Oil-impregnated Pressboard Research Articles

Investigation on electrical, thermal and mechanical properties of thermally aged pressboard impregnated with mixed mineral oil and synthetic ester fluid

The proposed study compares the electrical, thermal and mechanical performances of thermally aged cellulose pressboard material impregnated in mineral oil, synthetic ester fluid and mixed oil. The electrical insulation characteristics of oil-impregnated pressboard (OIP) are studied by means of surface discharge inception voltage (SDIV) using an ultra-high frequency technique and surface potential measurement analysis. It indicates that OIP aged in mineral oil has higher surface potential due to higher charge trap density compared to that of the mixed oil and synthetic ester fluid. Mechanical and thermal characteristics of the aged OIP are studied by means of tensile strength measurement and thermo-gravimetric analysis, respectively. The change in the chemical composition of OIP is studied by means of laser-induced breakdown spectroscopy, which indicates lower plasma temperature with mineral oil-based OIP indicating higher degradation state. To further understand the microscopic structure of the thermally aged OIP in different oils, scanning electron microscopy and X-ray diffraction studies have been performed. The pressboard thermally aged in synthetic ester fluid and the mixed oil has better electrical, thermal and mechanical properties as compared with the pressboard aged in mineral oil.

Electrical tree reconstruction method for oil-impregnated pressboards based on the inverse problem for the electrostatic field

There is still a lack of effective means of observation for electrical trees in opaque materials, which hampers experimental and theoretical research on the growth of such trees in the oil-impregnated pressboards of power transformers. This presents a great obstacle to the accurate diagnosis and prediction of partial discharge (PD) defects. This paper describes a method to monitor the growth process of electrical trees in pressboards based on solution of the inverse problem for the electrostatic field. The three-dimensional structure of the carbonized branches is reconstructed from the positions of PDs. Each PD is located by the charge remaining in the material. This charge is calculated from the electric field intensity or potential at some points in the field using the mirror image method and solution of the inverse problem for the electrostatic field. The electric field intensity is measured using an array of metal strips near the zero-potential border while PDs are occurring. High-voltage PD experiments are performed on oil-impregnated pressboard strips to verify the feasibility of the method. From the results of these experiments, the accuracy of PD site location is found to be as good as 0.35 mm in the χ direction (the horizontal coordinate), which is sufficient to describe the general structure of electrical trees in oil-impregnated pressboard. The movement of the PD positions indicates the growth process of tree branches. The calculated positions and lengths of the branches are consistent with observations of carbonized tracks after the pressboard has been torn open. The results obtained here will be of benefit to establishing a real-time monitoring method for three-dimensional reconstruction of electrical trees to reveal their internal growth mechanism in opaque materials such as oil-impregnated pressboards.

Research on the electrical aging characteristics of oil-impregnated pressboard under partial discharges

The insulating performance of oil-impregnated pressboard decreases due to partial discharges (PDs). Although methods of pressboard life assessment and its dependency on PD have been extensively investigated, little is known about the microscopic process and the physical mechanism of PD-induced pressboard failure. This study investigated the electrical aging characteristics of pressboard from the perspective of physico-chemical analysis. Pressboard samples were treated with the chosen numbers of PDs to induce electrical aging and the chemical composition of the PD-treated pressboard was analyzed. The results revealed the generation of carbonyl groups and the consumption of hydroxyl groups on the surface of the pressboard after PD treatment. These groups were generated by the cracking of the C2-C3 and C1-O bonds in the pyran ring, and the oxidation of hydroxyl group in cellulose molecules, respectively. Subsequently, the effect of PDs on the surface topography of the pressboard was examined. The results showed that the surface roughness of the pressboard changed with a U-shaped trend under the influence of PDs, first decreasing and then increasing immediately after. In addition, pits were found on the surface of the pressboard. Breakdown of the pressboard happened when a micro pit grew to a critical depth.

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.

Effect of Al2O3 nanorods on the performance of oil-impregnated pressboard insulation

The mineral oil (MO) in conjunction with paper is the main insulation components in oil-immersed transformers; their insulation properties play a significant role in the safe and stable operation of power transformers. To strengthen the insulation level of ultra-high voltage transformer and to reduce its size and weight, it is imminent to enhance the insulating performance of transformer oil and oil-impregnated cellulose. Recently, a unique novel effort of suspension of nanorods (NRs) into MO has been carried out and the results have exhibited improved insulation characteristics of transformer oil. The Al2O3 NRs with favorable features were prepared in our laboratory to develop transformer oil/paper insulation system with better insulation performance. The transformer oil-based nanofluids (NFs) were prepared with nanorod shape, oleic acid surface modification and 0.8 g/L concentration of NRs. The impregnated pressboards were prepared by impregnating them into dried oil and NF under vacuum beneath 1 kPa at 80 °C for almost 48 h to obtain the oil-impregnated pressboard (OIP) and nanofluid-impregnated pressboard (NIP), respectively. The interface at solid/liquid is considered weak link and is the main reason of oil-filled transformer collapse. The effect of Al2O3 NRs on creeping discharge and flashover traits of oil/pressboard (OP) interface under AC and impulse voltages were studied. Partial discharge and creeping flashover test of OIPs and NIPs were conducted. TSDC and PEA tests were applied to examine the space charge properties of OP interface before and after suspension of Al2O3 NRs.

Effect of TiO2 nanoparticles on DC breakdown performance of transformer oil-impregnated pressboard

Transformer oil-based nanofluid-impregnated pressboard (NIP) with TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> nanoparticles has been previously shown to exhibit substantially higher AC breakdown strength than that of pure oil-impregnated pressboard (OIP). In this study, OIP and NIP with TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> nanoparticles were prepared and experiments on DC breakdown strength of were performed. It was found that NIP possesses higher resistance to DC breakdown strength than that of OIP. To reveal the working mechanism, a pulsed electro acoustic (PEA) device was used to measure the space charge accumulation and dissipation characteristics in OIP and NIP. The results show that the density of deep level traps in NIP is visibly reduced owing to the addition of TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> nanoparticles and homopolar space charge accumulation on both sides of NIP is suppressed. Moreover, the charges injected from electrodes into NIP under DC voltage is also restrained owing to the increase of the Schottky potential barrier. Thus, the DC breakdown performance of NIP is considerably improved.

Power frequency breakdown characteristics of new 3-element mixed insulation oil and oil-impregnated pressboard

Mixed insulation oil has attracted widespread attention in recent years due to its excellent performance. In our previous works, we proposed a novel 3-element mixed insulation oil with superior physical and electrical properties. To promote the application of the mixed insulation oil to transformers, we studied the AC breakdown characteristics of the 3-element mixed insulation oil and the mixed insulation oil-impregnated pressboards and then performed a comparison to Karamay 25# mineral oil. Oil or oil-impregnated pressboard samples with different moisture contents were obtained by moisture absorption or by controlling the drying time. The breakdown tests were carried out at different temperatures in an oil bath. The results showed that the AC breakdown characteristics of the 3-element mixed insulation oil are better than those of mineral oil and that the mixed insulation oil can maintain breakdown strength over a range of moisture contents. The breakdown mechanism of insulation oil is discussed according to the experiment results. Moreover, the breakdown characteristics of the pressboard impregnated with the 3-element mixed insulation oil are also better than those of the pressboard impregnated with mineral oil. These advantages of mixed oil promote application of the 3-element mixed insulation oil to power transformers.

Vacuum ultraviolet laser induced breakdown spectroscopy for detecting sulphur in thermally aged transformer insulation material

Spatially resolved and temporally integrated vacuum ultraviolet laser induced breakdown spectroscopy (VUV LIBS) was used for contaminant analysis in thermally aged oil impregnated pressboard (OIP) insulation material taken from high voltage transformers. The signature of copper sulphide contamination was obtained through detection of copper and sulphur emission lines. The plasma emission also contained highly ionized oxygen and carbon emission lines. In contrast to the UV-VIS spectral region, sulphur emission lines were found to be strong in the VUV region. The concentration of sulphur was found to increase with the duration of ageing of the OIP material.

Investigation on Insulation Performance of Thermally Aged Natural Ester Oil Impregnated Pressboard

An attempt is made to understand the effect of thermal ageing on natural ester oil impregnated pressboard material. Surface discharge inception voltage of natural ester oil impregnated pressboard material is reduced drastically under AC, harmonic AC voltages and with DC voltages with high ripple content. Phase-resolved partial discharge studies show an increase in the number of discharges for natural ester oil impregnated pressboard (OIP) aged at higher temperature, under AC and harmonic AC voltages with different total harmonic distortions. Surface potential measurement study complies with results obtained by surface discharge inception voltage. It indicates the presence of high-density charge traps evolved as a function of ageing time and temperature. Laser-induced breakdown spectroscopy analysis is carried out to understand the characteristic variation with thermally aged OIP material. A decrease in plasma temperature is observed for sample aged at higher temperature. Differential scanning calorimetric studies indicate reduction in peak temperature with thermally aged specimens. Ageing observed to affect mechanical strength of the cellulosic material, as indicated by tensile test. Dielectric response spectroscopy study indicates that with an increase in thermal ageing, the permittivity, tan(δ), DC conductivity and relaxation time have increased. A reduction in activation energy of the aged material is observed, which indicates the status of the OIP material.

Effect of DC discharges in mineral oil on degradation characteristics of oil-impregnated pressboard

Oil discharge characteristics have been studied extensively, yet there have been few studies regarding the effect of oil discharges on pressboard. It is not yet known whether a single discharge of large magnitude or the accumulation of many small-magnitude discharges damage oil-impregnated pressboard, nor has any researcher to date identified the precise mechanism of pressboard failure. The present study was conducted to determine the oil discharge magnitude at which damage to pressboard occurs and the mechanism of the subsequent pressboard failure. The harmful oil discharge level was quantified by the comparison of the DC breakdown voltages between new and discharge-treated pressboards. Exposure to a 7.8 nC oil discharge repeated 100 times was found to cause a decrease in the DC breakdown voltage of the pressboard. Oil discharges of 13.1 nC repeated 50 times and of 26.8 nC repeated 10 times also degraded the pressboard. A single discharge of large magnitude will degrade the pressboard at a smaller integrated charge value than will several discharges of small magnitude. A dual-parameter grid diagram was proposed to numerically quantify the harmful oil discharge level. Further observation of the discharge-treated pressboard indicated that the oil discharge created a whitish area and wormhole-like traces on the surface of the pressboard, due to the vaporization/decomposition of the transformer oil and the carbonization of the cellulose, respectively. Subsequent discharges will propagate along the gaseous wormhole into the pressboard, and finally, break down the pressboard in bulk.

Study on the Influence of Temperature, Moisture and Electric Field on the Electrical Conductivity of Oil-Impregnated Pressboard

The main insulation of converter transformers consists of transformer oil and oil-impregnated pressboard. Under operating conditions, the valve-side winding of the converter transformer is subject to DC voltage components. Therefore, studies on the characteristics of oil-impregnated pressboard conductivity are necessary. In this paper, the temperature, moisture and electric field dependency of pressboard conductivity are investigated based on a specially designed three-electrode experimental chamber, which allows for a variation in temperature ranging from 25 °C to 120 °C and an electric field strength ranging from 0 to 30 kV/mm. The experimental results show that, within the experimental conditions, the conductivity of oil-impregnated pressboard increases exponentially with increasing moisture and temperature. High moisture and temperature will increase both the carrier concentration and carrier mobility, which explains the exponential correspondence. Furthermore, the electric field dependency of the conductivity is more obvious for wet pressboard than for dry pressboard. Protons in the wet pressboard are more easily accelerated by the electric field than the impurity ions in the oil of the dry pressboard, which leads to an obvious electric field dependency of the wet pressboard conductivity.

Assessment on Oil-Paper Insulation Aging of Transformer Based on Dielectric Response Model

For nondestructive assessment of insulation aging in power transformer, dielectric response technology is usually used to evaluate the oil-paper insulation aging. In this paper, a quantitative evaluation method of insulation aging based on Davidson-Cole model is proposed. The oil-paper insulation equivalent Debye model parameters of transformer are extracted by exponential function fitting. According to the complex permittivity of oil-paper insulation, the parameters of Davidson-Cole model are extracted by fitting algorithm. Under laboratory conditions, accelerated thermal aging test are carried out. The quantitative relationship between Davidson-Cole model parameters and aging of oil-paper insulation is analyzed. The experimental results show that the Davidson-Cole model parameters are directly related to the aging time. It is found that the parameters β and Δε of the Davidson-Cole model gradually decreases and the parameter τ gradually increases with the polymerization degree of oil-impregnated paper. Therefore, according to the quantitative characterization of the model parameters, the aging of transformer oil-paper insulation can be effectively evaluated, which is helpful to realize the nondestructive testing of polymerization degree in oil-impregnated pressboard (paper) power transformer.

Modified propagating behavior of creeping streamers at TiO2 nanofluid/pressboard interface

Nanotechnology provides potential to improve the insulation properties of oil/pressboard system. In this paper, propagating properties of creeping streamer at the interface of the pure oil-impregnated pressboards (OIPs) and nanofluid-impregnated pressboards (NIPs) are measured by the shadowgraph method. The results show that TiO2 nanoparticles significantly decelerate the propagation of creeping streamers. The decelerated creeping streamers are linked with the modification of streamer pattern, which is attributed to increased traps and reduction of permittivity mismatching coefficient of nanofluid/NIP. Increased trap density affects the electrodynamics of ionized charges involving in streamer propagation, therefore modifies the behavior of creeping streamers. Besides, the better permittivity matching, caused by additional polarization of TiO2 nanoparticles, decreases the electrostatic attractive force to creeping streamers. The squeezing effect of creeping streamer is alleviated, resulting in a stronger shielding effect and decelerated propagation of creeping streamers at NIP's interface.

Effect of Al2O3 nanorods on dielectric strength of aged transformer oil/paper insulation system

With the commencement of high voltage alternating current (HVAC) and high voltage direct current (HVDC), the demands on insulating reliability of power transformer is getting more and more imperative. The mineral oil (MO/TO) in conjunction with paper is the main insulation components in oil-immersed transformers; their insulation properties play a significant role in the safe and stable operation of power transformers. To strengthen the insulation level of ultra-high voltage transformer and to reduce its size and weight, it is imminent to enhance the insulating performance of transformer oil and oil-impregnated cellulose. Recently, a unique novel effort of suspension of nanoparticles (NPs) into mineral oil (MO) has been carried out and the results have exhibited improved insulation characteristics of transformer oil. The Al2O3 nanoparticles with favorable features were prepared in our lab to develop transformer oil/paper insulation system with better insulation performance. The transformer oil-based nanofluids (NFs) were prepared with nanorod shape, oleic acid (OA) surface modification and 0.8 g/L concentration of NPs. The pressboards were cut into the desired sizes (85 mm.55 mm). The thickness of the pressboards was 2 mm applied in flashover tests. The prepared pressboards were put into an oven at 105 °C for almost 48 h to carry out the hot air drying operation. Then they were put in the vacuum drying at 85 °C for 48 h. The impregnated pressboards were prepared by impregnating them into dried oil and NF under vacuum beneath 1 kPa at 80 °C for almost 48 h to obtain the oil-impregnated pressboard (OIP) and nanofluid-impregnated pressboard (NIP) respectively. Additionally, the prepared insulation system samples of liquid (MO and NFs) and solid (OIPs, NIPs) were thermally aged at 130 °C for 30 days. The effect of alumina NPs on thermal aging of transformer oil and pressboard were also investigated. The results showed that Al2O3 NPs can slow down the aging of oil and pressboard. The average AC and positive LI BDV of aged NFs was 11% higher than aged MOs. The average LI creeping FOVs of NIPs were 6% higher than OIPs. Additionally, average AC creeping FOVs of NIPs were 8% higher than that of OIPs. The enhancement in insulating performance of aged NFs and NIPs as compared to MO and OIPs is interpreted in light of electron traps theory.

Research on Creeping Flashover Characteristics of Nanofluid-Impregnated Pressboard Modified Based on Fe₃O₄ Nanoparticles under Lightning Impulse Voltages.

Creeping flashover of mineral-oil-impregnated pressboard under impulse stress is a common insulating failure in oil-immersed transformers, arousing increasing attention. Recent studies have shown that the breakdown strength of transformer oil under positive lightning impulse voltage can be significantly improved through nanoparticles-based modification, and Fe3O4 has shown the best improvement in breakdown strength compared to other nanoparticles that have been used. This paper presents the creeping flashover characteristics of pure oil-impregnated pressboard (OIP) and nanofluid-impregnated pressboard (NIP) based on Fe3O4 nanoparticles under positive and negative lightning impulse voltages, respectively. It was found that NIP possessed higher resistance to creeping flashover than OIP. The relative permittivities of oil and oil-impregnated pressboard before and after nanoparticles-based modification were measured, and the results revealed that the addition of nanoparticles led to a better match in relative permittivity between oil and oil-impregnated pressboard, and a more uniform electric field distribution. Furthermore, the shallow trap density in NIP was obviously increased compared to that of OIP through the thermally stimulated depolarization current (TSDC), which promoted the dissipation of surface charges and weakened the distortion of the electric field. Therefore, the creeping flashover characteristics of oil-impregnated pressboard were greatly improved with Fe3O4 nanoparticles.

Multilayers oil and oil-impregnated pressboard electric field simulation based on space charge

Space charge build-up has been considered as the major issue in the development of HVDC apparatus such as the converter transformer. The existence of space charge can distort the local electric field, which could lead to the degradation and even breakdown of insulation materials. Therefore, it is vital to investigate factors that can affect space charge formation and dissipation characteristics such as the temperature, moisture, ageing, thickness, multi-layered structure and electric fields. This paper mainly focuses on the effect of multilayers and thickness on space charge behaviour of oil and pressboard insulation system. Space charge was measured using the pulsed electroacoustic technique (PEA) method. The space charge results are quantitatively analysed to establish the relationship between interfacial charge density and different pressboard and oil thickness ratios. A new space charge interpolation methodology is utilized to input space charge into the multilayers oil and pressboard model using COMSOL software. The local electric field of multilayers oil-impregnated pressboard and oil could be simulated, with the emphasis on the electric field after the polarity reversal operation. From space charge results, they indicate that the increased thickness of pressboard could prohibit the interfacial charge increase while the increased oil thickness could facilitate interfacial charge increase. Moreover, from the electric field simulation results, they indicate that there is the electric field gradient caused by the space charge for multilayers oil and oil-impregnated pressboard structure. After the polarity reversal, the maximum electric field of the oil caused by the space charge is higher than electric field calculated based on the Maxwell-Wagner theory.

Effects of temperature on creepage discharge characteristics in oil-impregnated pressboard insulation under combined AC–DC voltage

Due to the complexity of the valve side winding voltage of the converter transformer, the insulation characteristics of the oil-impregnated pressboard (OIP) of the converter transformer are different from those of the traditional AC transformer. The study on effect of temperature on the creeping discharge characteristics of OIP under combined AC–DC voltage is seriously inadequate. Therefore, this paper investigates the characteristics of OIP creepage discharge under combined AC–DC voltage and discusses the influence of temperature on creepage discharge characteristics under different temperatures from 70 °C to 110 °C. The experimental results show that the partial discharge inception voltage and flashover voltage decrease with increasing temperature. The times of low amplitude discharge (LAD) decrease and amplitude of LAD increases. Simultaneously, the times of high amplitude discharge (HAD) gradually increase at each stage of creepage discharge with higher temperature. The analysis indicates that the charge carriers easily accumulate and quickly migrate directional movement along the electric field ahead of discharging. The residual charge carriers are more easily dissipated after discharging. The ‘hump’ region of LAD moves to the direction of higher discharge magnitude. The interval time between two continuous discharges is shortened obviously. The concentration of HAD accelerates the development of OIP insulation creepage discharge. The temperature had an accelerating effect on the development of discharge in the OIP under applying voltage.

Measurements on pressboard to understand the effect of solid insulation condition on monitoring of power transformer winding clamping pressure

Pressboard is the main building material of a power transformer winding structure. Through-thickness compression behaviour of pressboard has a direct influence on the transformer winding clamping pressure. Being a cellulose based polymeric material with a porous structure, the through-thickness compression behaviour of pressboard is highly non-linear. Also, the mechanical properties of pressboard are moisture, temperature, and ageing status dependent. However, information on the exact behaviour of the through-thickness compressibility of pressboard under typical operating conditions of a power transformer has not been well documented in the literature. This has posed difficulties in developing reliable techniques to detect loose clamping conditions in power transformer windings. This study presents the results of laboratory measurements on the effect of moisture, temperature and ageing on the through-thickness compression behaviour of oil-impregnated pressboard. From the measured results, it was found that the compressibility of pressboard could increase with increasing temperature, ageing and moisture ingress. It was also found that moisture and ageing tend to exaggerate the thermal softening of pressboard. Subsequent finite element analysis confirmed that transformer clamping pressure has significantly altered with the pressboard condition.

Analysis of creeping discharges on oil-impregnated pressboard under combined AC and DC voltages

This paper presents a comparative analysis of the creeping discharge characteristics on mineral oil-impregnated pressboard (OIP) under combined AC and DC voltages. The creeping discharge behavior at various voltage levels to failure were analyzed using partial discharge (PD) measurements, camera observations, and electric field simulations. It was found that the creeping discharge distribution was significantly influenced by the DC component, and it was difficult to induce failure even on OIP with high moisture under combined AC and DC voltage. The flashover along the free-oil boundary layer above the OIP during creeping discharge occurred much more easily under combined AC and DC than under AC voltage, and the quality of the oil was important to avoid flashover. These differences were attributed to the combined influence of the electric field distribution under DC and space charge accumulation.

Molecular Dynamics Simulations of Molecular Diffusion Equilibrium and Breakdown Mechanism of Oil-Impregnated Pressboard with Water Impurity.

The water molecule migration and aggregation behaviors in oil-impregnated pressboard are investigated by molecular dynamics simulations in combination with Monte Carlo molecular simulation technique. The free energy and phase diagram of H2O-dodecylbenzene (DDB) and H2O-cellulose mixtures are calculated by Monte Carlo technique combined with the modified Flory-Huggins model, demonstrating that H2O molecules can hardly dissolved with infinitesimal content in cellulose system at temperature lower than 650 K, based on which the oil/cellulose layered structure with water impurity representing three-phase coexistence in oil-impregnated pressboard are modeled and performed for molecular dynamics. The molecular dynamics of H2O/DDB/cellulose three-phase mixture simulating oil-paper insulating system with H2O impurity indicates that DDB molecules can thermally intrude into the cellulose-water interface so as to separate the water phase and cellulose fiber. The first-principles electronic structure calculations for local region of H2O/DDB interface show that H2O molecules can introduce bound states to trap electrons and acquire negative charges, so that they will obtain sufficient energy from applied electric field to break DDB molecular chain by collision, which are verified by subsequent molecular dynamics simulations of H2O−/DDB interface model. The electric breakdown mechanism under higher than 100 kV/m electric field is presented based on the further first-principles calculations of the produced carbonized fragments being dissolved and diffusing in DDB phase. The resulted broken DDB fragments will introduce impurity band between valence and conduction bands of DDB system, evidently decreasing bandgap as to that of conducting materials in their existence space. The conductance channel of these carbonized DDB fragments will eventually be formed to initiate the avalanche breakdown process by the cycle-feedback of injected charge carriers with carbonized channels.