Cellulose Pressboard Research Articles

A new technique for recovering aged mineral oil to promote lifetime and sustainability

Using mineral oil (MO) derived from petroleum, e.g., transformer insulation, raises environmental risk and affects sustainability. Due to its affordability and easy availability, MO will ineluctably be preferred over other biodegradable oils. Therefore, tools must be developed to recover aged MO, extend the life cycle, and increase its sustainability. The work presented here is aimed at the investigation of the electrical insulation properties of thermally-aged recovered-aged mineral oil (RAMO), as well as of aged cellulose pressboard impregnated by fresh mineral oil (FMO), aged mineral oil (AMO), and RAMO. The AMO is recovered using the combined adsorbents, A* and B*, which are Chitosan-activated Bentonite (CTN-aB) and Polyaniline coated Kapok Fibre (PANI-c-KPF) mix. According to FTIR, the recovered oil, RAMO, displays less C=C double bonds and carbonyl vibrations at 140°C, indicating lower oil ageing compared to AMO. The distribution of the crystalline and amorphous peaks is uniform for aged oil-impregnated pressboard (PB) with FMO and RAMO. Overall, it has been discovered that PB is impregnated with RAMO, resulting in an extended lifetime for the solid insulation. As a result, the increase in emissions slows down, and global sustainability targets are met.

Electrical Aging Effect on PD Characteristics of Cellulose Pressboard Modified by Nano-TiO2

Cellulose pressboard has been widely used as internal insulation in oil-immersed transformers. Partial discharge (PD) is the main cause of the degradation of the insulation properties for electrical equipment. Nano-modification is one of the methods to improve the electrical properties of the oil–paper insulation. In this article, the effects of the oil-immersed pressboards modified by nano-TiO2 particles on PD characteristics were studied. The pressboards modified by different mass fractions of nano-TiO2 (10-nm particle size) were prepared. The electrical aging experiment was carried out on the oil-immersed pressboard. Experimental platforms for partial discharge inception voltage (PDIV) and relative permittivity were established to measure the dielectric performance of the pressboard. Phase-resolved partial discharge (PRPD) spectrograms and PD current pulses of the samples under different electrical aging states were obtained for different PD time. The surface morphology of the pressboards was observed by scanning electron microscope (SEM) to verify the effects of nano-TiO2 particles. The results showed that the values of electric field strength for the samples were 4.58 and 3.6 kV/mm, respectively, when the PD occurred in the samples modified by nano-TiO2 with 3 wt% and the unmodified ones. Compared with the unmodified pressboard, the modified one had a lower value of relative permittivity. The pulse current of the pressboard modified by 3 wt% nano-TiO2 particles under different electrical aging states was smaller than that of the other samples. The addition of 3 wt% nanoparticles was the optimum ratio to suppress the PD of the samples in the ac electrical field.

Influencing Mechanism of Adhesive on Partial Discharge of Pressboard Block under AC/DC Composite Voltage

The phenolic adhesive and casein adhesive are two kinds of adhesives used in pressboard block production. The partial discharge characteristics of pressboard block containing the two types of adhesives under AC, DC and AC/DC composite voltage are researched in this paper. Using the block samples with phenolic adhesive or casein adhesive, the partial discharge characteristics, flashover characteristics and the influence of the adhesive layer orientations were obtained. The results show that the use of adhesive may introduce a weak zone of insulation performance and reduce the insulation strength of the pressboard block. There exist polar effects on partial discharge and flashover characteristics under DC voltage, and there is no significant difference in the partial discharge characteristics and flashover voltages between the pressboard blocks with casein and phenolic adhesive. The phenomena are analyzed in terms of molecular characteristics of adhesives and cellulose pressboard. The research results of this paper provide significant theoretical and practical reference for the optimization of transformer insulation materials.

Thermal Aging Effect on Properties of Pure and Doped Nano-TiO2 Cellulose Pressboard

In this research, nano-TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> of various diameters is added to cellulose and pressboard is made to enhance its anti-thermal aging characteristics. After thermally aging of the pressboard samples, partial discharge inception voltage (PDIV), AC breakdown strength and relative permittivity are measured. The results show that the dielectric properties of the pressboard made with 10 nm TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> has the highest AC breakdown due to closer match of the relative permittivity. The mechanical properties are higher than the unfilled cellulose samples. The change observed of the surface microstructure of the samples show that aging resistance of the filled samples are better than the unfilled samples.

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.

Quantitative aging assessment method for cellulose pressboard based on the interpretation of the dielectric response mechanism

Cellulose pressboard is used as a key element in various power equipment insulation structures and determines the operational safety of electric equipment and power systems. This paper aims to increase the application of a pressboard aging assessment method based on frequency domain spectroscopy, a promising nondestructive method for power equipment diagnosis, through in-depth theoretical analysis and accurate data fitting. To achieve this purpose, first, based on classic theoretical models in the field of dielectric physics, a mathematical deduction is proposed to separate each dielectric process in the overlapping frequency band. Second, a novel analysis approach that considers both the pressboard dielectric response and the thermally stimulated current is proposed and demonstrated to be effective in determining the physical meaning of pressboard microscopic dielectric processes. Third, a function model based on a specific microscopic mechanism is established to fit the frequency domain spectra of pressboard samples with different aging degrees. Based on this approach, the variations in charge carrier motion characteristics in the frequency domain with increasing pressboard aging degree are clarified. Moreover, the function model provides characteristic parameters that are clearly related to the degree of polymerization of the material, which represents a quantitative method for evaluating the aging degree of cellulose pressboard by indirectly obtaining its frequency response curve.

Effects of thermal aging on cellulose pressboard using terahertz time-domain spectroscopy

We demonstrate a method for examining the effects of thermal aging of cellulose pressboard using terahertz time-domain spectroscopy. The change of refractive indexes at the frequencies of pulsed terahertz waves while applying heat reveals that the degree of thermal aging and the condition of thermal breakdown can be clearly analyzed by performing terahertz measurements. The visual examination method only allows evaluation of the degree of blackness on the external surfaces of cellulose pressboard. However, using terahertz techniques for non-destructive inspection enables determination of the dielectric integrity to check the strength of the electrical insulation inside the cellulose pressboard. The results indicate that the terahertz techniques could be very useful, especially in the electric power industry, which requires a variety of methods for internal non-destructive testing to ensure the safety and reliability of insulation materials in electric power facilities and components.

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.

Improved method for measuring moisture content of mineral-oil-impregnated cellulose pressboard based on dielectric response

The moisture content in transformer insulation pressboards is an important index to evaluate the insulation aging in the transformer. The dielectric-response technique based on frequency domain spectroscopy is used to accurately measure the moisture content in transformer insulation pressboard. In this study, according to the distribution of water molecules in oil-impregnated cellulose pressboard and the polarization behaviour of water molecules at different frequencies, a theoretical calculation method for moisture content in a composite model of cellulose pressboard, mineral oil, and moisture based on a typical transformer structure is proposed. To verify the effectiveness of the numerical iterative calculation method further, a typical structure model of a transformer with different proportions was prepared under laboratory conditions of natural moisture absorption and accelerated aging. Three groups of comparison tests were done. In the stage temperature rise test (40–90 °C), the moisture contents are compared and analysed by the method proposed in this research and the IDAX-300 software fitting method. It was found that the fitting of the dielectric loss factor curve is more accurate in the high-temperature and low-frequency region. In a room temperature (25 °C) test, the moisture contents obtained by the method proposed in this study, the IDAX-300 instrument test method, and the traditional Karl–Fischer titration method are compared and analysed. After testing the XY model with different moisture absorptions at 25 °C, it was found that the accuracy of the algorithm is higher. Testing the XY model at different degrees of aging at 25 °C, showed the effect of the aging of the oil-impregnated cellulose pressboard. The selected fitting frequency range should be (10−1–103) Hz, and the accuracy of the numerical iterative calculation method is high.

Comparative analysis of cellulose pressboard and aramid paper used in air insulation systems of high-voltage devices

The paper presents results of a comparative analysis of parameters of two kinds of solid dielectrics used in air insulation systems to prevent occurring partial discharges. The research works regarded materials made of: cellulose pressboard and aramid paper. All measurements were performed under laboratory conditions by changing the value of partial discharges generation voltage until breakdown occurred in the inhomogeneous environment that was simulated using needle-plate (made of copper) electrode system. The main contribution which resulted from studies is a statement that potential use of aramid paper as a dielectric can extend the life of a high voltage electric device as compared to standard cellulose pressboard usage due to higher electric resistances to breakdown or detection of corona voltage. Results shown that the aramid paper has greater electric resistance to breakdown in comparison to cellulose with no difference between both on detecting corona of partial discharge.

Preventive Measurement and Online Insulation Evaluation of Power Transformer

Cellulose based non conducting materials and mineral oils have widely been used in liquid filled transformers. Environmental and fire-safety concerns have accelerated the use of Natural Esters (NE) as a sustainable alternative to mineral oil. To ensure a safe and long-term operation of the vegetable oil filled transformers, it is need to clarify the knowledge gaps with respect to the ageing of vegetable oil impregnated cellulose and insulation diagnostic methods. The condition monitoring of mineral oil filled transformers has been well characterised, due to the numerous laboratory and field studies conducted during the past several decades. The application of these available condition monitoring techniques for NE filled transformers is yet to be validated as the use of NE for power transformers is in the tested stage. In this paper the performance of NE as insulation in transformers by evaluating the ageing of NE impregnated cellulose pressboard is presented. For this purpose, an accelerated ageing experiment has been carried out in sealed tubes at 120 ⁰C. The ageing of oil impregnated pressboard has been characterised by decrease in the degree of polymerisation (DP) of pressboard, and the formation of both 2-furfuraldehyde and dissolved carbon dioxide (CO2) in the oil. The structural changes of cellulose have been supported by fuzzy logic algorithm. Moreover, the results have been compared with cellulose insulation samples aged under similar conditions in conventional mineral oil.

INVESTIGATION ON THE DEGRADATION BEHAVIOR OF CREEPAGE DISCHARGE ON PRESSBOARD IMMERSED IN PALM FATTY ACID ESTER (PFAE) OIL

Creepage discharge at the oil-pressboard interface is known as the development of a conducting path which is characterized by white and carbonized marks. This phenomenon tends to cause damage on the cellulose pressboard insulation which is subsequently promote to the catastrophic failure of a transformer. One of major defects that may cause the creepage discharge to occur along the pressboard surface is an excessive moisture content in pressboard insulation. Previously, there is an extensive research concerning the effect of moisture content in pressboard on the degradation of creepage discharge at mineral oil-pressboard interface. However, none has been found on the study of the effect of moisture content in pressboard on creepage discharge using ester oil. Therefore, this paper attempts to present the investigation on the degradation behavior of creepage discharge at Palm Fatty Acid Ester (PFAE) oil-pressboard interface by using a needle-bar electrode configuration under a constant AC voltage. Dry and wet pressboard samples (moisture content of 3 %) are used in this experiment to differentiate the degradation behavior. The development of creepage discharge is analyzed by correlating the visual records of creepage discharge and phase-resolved partial discharge (PRPD) pattern. An unexpected result has been found that the number of partial discharges (PDs) for wet pressboard is lower than the dry one as the formation of white mark with bush-like pattern is observed. Eventually, as the white mark is seen about halfway of the gap distance, the PDs number for wet pressboard sample significantly dropped, i.e. by 99 % compared to the dry board which is only 38 %. Variations in the PRPD data is due to constructive and destructive superpositions of electric field. The results suggest that the destructive effect is higher on the wet pressboard compared to the dry one.

Understanding the ageing aspects of natural ester based insulation liquid in power transformer

Cellulose based insulation materials and mineral oils have widely been used in liquid filled transformers. Environmental and fire-safety concerns have accelerated the use of Natural Esters (NE) as a sustainable alternative to mineral oil. There is a rapid growth of their application for distribution level transformers. To ensure a safe and long-term operation of the vegetable oil filled transformers, it is necessary to clarify the knowledge gaps with respect to the ageing of vegetable oil impregnated cellulose and insulation diagnostic methods. The condition monitoring of mineral oil filled transformers has been well characterised, due to the numerous laboratory and field studies conducted during the past several decades. The application of these available condition monitoring techniques for NE filled transformers is yet to be validated as the use of NE for power transformers is in the evaluation stage. In this paper the performance of NE as insulation in transformers by evaluating the ageing of NE impregnated cellulose pressboard is presented. For this purpose, an accelerated ageing experiment has been carried out in sealed tubes at 120 °C. The ageing of oil impregnated pressboard has been characterised by decrease in the degree of polymerisation (DP) of pressboard, and the formation of both 2-furfuraldehyde and dissolved carbon dioxide (CO2) in the oil. The structural changes of cellulose have been supported by Fourier Transform Infrared (FTIR) spectra. Moreover, the results have been compared with cellulose insulation samples aged under similar conditions in conventional mineral oil. Acidity value, dissipation factor and viscosity measurements have been used to compare the stability of NE in a transformer operating environment to mineral oil.