously, the 3D reconstruction technique was used to observe the 3D morphology of fibres at different discharge stages. The study found that with the deepening of the discharge process, the discharge of the pressboard increased, and the white marks of the pressboard continued to expand from point to surface. In addition, the diameter of the fibre of the insulating pressboard decreased obviously with the increase of the pressing time, and the fibre diameter was 89.6% after the breakdown. Moreover, the electrical stress had a great effect on the expansion of the interlayer pores of the pressboard; the cross-section porosity of the insulating pressboard gradually increased with the deepening of the discharge process, and after breakdown, the interlayer porosity reached 12.5%.

Effect of Moistures on Space Charge and Trap Level Characteristics of Oil-Impregnated Pressboards

Oil-paper insulation as an irreplaceable insulation system in the electrical power transformer is inevitably suffering the water intrusion on working condition, which will make dielectric performance degraded and insulation aging accelerated. In high interest of explaining the deterioration effect of absorbing moisture on the dielectric properties of oil-paper insulation system and corresponding mechanism, the space charge and trap level characteristics are investigated for the specially prepared oil-impregnated pressboards with different moisture contents. The Pulsed Electro Acoustic and Isothermal Surface Potential Decay experiments are performed to test the space charge distributions and evaluate the trap level depths, respectively. It is indicated that the polarity of injected space charges is alternated with the decaying rate being raised when the absorbing moisture is aggravated in oil-impregnated pressboards. The charge carriers are more prone to be captured by the shallow-level traps that have been introduced by water permeation near pressboard surface than the intrinsic deep-level traps inside pressboard. The captured charge carries accounts for the significantly exacerbated space charge accumulation as manifested by the surface charges of oil-impregnated pressboards.

AC Flashover Characteristic at the Triple Junction of the Oil-Pressboard Insulation

Oil-pressboard insulation in transformers is prone to surface tracking or creeping discharge during operation. The oil-pressboard interface is believed to be one of the weakest points in the insulation system. In this paper, AC flashover characteristic at the triple junction of the oil-pressboard interface under a slightly inhomogeneous electric field has been studied. The results showed that the creeping discharge always started at the triple junction, then developed along the pressboard (PB) surface, and finally reached the opposite electrode. The reason was that the triple junction was subjected to the maximum electric field strength. The field strength corresponding to the breakdown of the pure oil gap (POG) was decreased with the increment in the gap distance between electrodes due to the volume effect. As a comparison, the field strength corresponding to the breakdown for PB only changed slightly with the gap distance. Besides, the breakdown electric field of both POG and PB hardly changed when the electrode diameter increased. The above results could be applied to the inverse calculation and estimation of the flashover voltage levels under longer gap distances with any electrode diameter. Moreover, the relative position of the oil gap and PB could influence the flashover voltage levels. The breakdown voltage with PB directly contacting the ground electrode was 10% higher than that with PB directly contacting the high voltage electrode. This could be explained by the different charge behavior. The obtained results could provide a reference for the insulation margin design in ultra-high-voltage transformers.

Comparison of creeping discharges propagating over pressboard immersed in olive oil, mineral oil and other natural and synthetic ester liquids under DC voltage

This paper presents an experimental study on the characteristics of creeping discharges propagating over pressboard immersed in olive oil, rapeseed oil, tetra-ester, methyl oleate, and mineral oil, under both negative and positive DC voltages. The investigated characteristics are mainly the shape of discharges and their stopping lengths. Two experimental test cells are used: in the first one, the pressboard is inserted between pointe and plane electrodes so that the tip is perpendicular and at the center of the pressboard sample; and in the second one, the pressboard is placed between a point and bar electrodes in a way that allow a propagation in one direction (tangentially to pressboard). It is observed that the creeping discharges under DC voltage are not radial. Their shapes are different from those observed in previous work under AC and lightning impulse voltages. On the other hand, for given voltage and pressboard thickness, the stopping length L <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</sub> is shorter when the point is negative than when it is positive indicating that the flashover voltage will be greater with a negative point This also evidences a difference in the processes involved in each polarity especially the space charge resulting of the injected charges and the charge accumulated at the interface (including the charges resulting of double layer) as well as the evolution of the electric field in the vicinity of the point electrode and along the pressboard surface. For a given pressboard thickness, L <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</sub> increases with the voltage; it decreases when the thickness of pressboard is reduced.

Improvement of the Space Charge Suppression and Hydrophobicity Property of Cellulose Insulation Pressboard by Surface Sputtering a ZnO/PTFE Functional Film.

Oil-impregnated cellulose insulation polymer (oil-paper/pressboard insulation) has been widely used in power transformers. Establishing effective ways of improving the physical and chemical properties of the cellulose insulation polymer is currently a popular research topic. In order to improve the charge injection inhibition and hydrophobic properties of the cellulose insulation polymer used in power transformers, nano-structure zinc oxide (ZnO) and polytetrafluoroethylene (PTFE) films were fabricated on a cellulose insulation pressboard surface via reactive radio frequency (RF) magnetron sputtering. Before the fabrication of their composite film, Accelrys Materials Studio (MS) software was applied to simulate the interaction between the nanoparticles and cellulose molecules to determine the depositing sequence. Simulation results show that the ZnO nanoparticle has a better adhesion strength with cellulose molecules than the PTFE nanoparticle, so ZnO film should be sputtered at first to fabricate the ZnO/PTFE composite film for better film quality. The sputtered, thin films were characterized by X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and X-ray diffraction (XRD). The space charge injection behavior and the hydrophobicity performance of the untreated pressboard; and the cellulose insulation pressboard with sputtered nano-structure ZnO, PTFE, and the ZnO/PTFE functional films were compared with each other. X-ray photoelectron spectroscopy results showed that ZnO, PTFE, and ZnO/PTFE functional films were all successfully fabricated on the cellulose insulation pressboard surface. Scanning electron microscopy and XRD results present the nano-structure of the sputtered ZnO, PTFE, and ZnO/PTFE functional films and their amorphous states, respectively. The ZnO/PTFE composite functional film shows an apparent space charge suppression effect and hydrophobicity. The amount of the accumulated space charge in the pressboard sputtered ZnO/PTFE composite functional film decreased by about 40% compared with that in untreated cellulose insulation pressboard, and the water contact angle (WCA) increased from 0° to 116°.

Fabrication of ZnO-Al2O3-PTFE Multilayer Nano-Structured Functional Film on Cellulose Insulation Polymer Surface and Its Effect on Moisture Inhibition and Dielectric Properties

After a century of practice, cellulose insulating polymer (insulating paper/pressboard) has been shown to be one of the best and most widely used insulating materials in power transformers. However, with the increased voltage level of the transformer, research has focused on improving the insulation performance of the transformer’s cellulose insulation polymer. Considering the complex environment of the transformer, it is not enough to improve the single performance of the insulating polymer. In this study, a nano-structured ZnO-Al2O3-PTFE (polytetrafluoroethylene) multifunctional film was deposited on the surface of insulating pressboard by radio frequency (RF) magnetron sputtering. The effect of the multilayered ZnO-Al2O3-PTFE functional film on the dielectric and water contact angle of the cellulose insulating polymer was investigated. The scanning electron microscopy/energy dispersive spectrometry (SEM/EDS) showed that the nano-structured ZnO-Al2O3-PTFE functional film was successfully deposited on the cellulose insulation pressboard surface. The functional film presented an obvious stratification phenomenon. By analyzing the result of the contact angle, it was found that the functional film shields the hydroxyl group of the inner cellulose and improves hydrophobicity. The AC breakdown field strength of the treated samples was obviously increased (by 12 to ~17%), which means that the modified samples had a better dielectric insulation performance. This study provides a surface modification method to comprehensively improve electrical properties and the ability to inhibit the moisture of the cellulose insulating polymer, used in a power transformer.

PD pulse waveform and charge distribution in oil-pressboard insulation with needle-plate model at positive DC voltage: Their characteristics and relationship

A better understanding of the complicated characteristics and interrelationships between the charge distributions and partial discharge (PD) pulse waveforms occurring in oil-pressboard insulation can be used to characterise the PD mechanism as a basis for the optimisation of the insulation structure to minimise PD activity. These characteristics were studied by using a non-inductive resistor to measure the PD pulse waveforms produced in oil-pressboard insulation by a needle-plate at positive DC voltage and through additional simulations based on bipolar charge transport and hydrodynamic drift-diffusion models. It was found that the PD pulse waveforms are unimodal, and with increased applied voltage, their rising and falling edges and equivalent times decrease, and their apparent charge amplitudes and equivalent frequencies increase. These parameters have a saturation state at high voltages (>45 kV) and the oil streamers have voltage-dependent shapes and evolution velocities (vs); the higher the voltages, the higher the values of vs, and the wider are the streamer bodies. Accumulated charge can form a shield layer that hinders streamer evolution. Analysis of the PD mechanism in terms of the relationships between charge distribution and PD pulse waveform reveal that the rising and falling edges of a PD pulse correspond to streamer propagation in the oil gap and charge accumulation at the pressboard surface (i.e., the charging process in the pressboard), respectively. The peak value of the PD pulse waveform is shown to be related to the conduction current in the coupling loop as reflected by the current density at the streamer head.

Development Processes of Surface Trucking and Partial Discharge of Pressboards Immersed in Mineral Oil: Effect of Tip Curvatures

The pressboard surface is the electric weak link of the oil-paper insulation in transformers, and long-term partial discharge (PD) erosion is the dominant cause of degradation in pressboard. To explore the development processes of surface tracking under the effect of tip curvature, the typical needle-plate model was selected to initiate an electric field with a high tangential component on pressboard surface under needle tip curvature of 4~42 μm. With the help of a high-speed camera and a PD detecting system, the development processes of surface tracking and PD were recorded under a sustained AC voltage. A profound difference between surface tracking under different curvatures was discussed. Pressboard surfaces after tests were observed under a scanning electron microscope (SEM), and the damage degree of cellulose fibers was dependent on the tip curvature.

Effects of impurities on surface discharges at synthetic ester/cellulose board

An electrical discharge that occurs at an oil/pressboard interface or pressboard surface is called surface partial discharge. The surface discharge activity can cause irreversible damage to the insulation due to its thermal and chemical actions. It is visualized as white marks or tree-like structures on the surface of pressboard. In this paper, surface discharge characteristics of synthetic ester/cellulose-board have been investigated to study the influence of moisture, copper particles, and gas bubbles. The partial discharge (PD) experiments were carried out using a needle-plane electrode. The measurements of partial discharge inception voltage (PDIV), apparent charge and phase-resolved partial discharge patterns were acquired. In order to compare the effect of moisture over two independent insulations, separate PD measurements of the moisture-in-oil and the moisture-in-pressboard were conducted. The experimental results show that the moisture-in-oil samples have low PDIV values and more discharge pulses over the moisture-in-pressboard samples. Further, in the case of metallic particles, the PDIV and the number of repetitive pulses were increased when copper particles on the pressboard surface. The measurements of PD patterns of bubble samples are in good agreement with available literature on field measurement of 132/11 kV rated transformer. The findings of this study will assist in understanding the behavior of surface PDs under various conditions and would be of interest to asset managers considering the effects of such conditions on the transformer insulation diagnostics.

AC Surface Flashover and Gas Generation Difference of the Cellulose Insulation Pressboard Immersed in Novel 3-Element Mixed Oil and Mineral Oil

Surface flashover fault is one of the most challenge issues in oil-cellulose insulation pressboard system used in power transformer. In this study, a comparative study of the AC surface flashover properties of the novel 3-element mixed oil-cellulose insulation pressboard (3EMO-IP) and mineral oil-cellulose insulation pressboard (MO-IP) was performed under needle-plate and finger-finger electrode, respectively, measurement including dielectric property, surface flashover voltage, damage of cellulose pressboard surface and the gas generation behaviors after multiple surface flashover. Results show that the cellulose insulation pressboard immersed in the novel 3-element mixed insulation oil (3EMO) has higher relative permittivity and dielectric loss factor at 50 Hz, and also has slightly lower surface resistivity. The AC surface flashover voltage of the 3EMO-IP is higher than that of MO-IP under needle-plate and finger-finger electrode (electrode distance 5 mm, 10 mm, 15 mm and 20 mm). Compared to MO-IP, the lower electric field intensity at the oil-pressboard interface, as well as more difficult for surface charge accumulation of 3EMO-IP and the higher breakdown voltage of 3EMO lead to the higher AC surface flashover voltage of 3EMO-IP. Moreover, the carbonization of fibers in 3EMO-IP is slightly less. After multiple surface flashover, C2H2 and total hydrocarbon gases are the main differences between 3EMO-IP and MO-IP, which is more marked with the increase of flashover times. This study offers a reference for improving the surface flashover property of oil-pressboard insulation system by using 3EMO.

Effect of nanoparticles on streamer propagation and breakdown of vegetable oil-pressboard interface in non-uniform electric field

Electrical equipment is always subjected to various of operating conduction and voltage waves which cause insulation failure on the surface of pressboard (PB). In this paper, pre-breakdown streamers of surface discharge on the interface of PB in vegetable oil based Fe3O4 nanofluids (NFs) were observed by the shadowgraph method under lightning impulse voltage. The images indicate that streamers of NFs impregnated PB show more branches than that of pure oil impregnated PB. The stopping length of streamers propagation is calculated by shadowgraph images for NFs and pure oil. Results suggest that the stopping length of NFs impregnated PB is shorter under the same extra voltage. Secondary reverse streamer is generated at the dissipation process by the reverse electric field, which is caused by residual space charge imparted by primary streamer. Results indicate that the Fe3O4 nanoparticles have reduced the length of secondary reverse streamer. The positive and negative lightning breakdown voltage of Fe3O4 NFs impregnated PB is increased by 24% and 12% at 50mm gap, respectively. In addition, nanoparticles have effectively changed the electric field distribution resulting in the alleviation of streamer concentrated along the parallel direction of PB, and increased the lightning impulse breakdown voltage.