Oil-paper Interface Research Articles

Numerical Simulation of Multibranching Positive and Negative Streamer Discharges Near the Oil-Paper Interface in Dielectric Liquid During Pulsed Voltage

Numerical Simulation of Multibranching Positive and Negative Streamer Discharges Near the Oil-Paper Interface in Dielectric Liquid During Pulsed Voltage

Effect of Insulating Oil Type on Double Electric Layer at Oil-Paper Interface

Effect of Insulating Oil Type on Double Electric Layer at Oil-Paper Interface

Effect of Insulating Oil Type on Charge Transportation at Oil-Paper Interface

Effect of Insulating Oil Type on Charge Transportation at Oil-Paper Interface

Probing and Modulation of the Electric Double Layer at the Insulating Oil-Paper Interface.

Charge accumulation in the insulating oil-paper system determines the operating safety of the converter transformers in high-voltage direct current (HVDC) transmissions. However, it has been a long-standing challenge to reveal the charge distribution of the electric double layer (EDL) at the insulating oil-paper interface and relate it to charge transport. In particular, the EDL and charging mechanisms at the oil-paper interface have not been fully understood. We herein demonstrate that the charge distribution of EDL at the oil-paper interface is probed through Kelvin probe force microscopy (KPFM). The origin charge distribution of EDL without any additives shows that the negative charge gathers on the insulating paper surface, while the positive charge diffuses in the insulating oil, which is derived from the electron affinity difference between insulating oil and insulating paper and acts as an additional obstacle to charge transportation at the oil-paper interface. Interestingly, the additive 3-amino-2,4-triazole (ATA) can tune the charge distribution of EDL by bringing extra hole traps, which significantly decreases the interface barrier and reduces the charge accumulation at the oil-paper interface. As well as increasing charge mobility in oil-paper insulation, ATA also ensures stabilization of operation under polarity inversion conditions by accelerating the dissipation rate of accumulated charge.

Influence of Paper Surface Morphology on Streamer Propagation at Oil-Paper Interface

Influence of Paper Surface Morphology on Streamer Propagation at Oil-Paper Interface

Molecular-level evaluation of thermal field confinement effects on moisture distribution at the interface of natural ester oil-paper insulation: An investigation of electric field correlation

Oil-paper insulation is commonly used in transformers and is subject to dynamic changes in moisture content and distribution due to the influence of multiple physical fields such as temperature and electric fields. Moisture at the interface of oil-paper insulation can distort the interfacial electric field and cause insulation failure, but it is difficult to measure and characterize. In this study, molecular dynamics was used to simulate the diffusion behavior of water molecules at the interface of natural ester oil-paper insulation at temperatures ranging from 298 to 373 K, and the effect of electric fields on water diffusion was also analyzed. The results showed that without an electric field, water molecules in natural ester insulating oil tend to cluster at 298 K. Water molecules gradually diffuse towards the interface of the natural ester oil-paper insulation, but most remain at the interface and are difficult to penetrate into the cellulose insulating paper. In addition, normal diffusion is gradually enhanced at 298–328 K, reaching a minimum at 343 K. The interaction energy between water molecules and natural ester insulating oil decreases with increasing temperature, while the interaction energy with cellulose insulating paper decreases and then increases, reaching a minimum at 343 K. The combined influence of temperature and electric fields leads to an increase in the number of water clusters at the oil-paper interface. The interaction energy between water molecules and both natural ester insulating oil and cellulose insulating paper is enhanced, inhibiting the diffusion of water into the insulating paper. It was also found that the ability of water to diffuse differs significantly when the electric field direction is normal to the interface compared to when it is tangential to the interface. This is due to the fact that the water molecules when subjected to electric field action turn to polarize, and their electric field direction diffusion coefficient decreases, but still much higher than the non-electric field action direction. Further, the study found that the natural ester oil-paper insulation interface polarization is directional, and water molecules polarized in the direction normal to the insulation interface require a higher electric field strength.

Initial temperature of bubble formation at Oil-Paper interface based on bubble dynamics

The initiation of bubbles at the surface of insulating paper under high temperature may result in partial discharge and ultimately lead to the insulation failure of oil-immersed transformers. To study the formation mechanism of bubbles, the process of bubble growth at the oil-paper interface is established by using Rayleigh-Plesset equation. The Ideal Gas Law as well as the Hertz-Knudsen relation is used to quantify the flux of water vapor entering the bubble. The growth curve of bubbles in the oil-paper during temperature rise is calculated. And the Initial Temperature of Bubble Escape (ITBE) is then obtained based on the force analysis of the bubble growth. The results show that the bubble detaches from the surface of paper when its radius reach 198 μm, which is consistent with experimental observations. And the minimum average relative error between the predicted ITBE and experimental results is 1.11%. The effect of influence factors on the ITBE of oil/paper are then studied in detail. It is shown that the increased moisture content in paper accelerates the growth rate of bubbles, then reduces the ITBE. And the higher pressure inside oil tank restrains the bubble growth through the overall process. Additionally, as the cavity size of paper increase, the surface tension of bubble increases. Consequently, the bubbles require a larger size to achieve sufficient buoyancy for their escape from the aged paper.

Study on surface discharge characteristics and design method of large size oil-paper insulation of electrical transformer

When partial discharge occurs in the power transformer, the arc can further develop on the oil-paper interface between high and low potentials, and then form a path, resulting in large-area flashover. This will lead to a wide range of insulation damage, insulation failure, equipment shutdown and even large-scale economic losses. For the insulation damage caused by surface discharge of oil paper insulation, relevant scientific research institutions have carried out a large number of experimental studies. However, limited by the test conditions, the research focuses on the small-size model below 50mm, and mostly adopts the test device of uneven electric field. The research on the “large-size“ of the same scale inside the transformer is still blank. In this study, an oil-paper insulation surface discharge test platform with a large size of 50-150mm is built, the electric field conditions at the oil paper interface during the operation of transformers in engineering are simulated, and the surface discharge test of insulating paperboard is carried out by taking the same sample processing means as the engineering. Combined with the test results, a typical design optimization case of setting along surface corner ring to improve the allowable value of along surface discharge field and enhance along surface insulation margin is proposed. It lays a foundation for enriching the transformer oil-paper insulation design parameter system and improving the transformer oil paper insulation performance and operation reliability.

Estimation of de‐trapped charge for diagnosis of transformer insulation using short‐duration polarisation current employing detrended fluctuation analysis

Researchers have shown that the value of charge carriers, de-trapped from the oil–paper interface of power transformer insulation, is useful in carrying out the diagnosis. However, the evaluation of the de-trapped charge requires the analysis of polarisation–depolarisation currents. Being an off-line time-consuming process, the measurement and analysis of polarisation and depolarisation current (PDC) data are not practically advantageous. The study presents a detrended fluctuation analysis-based technique to estimate the magnitude of normalised de-trapped charge using the polarisation current measured for a short duration. Using the proposed technique, the requirement of measuring the complete PDC data, for diagnosis purposes, can be eliminated. Further, the technique also eliminates the requirement of depolarisation current which in turn facilitates a reduction in equipment shutdown time. The applicability of the proposed technique is tested on the data obtained from several real-life power transformers.

Molecular dynamics simulation on the distribution and diffusion of different sulfides in oil-paper insulation systems

Transformer faults frequently occur because of a deterioration of the insulation performance caused by corrosive sulfides. By studying the distribution and diffusion of different sulfides in oil paper, the mechanism of sulfur corrosion can be further investigated. Here, the diffusion of macromolecular sulfides (represented by DBDS and n-hexadecanethiol) and small molecular sulfides (represented by hydrogen sulfide) in oil-paper insulation systems were studied based on molecular dynamics simulation methods. The simulation results showed that cellulose had a strong interaction with hydrogen sulfide, while oil had a strong interaction with DBDS and n-hexadecathiol. This makes the trajectory and relative concentration of the various sulfides in the oil-Paper system different: DBDS was mainly distributed in oil, n-hexadecathiol was distributed at the oil-paper interface, and hydrogen sulfide was mainly distributed in cellulose. In addition, the diffusibility of DBDS and n-hexadecathiol in oil and cellulose were negatively correlated with the interaction energy: a larger interaction energy resulted in lower diffusion. The diffusion of hydrogen sulfide in oil paper was determined by the free volume fraction. A larger free volume fraction resulted in more intense molecular diffusion. The results of this work can provide a reference for prevention of sulfur corrosion.

Use of Interfacial Charge for Diagnosis and Activation Energy Prediction of Oil-Paper Insulation Used in Power Transformer

Activation energy is popularly used for the estimation of remaining life of transformer insulation. It is defined as the average rate of all reactions that happen with cellulose. Existing literature shows that the activation energy of oil–paper insulation can be obtained from polarization depolarization current (PDC) and return voltage measurement (RVM) data that are measured at a specific temperature. It is practically difficult to ensure the same measurement temperature for both PDC and RVM data. On the other hand, PDC data and its analysis get influenced by de-trapping current. This de-trapping current is generated by ionic charge carriers that get freed from trap sites during PDC measurement process. Formation of these trap sites is related to physical, chemical reactions that happen at oil–paper interface. This paper proposes a methodology which uses de-trapped charge, dislodged from deep and shallow traps, to assess insulation condition and for the prediction of activation energy. Thus, eliminating the need of RVM data. The proposed method is tested using data collected from various real-life in-service transformers.

The Influence of Pressure on the Discharge along Oil-Paper Interface under AC Stress

This study explores the influence of hydrostatic pressure on the discharge along the oil-paper interface under AC voltage, especially for the normal operating condition and breakdown. In this paper, an experimental platform was set up to record the partial discharge (PD) parameters of the test sample under different hydrostatic pressures, while the applied AC voltage was increased to final flashover voltage step by step. Experimental results showed that higher hydrostatic pressure had different effects on PD under different voltages. Higher pressure decreased the PD energy and increased the flashover voltage. Furthermore, under higher hydrostatic pressure, discharge traces (white mark) were found on the surface of the samples after intense discharging on the oil-paper interfaces, indicating that the hydrostatic pressure can affect the gas generation and dissipation process underneath the surface of the pressboards. Finally, the mechanism of how hydrostatic pressure influences the PD, flashover voltage, and white mark was interpreted based on the bubble theory. The results derived in this paper can be helpful for an optimal design and reasonable operation of oil-paper insulation systems, especially for power transformers.

Corrigendum: Effect of charge accumulated at oil–paper interface on parameters considered for power transformer insulation diagnosis

Corrigendum: Effect of charge accumulated at oil–paper interface on parameters considered for power transformer insulation diagnosis

Water molecules migration at oil–paper interface under the coupling fields of electric and temperature: a molecular dynamics study

ABSTRACTMoisture is an important factor affecting the insulation properties of transformers. Due to the limitations of macroscopic experimental methods, the diffusion of water at oil–paper interface cannot be accurately measured. Therefore, molecular dynamics method was used in this work to establish oil–paper layer model of 105 atoms. Through jointly analysing the aggregation degree, diffusion coefficient, free volume as well as radial distribution function of water molecules, the diffusion mechanism of water molecules at oil–paper interface was studied. The results show that when the initial water content in paper was high, water molecules would accumulate at oil–paper interface to form the local high-water region during heating. The polarisation of the electric field strengthened the hydrogen bonding interaction between water molecules and increased the probability of occurrence of the high-water region. Meanwhile, electric field reduced the free volume and diffusion coefficient of water molecules and rendered its diffusion coefficient anisotropic. What’s more, when the electric field was combined with the temperature field, the electric field played a leading role in the diffusion of water molecules while the temperature field was less affected. Diffusion coefficients of water molecules at different temperatures from molecular dynamics simulations were well consistent with experimental results, which verified the rationality of the model.

Water molecules migration at the oil-paper interface during heating: a large-scale distributed computing study

In view of the small molecular model established in the field of high voltage insulation, the actual operation of transformer cannot be fully reflected at micro-level. Therefore, this paper aims to improve the performance of computing environment and expand molecular scale. Firstly, two servers were connected through a high-speed communication network as the initial cluster architecture. Secondly, spatial decomposition and load balancing algorithms were used to improve the operation efficiency of cluster. Meanwhile, the oil-paper composite media model of 105 atoms could be established based on this cluster, but it consumed a lot of time. Therefore, we analysed the relationship between operation efficiency and four characteristic quantities such as central processing unit performance, number of cores, simulation time and number of nodes. Then the highest point of cluster operating efficiency was found through continuous optimisation. It can be summarised that the calculating speed of cluster is nearly 10 times faster than that of the large server. Meanwhile, according to the results based on this cluster, it can be concluded that water molecules would migrate towards the oil during heating. When the initial moisture content in paper is high, the high water region would appear at the oil-paper interface.

Effect of charge accumulated at oil–paper interface on parameters considered for power transformer insulation diagnosis

Polarisation and depolarisation current (PDC) measurement and analysis is one of the popular tools for effective diagnosis of power transformer insulation. Normally, it is assumed that polarisation current is the combination of the current due to dipole movement and conduction current. Similarly, the depolarisation current is only due to the relaxation of dipoles. However, it is found that after eliminating the effect of dc conduction from polarisation current the resulting current is not similar to that of measured depolarisation current. This shows some non-linearity is present in the system. This non-linearity occurs due to movement of trapped charge that resides in the interfacial region of oil–paper insulation. This study shows the effect of de-trapping charge on various performance parameters that are used for insulation diagnosis like paper moisture and dielectric dissipation factor (tanδ).

Effect of High Voltage Impulses on Surface Discharge at the Oil-Paper Interface

High voltage transients in electrical power systems are mainly caused by lightning strikes and switching operations. The high voltage transients above the basic insulation level (BIL) would degrade the electrical insulation of the equipment.In this work, the effect of high voltage impulses on the trend of the surface discharge at the oil-paper interface was investigated. The partial discharge patterns were recorded by the Insulation Condition Monitoring (ICM) system. The HV impulses were generated by a standard 1.2/50 μs HV impulse generator. The measurement results presented in this paper seem to show that surfacedischarge would cause complete breakdown (continuous flashovers) at the oil-paper interface much faster when the test sample is first aged by HV impulses comparedto when it is unaged.

Assessment of interfacial charge accumulation in oil-paper interface in transformer insulation from polarization-depolarization current measurements

Accumulation of interfacial space charge in oil-paper interface is a critical issue in insulation diagnostics of transformers. This interfacial charge mainly accumulates due to the conductivity difference of oil and paper. Accumulation of interfacial charge leads to localized field enhancement, which further leads to partial discharges and acceleration in the aging of insulation. Therefore, from the point of view of transformer insulation diagnostics, assessment of interfacial charge is very important. However, it is not easy to estimate interfacial space charge behavior from the transformer diagnostics methods currently in use. In case of Polarization-Depolarization Current (PDC) measurement, a well known method for transformer condition monitoring, the effect of interfacial charge is reflected in the non-linearity of current response during polarization and de-polarization. During de-polarization process, a part of the interfacial charge accumulated during polarization period is absorbed by the electrodes producing a current, which is difficult to separate using conventional linear dielectric theory. In this paper, an attempt has been made to separate this current component from de-polarization current through considering charge de-trapping mechanism. Terming this current component as de-trapping current, its relationship with other parameters of transformer insulation is discussed. The developed methodology has been applied on several practical transformers. It was observed that the time constant of de-trapping current is related to the paper conductivity, oil conductivity, dissipation factor and age of the insulation.

Comparison of oil-impregnated papers with SiO<inf>2</inf> and ZnO nanoparticles or high lignin content, for the effect of superimposed impulse voltage on AC surface PD

Surface discharge behavior of modified oil-impregnated paper (OIP) with nanoparticles (NPs), has been investigated under AC voltage with superimposed impulses. Surface Partial Discharges (PD) can develop at an oil-paper interface and lead to its degradation. Modified paper, made from fibers with adsorbed nanoparticles, can affect the partial discharge behavior of a paper in combination with oil at the interface between oil and fibers. Papers with two different concentrations (2 wt% and 6 wt%) of silica (SiO 2 ), and paper with silanized zinc oxide (ZnO) nanoparticles (1 wt%) have been studied. Papers with SiO 2 NPs showed lower impulse-induced surface PD activity. However, thorough purification during the production of SiO 2 filled papers was necessary to achieve a good performance. With less purification, paper with 2 wt% of SiO 2 did not show such significant improvements. Paper with 6 wt% of SiO 2 NPs showed a large number of AC surface PDs, but low influence of impulse voltage on subsequent PD. Papers containing 1 wt% of silanized ZnO showed reduced relative permittivity, but no significant difference in surface PD behavior. The effect of high lignin content in Kraft paper has also been studied. Paper with higher lignin content showed better surface PD characteristics under the impulse. Paper with low concentrations of pure SiO 2 NPs, and paper with high lignin content thus appear good candidates for further studies to improve the surface PD behavior of OIP.

Interface charge behavior of multi-layer oil–paper insulation under DC and polarity reversal voltages

Transformer oil, insulating paper, high voltage direct current (HVDC) bushings and on-load tap changer (OLTC) are essential parts of converter transformers. When the power flow of the HVDC system is reversed or a sudden outrage occurs, the charge field will deform the dielectric internal electric field, thus easily leading to the occurrence of discharge or insulation damage. In this case, it is necessary to carry out the characterization of the oil–paper interface to avoid these consequences. This paper analyses the interface charge accumulation and dissipation characteristics of an oil–paper insulation system under DC and polarity reversal voltages. In this paper, induced electrification was used to charge the composite oil–paper insulation. The amplitude of DC and polarity reversal voltages applied in this paper was 4 kV, and paper with thicknesses of 0.08 and 0.13 mm was used. This research offers a theoretical exploration to optimize the transformer oil– paper insulation structure and prevent damage to its insulation systems. From the research on different layers and thicknesses of paper, it can be concluded that the decay is fast in the initial period and then becomes slower with the lapse of time until finally the decay curve tends to become flat. With an increase in the number of the paper layers, the interface charge density decreases. So in real life multilayer paper is used to avoid failures owing to its structural weakness. Regarding the DC polarity reversal effect, increasing the thickness of the paper is equivalent to adding paper layers. The dissipation rate of the positive interface charge is quicker than that of the negative one. In the polarity reversal test, the dissipation rate becomes smaller as the reversal time gets longer.