Oil-impregnated Pressboard Research Articles

Influence of TiO2 nanoparticle size on creeping flashover property of transformer oil-impregnated pressboards

Nanoparticles have exhibited great potential in improving insulating property of liquid and solid dielectrics. To explore their modification mechanism on transformer oil-impregnated pressboards, TiO2 nanoparticles with different diameters of 5nm, 10nm, and 15nm were synthesized and used to prepare transformer oil-based nanofluids. The creeping flashover characteristics of both pure oil-impregnated paperboards (OPs) and nanofluid-impregnated pressboards (NPs) were examined under lightning impulse and AC voltages. It was found that the creeping flashover property of NPs is significantly improved with the decreasing of nanoparticle size. Especially, it is enhanced by 2.44 times under positive lighting impulse as nanoparticle size reduced from 15 nm to 5 nm. The results of space charge decay and trap characteristics reveal the decrease of nanoparticle size can lower trap depth and greatly enhance the space charge dissipation rate, leading to more uniform electric field distribution at the interface between nanofluid and pressboards and higher creeping flashover property.

Phenomenon analysis and state classification of surface discharge on oil-impregnated pressboard under AC-DC combined voltage

Surface discharge may cause irreversible damage to turn-to-ground insulation in valve windings of converter transformer, where withstand with AC-DC combined voltage. This paper analyzes the phenomenon and characteristics of surface discharge on oil-impregnated pressboard (OIP) under AC-DC combined voltage, and develops a discharge state recognition method. The cylinder-plate discharge model was used to simulate surface discharge. The results showed that discharge development and OIP failure were significantly accelerated by white marks on OIP which were essentially gaseous channels. The discharge characteristics before and after white mark occurrence were both dominated by AC component because of its bigger contribution to electrical field distribution (EFD), and the DC component had obvious effect on accelerating OIP failure. A set of features representing discharge state after white mark occurrence was selected out by the entropy weight method (EWM), based on which the discharge process was classified into three states (stable, fast development and pre-breakdown state) by fuzzy C means clustering method (FCM). A support vector machine (SVM) classifier to recognize discharge state was trained and showed a good performance, whose average assessment accuracy was up to 91.98%. Moreover, the ratio between negative and positive discharge numbers could be used as an auxiliary indicator of pre-breakdown state.

Feasibility of a universal approach for temperature correction in frequency domain spectroscopy of transformer insulation

Over the past few years, frequency domain spectroscopy (FDS) has been widely utilized to assess transformer insulation. Many investigations have revealed that the FDS measurement has been observed to be dependent on service conditions (aging, moisture, geometry, and temperature). During the onsite FDS measurement, the inner insulation temperature of the de-energized transformer gradually decreases, which affects the FDS results due to moisture migration from oil to the cellulose insulation. This process gives rise to misinterpreting of the condition of the transformer insulation. Presently, the master curve technique reported by several researchers is utilized for temperature correction in FDS measurements. In fact, various researchers have obtained different master curves, so making a universal approach to temperature correction has become an important topic. In the present investigations, a series of experiments was performed on oil-impregnated pressboard insulation under laboratory conditions which was followed by FDS measurements at lower than operating temperature and a universal master curve for temperature correction was studied. Activation energies from oil-impregnated pressboard tests support the effectiveness of the proposed universal approach for temperature correction.

Effect of Temperature on Space Charge Distribution of Oil–Paper Insulation under DC and Polarity Reversal Voltage

The electric field distortion caused by space charge is an important factor affecting the operation reliability of oil–paper insulation in a converter transformer. To study the accumulation and decay characteristics of the space charge within oil-impregnated pressboard under DC and polarity reversal voltage, and consider the possible operating conditions of the converter transformer, the space charge behavior of oil-impregnated pressboard was measured by the pulsed electro-acoustic (PEA) method in the temperature range from −20 °C to 60 °C. The effect of temperature on the accumulation and decay characteristics of space charge is also analyzed. The space charge accumulated within the pressboard at low temperature is mainly homocharge injected by the electrode, while heterocharge formed by ion dissociation counteracts some of the homocharge at high temperature. Thus, the space charge of pressboard first increases, then decreases, with an increase in temperature. However, slow decay of the space charge causes severe distortion of the electric field distribution in the pressboard during voltage polarity reversal.

Optical Sensing Techniques for Condition Monitoring of Transformer Insulation Material

Non contact optical sensing techniques—probing of surface flashover and probing of laser induced plasma, spectrally and temporally, were used to study the thermal ageing induced degradation of oil impregnated pressboard (OIP) insulation material of transformer. Optical probing of surface flashover discharge, gave parameter values comparable with, flashover discharge current measurements. Optical emission persistence time was increasing with ageing level of the insulation. Optical emission spectroscopy of impulse surface flashover gave atomic emissions from the copper sulphide contaminant, diffused in the OIP material due to ageing. Failed insulation samples showed larger Cu emission intensity. Optical emission intensity during AC, + DC and − DC flashover was used to indicate ageing and to identify initiation and arcing stages of surface flashover. Alternatively, laser induced breakdown spectroscopy (LIBS) indicated neutral Cu emission in the aged OIP material, indicating diffusion of Cu contaminants. Quantitative analysis was performed on OIP samples with surface deposited Cu and thermally aged specimen. A minimum detection limit of 4.5 µg/cm2 was achieved. LIBS plasma persistence time was identified as an alternative tool for understanding ageing level of OIP material of transformer.

Experimental Study on Breakdown Characteristics of Transformer Oil Influenced by Bubbles

Bubbles will reduce the electric strength of transformer oil, and even result in the breakdown of the insulation. This paper has studied the breakdown voltages of transformer oil and oil-impregnated pressboard under alternating current (AC) and direct current (DC) voltages. In this paper, three types of electrodes were applied: cylinder-plan electrodes, sphere-plan electrodes, and cone-plan electrodes, and the breakdown voltages were measured in both no bubbles and bubbles. The sphere-sphere electrodes were used to study the breakdown voltage of the oil-impregnated pressboard. The results showed that under the influence of bubble, the breakdown voltage of the cylinder-plan electrode dropped the most, and the breakdown voltage of the cone-plan electrode dropped the least. The bubbles motion was the key factor of the breakdown. The discharge types of the oil-impregnated pressboard were different with bubbles, and under DC, the main discharge type was flashover along the oil-impregnated pressboard, while under AC, the main discharge type was breakdown through the oil-impregnated pressboard.

Creeping flashover characteristics improvement of nanofluid/pressboard system with TiO2 nanoparticles

Creeping flashover easily occurs at the interface between oil and pressboard in transformer and thus results in outage of power transmission system. Investigations have shown that creeping flashover characteristics at oil/pressboard interface can be improved by the addition of TiO2 nanoparticles, but the mechanism is still not thoroughly known. In this work, creeping flashover performance at nanofluid/pressboard interface modified by different sizes of nanoparticles were studied and the mechanism was presented as well. Nanofluids with the same concentration but with different sizes of TiO2 nanoparticles were prepared, and pressboards impregnated with them were prepared as well. After that, their creeping flashover characteristics were measured and compared. Nanoparticle’s size affected the creeping flashover performance along oil/pressboard greatly under both AC and lightning impulse voltages. The highest creeping flashover voltage can be enhanced by as high as 12.2% and 32.0% respectively. The underlying electric field distribution and charge transportation behaviors were analyzed to demonstrate the influence of nanoparticle’s size. By the addition of nanoparticles with a smaller size, the dielectric constant of nanofluid was increased closer to that of the pressboard, thus they were matched better. Moreover, charge was easier to dissipate from the oil/pressboard interface and electric field distortion at the interface was consequently reduced. Therefore, the electric field was more like a uniform field and the forward development of flashover was more difficult, leading to a better performance of creeping flashover of oil-impregnated pressboard.

Influence of ambient medium on thermal ageing of pressboard in transformer oil containing dibenzyl bisulphide (DBDS)

Impact of thermal ageing of pressboard material in transformer oil containing dibenzyl disulphide (DBDS) under different ambience, such as air, N2, He and vacuum has been studied to understand the level of copper sulphide diffusion. The characteristic variations in electrical and thermal properties of copper sulphide diffused oil impregnated pressboard (OIP) material have been measured under different ageing conditions. Surface charge accumulation studies with AC and ±DC voltages showed that surface charge decay time was longer in air aged OIP material. Surface discharge studies by adopting ultra-high frequency (UHF) sensing indicated a reduction in the SDIV with the level of ageing although the characteristic variation in flashover voltage (FOV) under lightning impulse (LI) voltage is almost the same. Also the SDIV reduced with increase in frequency of supply voltage. Thermo-gravimetric analysis (TGA) indicated that maximum weight loss temperature (Tmax) of the cellulose constituent of OIP material decreased with ageing. Laser induced breakdown spectroscopy (LIBS) was adopted to quantify the diffused copper sulphide contaminant in OIP material and the results were compared with the inductively coupled plasma (ICP) spectroscopy based analysis. Based on these studies, deteriorating effect of ageing medium on several properties of OIP material was understood.

Understanding impulse surface flashover phenomenon on thermally aged oil impregnated pressboard material

Surface flashover phenomena under lightening impulse voltage of positive and negative polarity on thermally aged Oil Impregnated Pressboard (OIP) material used in transformer is studied. OIP material with surface deposited Cu at different concentrations are also used for comparison. Total energy deposited on the OIP material and the magnitude of discharge current increased with ageing duration. The decay characteristics concerning optical emission from the surface flashover matched with the temporal decay of discharge current. The study also revealed some of the pre-breakdown phenomena before the main flashover breakdown. Optical Emission Spectroscopy (OES) revealed signature of diffused Cu and S in aged OIP material. Higher emission intensity for Cu was observed for higher aged OIP material, indicating larger possibility of degradation. OIP material collected from a failed transformer also showed optical emission lines matching with Cu emission, indicating possibility of employing the OES technique in contaminant detection. The temperature of the plasma during the surface flashover was also estimated.

Understanding the dielectric properties of pressboard material thermally aged in Dibenzyl Disulphide (DBDS) included transformer oil

Surface Discharge Inception Voltage (SDIV) of unaged and copper sulphide diffused Oil Impregnated Pressboard (OIP) material was studied by adopting Ultra High Frequency (UHF) technique. Copper sulphide diffusion into OIP material was carried out by thermal aging of pressboard material in Dibenzyl Disulphide (DBDS) added transformer oil. The SDIV reduces with increase of thermal aging temperature. The current pulse injected in positive and negative half cycle during the surface discharge of the AC shows the rise time of 0.8ns and 1ns respectively. The FFT analysis of UHF signal measured during surface discharge process has shown frequency content in the range 0.7–1.2 GHz with its dominant frequency at 0.9 GHz. The life of the charge presented on the pressboard increases with thermal aged specimen. Energy dispersive spectroscopy (EDS) analysis of OIP material indicates presence of copper and sulphur content. Frequency Domain Dielectric Spectroscopy (FDDS) studies indicate the permittivity(er), dissipation factor (tan(δ)), conductivity (σdc) increases and relaxation time (τ), coefficient (α) decrease with OIP material (in air / Nitrogen) aged at higher temperature in DBDS added transformer oil. Cole-Cole double relaxation model was used to understand the dielectric parameters of copper sulphide diffused OIP material. The DC conductivity of pressboard material aged in DBDS included transformer oil follows Arrhenius law and the activation energy decreases with increase in temperature of thermal aging.

Creeping Discharge Characteristics of Nanofluid-Impregnated Pressboards Under AC Stress

This paper presents experimental research on creeping discharge characteristics of TiO2 nanofluid-impregnated pressboards (NPs) under sustained ac voltage. It was found that NPs possess higher resistance to creeping discharge than that of pure oil-impregnated pressboard. The results of permittivity and surface charge accumulation and decay characteristics reveal that the modification of nanoparticles can not only make the electric field more uniformly distributed at the interface of nanofluid/pressboard due to a better matching in permittivity but also significantly increase the surface charge decay rate via a trapping and detrapping process in shallower traps. Thus, the creeping discharge characteristics of NPs are greatly improved by the modification of TiO2 nanoparticles.

From the editors' desk

The first article is "Transient and Steady-State DC Behavior of Oil-Impregnated Pressboard," by Fabian Schober, Stephan Harrer, and Andreas Kuchler of the University of Applied Sciences, Schweinfurt, Germany; Frank Berger of the Technische Universitat Ilmenau, Germany; and Wolfgang Exner and Christoph Krause of Weidmann Electrical Technology AG, Rapperswil, Switzerland. The article investigates the influence of pressboard density and oil conductivity on the conductivity of oil-impregnated pressboard, and both are shown to be important for achieving a desired conductivity of oil-impregnated pressboard for HVDC applications. Two pressboard types, low and high density, and two types of mineral oils, low conductive and high conductive, were used in the investigation. Both pressboard types exhibited similar apparent conductivities when impregnated with low-conductive oil, but when impregnated with high-conductive oil, an apparent conductivity 2.8 times higher was observed as a consequence of the larger oil volume. Yet, the numeric assessment of oil-impregnated pressboard conductivity by simple linear superposition of pressboard conductivity and oil conductivity does not provide the measured apparent conductivity. In addition, the influence of oil conductivity on the apparent oil-impregnated pressboard conductivity was evaluated. Pressboard impregnated with four types of mineral oil, spanning an apparent conductivity of 2.5 orders of magnitude, was found to be only in an apparent conductivity range of oil-impregnated pressboard of only 0.5 orders of magnitude. As an explanation for the observed phenomenon, it appears that new and modified charge transport paths in the oil volume altered the transport paths along pressboard fibers. Conduction processes therefore interact with each other, and this interaction opens a new and interesting field of further research. Selecting a suitable pressboard density and the right choice of mineral oil can be used to achieve the desired conductivity for the oil-pressboard insulation system, which may be an option for the optimization of HVDC insulation systems.

Transient and steady-state dc behavior of oil-impregnated pressboard

The use of renewable energy is a key topic in the global energy market. Wind parks are being built all over the world, especially offshore, and large hydroelectric power stations are producing enormous amounts of electrical energy. Yet, the regions abounding in renewable energy frequently do not coincide with the consumption sites of energy. Therefore, energy has to be transported over long distances. For this purpose, high voltage direct current (HVDC) transmission up to 1,100 kV is an efficient option.

Effect of moisture and thermal degradation on the activation energy of oil–paper insulation in frequency domain spectroscopy measurement

Frequency domain spectroscopy (FDS) is widely used to diagnose the condition of transformer oil–paper insulation. However, FDS measurements are highly temperature dependent. The master curve technique is the most common method to remove the temperature effect on FDS, but the reliability of this method is largely dependent on the value of the activation energy, which can be affected by moisture content and thermal degradation degree. To understand the effects of these two factors on the activation energy, FDS measurements at different temperatures were performed on oil-impregnated pressboards with different thermal degradation degrees and moisture contents. The activation energies of the pressboards were calculated using the Arrhenius equation. Results show that they vary between 0.88 and 1.06 eV along with thermal degradation degree and moisture. The two-way ANOVA indicates that the effects of these two factors on activation energy are not significant at 95% confidence level when the degree of polymerisation (DP) changes from 415 to 1300 and when the moisture does not exceed 4%. The average activation energy (0.95 eV) can be utilised to generalise the Arrhenius equation to better eliminate the temperature effects. Finally, FDS measurements on a model transformer are performed to verify the effectiveness of the proposed technique.

Space charge characteristics in oil and oil-impregnated pressboard and electric field distortion after polarity reversal

The formation of space charge in the oil/paper insulation under HVDC condition could influence the electric field distribution. In the case of HVDC converter transformer, the distortion of the electric field may affect its performance. It is, therefore, important to analyze factors that can affect space charge formation and dissipation characteristics such as temperature, moisture, electric stress and ageing process. This paper mainly focuses on the effect of electric stress and oil ageing on space charge characteristics in oil/paper insulation system. The pulsed electroacoustic technique (PEA) method was used to monitor space charge dynamics. This data was further incorporated into a model established using COMSOL software to estimate space charge impact on electric field distribution, with an emphasis on the electric field after polarity reversal operation. The results show that the electric field arising from the space charge formation is much higher compared to the average electric field after the polarity reversal operation. Moreover, the electric field caused by space charge decreases faster for the aged oil-impregnated pressboard compared to the fresh one, which could be related to different space charge dissipation characteristic related to the condition of the oil.

Effect of Surface Discharges on Lightning Impulse Breakdown Voltage of Oil-Impregnated Pressboard in Power Transformers

Power transformers inevitably experience impulse over-voltage stresses such as from lightning and/or switching events despite various protection systems that may be in place. At locations along oil/pressboard interfaces in the transformer, surface discharges may initiate due to various reasons. The surface discharges may or may not eventually lead to voltage flashover faults. This paper presents a study of how relatively small surface discharges affect the lightning voltage impulse (LI) strength of oil-impregnated pressboard insulation. It is found that surface discharges of magnitudes in the order of some hundreds of pC can reduce the LI withstand voltage of the oil-impregnated pressboard by up to 15%. Although the negative polarity LI withstand voltage of surface discharge-aged pressboard is higher than that of positive polarity, the reduction in the LI voltage strength is more pronounced for the negative LI impulse voltage. The findings suggest further scrutiny of the standardised acceptable levels of PD in power transformers.

Effects of DC prestressing on partial discharge in oil-impregnated pressboard insulation

The AC (50 Hz) partial discharge (PD) characteristics of oil-impregnated pressboard (OIP) insulation are severely influenced by the DC voltage component in the converter transformer. Therefore, this study investigates the effect of different DC prestressing voltage (DCPV) amplitudes, prestressing time, and repetitions times of the applied AC voltage on the OIP insulation AC PD characteristics. Meanwhile, the space charge characteristics of distribution and dissipation are measured by pulsed electro-acoustic method. The experimental results show that the PD inception voltage (PDIV) and PD extinction voltage (PDEV) after DCPV were lower than that under AC voltage without DCPV. The PDIV decrease with higher DCPV at prestressing time from 30 to 60 min. But it increase with higher DCPV at prestressing time from 90 to 120 min. However, PDIV initially increase then decrease with the increasing repetition times of the applied AC voltage. The space charge distribution in 1 mm OIP insulation takes 60 min to reach a balanced condition. The residual space charge is relatively higher at a longer prestressing voltage time and higher DC voltage when the DC voltage was turn off. The PDIV and PDEV after DCPV is improved by Joule heating with longer prestressing time and higher DCPV. The space charge had negative effect on them.

The process of creeping discharge-caused damage on oil/pressboard insulation

This paper presents experimental research on creeping discharge by using cylinder-plate electrode configurations under AC voltages. The process of creeping discharge-caused damage on the oil/pressboard insulation was studied. First, the electric field distribution was achieved by Multiphysics software simulation. Afterwards, the phenomena that occurred in the entire damage process, such as ``white smoke'', ``white mark'', and ``black mark'', were recorded and analyzed. Furthermore, the micromorphology of the oil-impregnated pressboard was observed via scanning electron microscope (SEM). Finally, the inner mechanism of the damage to the oil/pressboard insulation was explored according to the phenomena and the SEM morphologies. Results showed that during damage processes, the high electrical field strength (nearly 25.853 MV/m) at the weak-link point between the cylinder electrode and the pressboard directly caused the incipient discharge. The cavity, moisture, impurity, solid particle, and formation of a gaseous channel all contributed to the development of the damage. The ``white smoke'' consisted of gases that stemmed from the ionization of oil and evaporation of moisture. The ``white mark'' was the gas channel and pressboard carbonization was caused by discharge and high temperature, both of which were also the main causes of the emergence of ``black mark''. SEM images revealed that the pressboard successively experienced ``white solid'', ``crack'', and ``pitting'', which changed its surface roughness. The distorted electric field caused by gases, solid particles, and pitting further damaged the oil/pressboard. The pitting evolved into the starting point of the electrical trees and gradually led to the final breakdown.

Understanding the surface discharge characteristics of thermally aged copper sulphide diffused oil impregnated pressboard material

An attempt has been made to understand the impact of thermal aging with copper sulphide diffused oil impregnated pressboard (OIP) material for surface discharge inception voltage (SDIV) variation. The SDIV of Copper sulphide diffused OIP material reduces with increase in thermal aging. The UHF signals generated due to surface discharges with copper sulphide diffused OIP at the point of inception have frequency content in the range 0.7-1.5 GHz with its dominant frequency at 0.9GHz. HFCT measurement indicated that the rise time of injected current pulse due to surface discharge activity in the negative and positive half cycle of the AC voltage are about 0.8ns and 1ns respectively. It is also found that increase in thermal aging of copper sulphide diffused OIP, the amplitude and width of the current pulse increases. The magnitude of surface charge and its mean life is high with thermal aging temperature of copper sulphide diffused OIP material. Polarity of surface charge acquired by OIP due to AC corona charging depends on the surface condition of the pressboard material and its charge decay rate is lower than under DC voltage. Atomic Absorption Spectroscopy (AAS) and Energy Dispersive Spectrometry (EDS) study confirms the presence of copper sulphide on the surface of OIP and is found to be increased for copper sulfide diffused OIP material compare to unused OIP material. Scanning Electron Microscope (SEM) analysis indicates formation of cracks with thermally aged OIP material. Phase Resolved Partial Discharge (PRPD) study indicates that the surface discharge activity occurs near the peaks of the applied AC voltage at inception and at the rising portion of the applied AC voltage at higher voltages. It is found that the tensile and flexural strength reduces drastically for thermally aged copper sulphide diffused OIP material compare to unused specimen.

Effects of thermal aging on creepage discharge in oil- impregnated pressboard under combined AC-DC voltage

In this study, we investigate the characteristics of creepage discharge on oil-impregnated pressboard under combined AC-DC voltage conditions using a sphere-plane electrode configuration. In our experiments, we use four oil-impregnated pressboard samples subjected to different thermal aging time. Experimental results show that the creepage discharge inception voltage (CDIV) and creepage discharge flashover voltage (CDFV) are lowest under pure AC voltage and gradually increase with higher DC voltage components. Longer aging time may contribute to higher CDIV with low DC voltage components; however, with higher DC voltage components, the effect of aging time is less pronounced. The CDFV is not significantly influenced by aging time. The length of carbonized tracks and the development velocity of the creepage discharge increase and the endurance time decreases with longer aging time and higher DC voltage components. We divide the development of creepage discharge in aged pressboard into four stages according to changes in the discharge characteristic parameters. With longer aging time, characteristic parameters exhibit less fluctuation and development velocity exhibits greater sensitivity to the DC voltage component. Our experimental results demonstrate that the characteristics of creepage discharge are significantly influenced by the age of the pressboard and by DC voltage components.