Cellulose Insulation Research Articles

Experimental Validation of a Method of Drying Cellulose Insulation in Distribution Transformers Using Circulating Synthetic Ester

The article concerns a method of drying cellulose insulation of distribution transformers with the use of synthetic ester. This method uses the high solubility of water in the ester compared to other dielectric liquids. The method is based on the striving of the oil-paper insulation system for a state of moisture equilibrium. Water migrates from the cellulose insulation of the high moisture content to the synthetic ester which is subjected to continuous drying. The research was carried out on a complex laboratory model reflecting the transformer insulation system. In this model, the drying of the ester was carried out using a molecular sieve with an appropriately selected adsorber weight. To check the drying efficiency of the cellulose insulation, the water content in the pressboard strips taken from the model, both before and after the drying process, was determined. The water content was measured using the Karl Fischer titration method. The research showed the high efficiency of the proposed method of drying. With the ester moisture in the range of 110-130 ppm and the insulation system temperature of about 70 °C, the loss of water in the samples dried for 7 days was over one percentage point. The obtained test results constitute the basis for the validation of the method on distribution transformers.

Precise Measurements of the Temperature-Frequency Dependence of the Conductivity of Cellulose—Insulating Oil—Water Nanoparticles Composite

This article presents direct σDC and alternating σ(f) current conductivity measurements obtained by the frequency domain spectroscopy (FDS) method on cellulose-transformer oil–water nanoparticle composite with a moisture content of (5.0 ± 0.2)% by weight in a temperature range from 293.15 to 333.15 K with step of 8 K. The uncertainty of temperature maintenance during measurements was below ±0.01 K. The sample was prepared for testing in a manner as close as possible to the cellulose insulation moisturizing process in power transformers. For the analysis of the results obtained, a model of alternating and direct current hopping conductivity was used, based on the quantum phenomenon of electron tunneling between the potential wells and nanodrops of water. It was observed that on the d(logσ)/d(logf)-derived waveforms there was a clear low-frequency maximum, and a tendency to reach the next maximum in the high-frequency area was visible. On this basis it was established that the increase in conductivity takes place in two stages. It was found that the position of σ(f) waveforms in the double logarithmic coordinates is influenced by the temperature dependence both of the conductivity and of the relaxation time of the conductivity. These relationships are described with the appropriate activation energies of the conductivity and relaxation time of conductivity. Based on the analysis of experimental data using Arrhenius diagrams, average values of the activation energy of conductivity ΔWσ ≈ (0.894 ± 0.0134) eV and the relaxation time of conductivity ΔWτσ ≈ (0.869 ± 0.0107) eV were determined. The values were equal within the limits of uncertainty and their mean value was ΔW ≈ (0.881 ± 0.0140) eV. Using the mean value of the activation energy, the frequency dependence of conductivity, obtained at different temperatures, was shifted to 293.15 K. For this purpose, first the waveforms were shifted along the horizontal and then the vertical axis. It was found that after the shift the σ(f) waveforms for the different temperatures overlap perfectly. This means that the shape of the frequency dependence of the conductivity is determined by the moisture content of the pressboard. The position of the waveforms in relation to the coordinates is determined by the temperature relationships of the conductivity and the relaxation time of the conductivity.

Investigation on Formation Mechanisms of Methanol During Cellulose Insulation Aging Based on Molecular Dynamics Simulation

In recent years, methanol has been proposed as a chemical indicator to assess the aging condition of cellulose insulation in oil-immersed power transformers. However, the formation mechanisms of methanol during cellulose degradation are not clear enough. In this paper, such formation mechanisms were studied using molecular dynamics simulation. Three main formation pathways of methanol were found and discussed. Analysis of the Mayer bonds of cellulose revealed that each atom of cellulose contributes differently to methanol formation. The methylol groups on the 5-carbon atom of the 1-pyran ring and 4-pyran ring of cellulose are more readily detached to form methanol molecules. In addition, changes in the amount of methanol molecule with simulation time were investigated. The results indicated that the separate existence of initial moisture and oxygen has no significant effect on the formation of methanol, while the co-existence of initial moisture and initial oxygen catalyzes the conversion of cellulose degradation products to methanol. The findings reported in this paper can provide a valuable theoretical basis for further studies on methanol as an indicator to evaluate the residual life of cellulose insulation.

Frequency Domain Spectroscopy Prediction of Oil-Immersed Cellulose Insulation under Diverse Temperature and Moisture

A novel model for predicting the frequency domain spectroscopy (FDS) curve is reported in this work, which is capable of analyzing the synergistic effect generated by temperature and moisture. The innovation of this work is an exploration of the reported model as a potential tool for predicting the FDS (dispersion loss/permittivity) curves of oil-immersed cellulose insulation under diverse temperatures and moisture. In that respect, the verification experiments indicated that the average relative error of the prediction results in lab conditions is less than 17%.

Molecular simulation on the mechanical and thermal properties of carbon nanowire modified cellulose insulating paper

The quality of transformer insulation is related to the safe and stability operation of the power grid, and the life of the transformer oil-paper insulation system mainly depends on the state of paper insulation. With the continuous improvement of the voltage level, the requirements for insulating paper are becoming higher and higher. Based on the molecular dynamics simulation technology, the effects of carbon nanowires with three different structures on the thermodynamic properties of cellulose insulating paper were analyzed from the aspects of mechanical parameters, MSD (mean square displacement), CED (cohesive energy density) and hydrogen bond. The simulation results showed that the carbon nanowires with three different structures have improved the thermodynamic properties of cellulose. Among them, the hard chiral carbon nanowires have the most significant improvement in the thermodynamic properties of cellulose. The elastic modulus, bulk modulus, and shear modulus have been increased by 30%, 25.6%, and 32.9%, respectively. It was found that the thermal stability of cellulose insulation paper modified by three different structure carbon nanowires was improved.The increase of thermal stability was mainly due to the increase of the cohesive energy density and the number of intermolecular hydrogen bonds.

Synergistic enhancement effect of moisture and aging on frequency dielectric response of oil-immersed cellulose insulation and its degree of polymerization evaluation using dielectric modulus

Oil-immersed cellulose insulation is the heart of the power transformer, which will will gradually deteriorate in the operation process of transformer. Therefore, accurately evaluating the aging condition of the oil-immersed cellulose insulation is important for maintaining safe and stable operation of transformer. In this paper, the synergistic effect of moisture and aging on frequency dielectric spectroscopy (FDS) of oil-immersed cellulose insulation was analyzed in depth using the modified cole–cole model. A novel degree of polymerization (DP) evaluation method for oil-immersed cellulose insulation based on the dielectric modulus analysis and its curve translation method was proposed. Results show that the higher the moisture content and the lower the DP, the greater the frequency dielectric parameters, the smaller the α relaxation time, the greater the DC conductivity and hopping conductivity of the oil-immersed cellulose insulation. The novel DP evaluation method for oil-immersed cellulose insulation based on calculating the integral area of the imaginary part of dielectric modulus A(DP–mc%) and translating the A(DP–mc%) curve was proposed, which could simultaneously consider the synergistic effect of moisture and aging on FDS. The novel DP evaluation method for the oil-immersed cellulose insulation is suitable for moisture range is 0.5–5%, and the DP range is 250–1300. The verified effectiveness of the novel DP evaluation method presents that it is ideal for transformer aging condition assessment, which is superior to the existing evaluation methods.

Power transformer cellulosic insulation destruction assessment using a calculated index composed of CO, CO2, 2-Furfural, and Acetylene

Power transformer insulation is degraded and decomposed over time. Due to the importance of cellulose, indicators such as 2-Furfural, CO, CO2, and CO2/CO ratio are used to investigate cellulose aging status because these are cellulose degradation products. However, various studies have shown that using each of these indicators alone may lead to incorrect cellulose degradation diagnosis. Therefore, a new calculated index called CFA index (an abbreviation of Carbon, Furfural, and Acetylene) that considers the effects of four indicators (CO, CO2, 2-furfural, acetylene) together will be presented in this paper. The CFA index has been obtained from the analysis of 216 power transformer measurement data using the correlation coefficients calculation. The reason for considering acetylene is the importance of its production and the high correlation coefficient between acetylene and transformer age. In this method, if the transformer has the CFA index between −2 and −5.3, then it can be ensured that the transformer does not suffer from severe cellulose aging or decomposition. The normal range for the CFA index is obtained from investigating data from many power transformers, all with CO2/CO ratio between 3 and 10. Finally, to verify the accuracy of the CFA index and its ability to detect cellulosic insulation degradation and the inefficiency of other methods in some cases, dissolved gas analysis (DGA) test results of 306 power transformers will be investigated, and results will be fully presented.

Thermal ageing study of ZnO nanofluid–cellulose insulation

Trends in the studies of mineral oil (MO)-based nanofluids (NFs) show that most of the conducted works have focused only on the thermal and dielectric properties but few numbers on the ageing performance. In the present study, ZnO NF, in combination with cellulose insulation experienced accelerated thermal ageing at 120 °C for 20 days to study the ageing performance and it was compared with that of MO–cellulose insulation. The deterioration rate of cellulose was evaluated through tensile strength, breakdown voltage (BDV) and dielectric dissipation factor properties. Whereas oils deterioration was evaluated through BDV, interfacial tension, kinematic viscosity, acidity and colour. The results demonstrate that for cellulose aged in NF (NFIP), the tensile strength and BDV are 3 and 6.9% higher, respectively; than those aged in MO. For aged oils, NF exhibits higher values of the viscosity and acidity by 3 and 33.3%, respectively, than MO. The BDV of NF is superior to that of MO in the initial ageing period, after that; it shows a lesser reduction tendency with ageing. The most important observation from this study is that despite this increment of ageing indicators for NF, it could improve the anti-ageing properties of cellulose insulation.

Fire resistance of wood stud wall assemblies

SummaryThis paper presents the fire resistance results of seven full‐scale wall assemblies that were conducted at the National Research Council of Canada (NRC) jointly with the North American construction industry and other Canadian government departments. Parameters investigated for protected interior wall assemblies with a single‐row of wood studs included: insulation types and mid‐height blocking. Parameters investigated for protected interior wall assemblies with a double‐row of wood studs on a separate plate included: insulation types, wood studs spacing and mid‐height blocking. The impact of the investigated parameters on the fire resistance of wall assemblies is discussed. Results of the parameters studied showed that the installation of mid‐height blocking in wall assembly with a single‐row of wood stud and filling the wall cavities with cellulose insulation in wall assembly with a double‐row of wood studs improved the fire resistance by 10 to 15 minutes, respectively. However, the other parameters studied had improved the fire resistance slightly by up to 6 minutes.

Analysis of factors affecting the effectiveness of drying cellulose materials with synthetic ester

This article deals with the application of synthetic ester for drying cellulose insulation of distribution transformers. The most important advantages and disadvantages of the commonly used methods of drying transformer insulation on site are discussed. The emphasis is placed on the analysis and research of the most important factors affecting the drying efficiency of cellulose insulation using a synthetic ester, such as: insulation temperature, water content in ester, weight ratio of cellulose materials to ester, thickness of pressboard, and water content in solid insulation. The choice of values range of the individual factors which are used during tests on drying efficiency in laboratory conditions are justified. It is found on the basis of the obtained test results that to ensure the satisfactory drying effect an average insulation temperature should be at least of 70°C and the water content in the ester should not exceed 150 ppm. The results give the opportunity to estimate the drying efficiency depending on the conditions of the process. Moreover, the results are used to develop the procedure of drying of distribution transformers by means of synthetic ester.

Effect of a cold delignification prior to determination of the IEC 60450 viscometric degree of polymerization of cellulosic insulation materials

The objective of this work is to propose a sample preparation procedure implementing a preliminary cold delignification (CD) for the IEC 60450 DPv determination in order to be able to dissolve any sample independently of its history (new, dried, or aged). This work shows that a cold delignification enables the full dissolution of samples known to be reluctant to it. The cold delignification is compared to other strategies known to ease the dissolution as a mechanical pretreatment, increasing the time of dispersion in water or of dissolution in the solvent. The comparison is conducted on new cellulosic pulp samples. A cold delignification enables the full dissolution of the sample after only 30 minutes of dissolution in the solvent and induces a DPv loss of 2.3%. On the other hand, extending the dissolution time in the solvent from 30 minutes to 4h leads to a DPv loss of 13%.

Measurements and performance evaluations of natural ester and mineral oil-immersed identical transformers

The use of natural esters as an insulating liquid has become popular since the early 2000s. Studies in the literature emphasize the superiority of natural esters as compared to mineral oils. In these studies, the prominent properties of natural esters are presented as being environmentally friendly, safe, and delaying the aging of cellulosic insulation material. Although there is no doubt in terms of their characteristics, such as being environmentally and safe, exaggerated comments are made about the delay of aging of the paper material used in the insulation of transformer windings. This is because comparative experimental applications on mineral oil and natural esters are carried out under artificial and equal conditions provided in the laboratory environment. Whereas, performing these experiments on identical transformers will give more realistic results. In this study, two identical transformers were designed and equipped with precision fiber optic temperature sensors. Temperature differences that vary depending on the load between high voltage and low voltage windings were measured with a high degree of accuracy for both identical transformers. Besides these measurements, physical and chemical changes were analyzed on samples taken from insulation liquids. It was observed that the physical and chemical properties of the insulation liquids directly affect the operating performance of the transformers. Thanks to the differences in viscosity of insulating liquids, the heat dissipation due to losses in the windings were different for both transformers. Both heat distribution and temperature differences in the windings varied depending on the loading factors. Considering all these factors together, comprehensively evaluations were made on the aging rate of cellulosic insulation and the service life of the transformers.

Multi-Objective Optimization of Building Life Cycle Performance. A Housing Renovation Case Study in Northern Europe

While the operational energy use of buildings is often regulated in current energy saving policies, their embodied greenhouse gas emissions still have a considerable mitigation potential. The study aims at developing a multi-objective optimization method for design and renovation of buildings incorporating the operational and embodied energy demands, global warming potential, and costs as objective functions. The optimization method was tested on the renovation of an apartment building in Denmark, mainly focusing envelope improvements as roof and exterior wall insulation and windows. Cellulose insulation has been the predominant result, together with fiber cement or aluminum-based cladding and 2-layered glazing. The annual energy demand has been reduced from 166.4 to a range between 76.5 and 83.7 kWh/(m2 y) in the optimal solutions. The fact that the legal requirements of 70 kWh/(m2 y) are nearly met without building service improvements indicates that energy requirements can be fulfilled without compromising greenhouse gas emissions and cost. Since the method relies on standard national performance reporting tools, the authors believe that this study is a preliminary step towards more cost-efficient and low-carbon building renovations by utilizing multi-optimization techniques.

Study on non-isothermal moisture transfer characteristics of hygroscopic building materials: From parameter characterization to model analysis

The heat and moisture transfer in building materials has significant effects on the energy consumption and indoor thermal comfort. In this paper, an improved heat and moisture transfer model based on temperature gradient coefficient (TGC) and phase change criterion (PCC) was built. The effects of relative humidity and temperature on liquid water conductivity, moisture diffusivity and TGC were considered and discussed. A new moisture transfer characteristic index called the temperature gradient factor (TGF) was proposed. It presents the ratio between the moisture flux due to temperature gradient and the total moisture flux. The PCC was introduced to quantify the significance of the vapor flux relative to the total moisture flux. Based on the improved model and two moisture transfer characteristic indexes (i.e., TGF and PCC), two series simulations were conducted to investigate the non-isothermal moisture transfer characteristics of cellulose insulation. The results showed that the liquid water conductivity was independent of temperature. The liquid water transfer could be ignored when the relative humidity in the cellulose insulation was less than 60%. Also, the liquid water transfer due to temperature gradient could be ignored when the relative humidity was larger than 60%.

Investigation on the Interaction between Cellulosic Paper and Organic Acids Based on Molecular Dynamics.

Organic acid is an important factor that accelerates the aging of cellulosic insulation materials. In this study, the interactions between cellulose and five acids, representative of what may be found in an aging transformer, were studied using molecular dynamics. The adsorption process of the five acids onto the surface of crystalline cellulose shows that the three low molecular acids are more readily adsorbed onto cellulose than the two high molecular acids. The deformation and adsorption energies of the acids all increase with an increase in molecular weight when they are stably interacting with cellulose. However, the differences between adsorption energies and deformation energies are positive for the three low molecular acids, whereas they are negative for the two high molecular acids. This indicates that the attachments onto cellulose of low molecular acids are considerably more stabilized than those of the high molecular acids. This is consistent with the experimental results. Furthermore, based on the calculated solubility parameters of acids, the experimental result that the three low molecular acids are to a large degree absorbed onto the cellulose, whereas the two high molecular acids remain in the oil, was theoretically elucidated using the theory of similarity and intermiscibility.

Performance Assessment of Oil-Immersed Cellulose Insulator Materials Using Time–Domain Spectroscopy under Varying Temperature and Humidity Conditions

The measurement of polarization and depolarization currents (PDC) based on time–domain response is an effective method for state assessment of cellulose insulation material in oil-immersed electrical equipment. However, the versatility of the data obtained at different temperatures is limited because of the temperature dependence of the PDC. In this respect, the universal conversion of PDC data at different temperatures is an essential aspect to improve the accuracy of the determination of insulating properties of cellulose materials immersed in the oil. Thus, an innovative temperature conversion method based on polarization time-varying current (PTC, obtained by multiplying the polarization current and time) is proposed in this article. In the current work, the PTC data at different temperatures are obtained from the oil-immersed cellulose pressboards with different moisture. Afterwards, the functional model based on the power series theory is used to simulate the PTC data, through which the coefficients of the power series are found related to the test temperature of the PTC and the moisture content (mc%) of the oil-immersed cellulose pressboards. Furthermore, the functional relationship among moisture, test temperatures, and the feature parameter calculated by these coefficients is established. Thus, the PTC data at various temperatures can be calculated by the established function. The potential application ability of the proposed method is verified by comparing the calculated results with the measured results obtained from the various samples.

Prediction of Moisture and Aging Conditions of Oil-Immersed Cellulose Insulation Based on Fingerprints Database of Dielectric Modulus.

Frequency-domain spectroscopy (FDS) is demonstrated to be affected by electrode polarization and conductance behavior in the low-frequency ranges, which causes the unreliable prediction results of transformer cellulose insulation. In order to solve this issue, a fingerprint database based on the dielectric modulus is reported to predict the degree of polymerization (DP) and moisture content of cellulose insulation. In the current work, the relevant fingerprints that characterize the insulation conditions are obtained by studying the dielectric modulus curves of cellulose insulation with various insulation conditions, as well as the DC conductivity of transformer oil. Then, the dielectric modulus fingerprint database is established in the lab, and the accuracy of the reported fingerprint database is later verified. As a potential tool, the dielectric modulus fingerprint database is tested by several samples, and the results demonstrate that the accuracy of this method is more than 80%. In that respect, an interesting discovery of this paper is that the dielectric modulus fingerprint database may be a helpful tool for conditions prediction of the transformer cellulose insulation system.

Optimization of Ethanol Detection by Automatic Headspace Method for Cellulose Insulation Aging of Oil-immersed Transformers.

The method using ethanol to evaluate the cellulose insulation aging condition of oil-immersed transformers has been proposed. At present, the dominating method for detecting ethanol in insulating oil is to use headspace–gas-chromatography–mass-spectrometry (HS-GC-MS). However, the problem of quantitative inaccuracy will be sometimes encountered in the actual detection process due to improper instrument parameter setting and improper manual operation. In this study, as an aging marker, ethanol in transformer insulating oil was separated by using VF-624 ms capillary column. The effects of gas-chromatography–mass-spectrometry (GC-MS) optimization conditions, headspace equilibrium temperature, headspace equilibrium time and standard solution preparation method on the determination of ethanol content in oil were discussed, and optimized measures were proposed. The experimental results showed that the measurement can be more accurate under the headspace temperature of 80 °C and the headspace time of 40 min, and relative standard deviation percentage (RSD%) could reach to 4.62% under this condition. It was also pointed out that, for the preparation of standard solution, the method which controlled the sampling volume of anhydrous ethanol by microliter syringe could make the peak area of ethanol chromatogram have a better linear relationship with the standard curve. Under the similar linear range, the goodness of fitting curve without diluting process could be as high as 0.9993, while the method of preparing the stock solution and diluting stepwise to obtain the fitting curve only had a goodness of 0.9910. The method was validated by standard addition recovery test, and the recovery values obtained were between 90.3% and 95.8%. The optimized method is of great significance for the measurement of ethanol dissolved in insulating oil.

Laboratory Model Studies on the Drying Efficiency of Transformer Cellulose Insulation Using Synthetic Ester

This paper presents the results of laboratory tests of cellulose insulation drying with the use of synthetic ester. The effectiveness of the drying process was investigated depending on the initial moisture of cellulose samples (2%, 3%, and 4%), ester temperature (55, 70, and 85 °C), initial moisture of the ester (70, 140, and 220 ppm), drying time (48, 96, and 168 h), and the weight ratio of cellulosic materials to ester (0.067 and 0.033). A large influence of temperature and time of drying on the efficiency of the drying process was found. This is important information due to the application of the results in the transformers drying procedure. The heating and drying ester unit should provide the highest possible temperature. For the assumed experiment conditions the initial moisture of the ester had little effect on the drying efficiency. An ester with a moisture content below 140 ppm can still be considered as meeting the requirements for drying cellulose with significant moisture. The weight ratio of cellulose products to ester has no major effect on drying efficiency during cellulose drying by circulating dry ester.

Failure mechanism of transformer oil-immersed cellulosic insulation induced by sulfur corrosion

The life of transformer oil-paper insulation is determined by the insulation property of transformer cellulosic insulation. Existing studies have shown that sulfur corrosion poses a serious threat to the properties of transformer oil-immersed cellulosic insulation due to the reaction of corrosive sulfur dissolved in oil with copper winding. This paper investigated the effect of sulfur corrosion on the characteristics of transformer cellulosic insulation, cellulosic insulation winding samples with different degrees of sulfur corrosion were obtained through an accelerated thermal aging experiment. Then, short-/long-term withstand voltage tests for the cellulosic insulation winding were conducted. Experimental results showed that the contamination of sulfur corrosion had a minimal effect on the power–frequency breakdown voltage of the cellulosic insulation winding but considerably decreased its long-term electrical life. Combined with the analysis of the electrical/physical and chemical parameters of oil-immersed cellulosic insulation, the failure mechanism of transformer oil-immersed cellulosic insulation induced by sulfur corrosion was synthetically discussed and analyzed.