Transformer Oil-paper Insulation System Research Articles

Bubble Formation and Motion Characteristics in the Transformer Oil-Paper Insulation System Under the Extremely Inhomogeneous Electric Field

Bubble Formation and Motion Characteristics in the Transformer Oil-Paper Insulation System Under the Extremely Inhomogeneous Electric Field

Effect of doped nano-SiO2, Al2O3, and polyhedral oligomeric sesquisiloxane on the thermodynamic properties of cellulose

Cellulose insulation paper is an essential part of the transformer oil-paper insulation system, and many types of nano-modified insulating paper are available. For studying the effects of various nano-modified molecules on cellulose and the differences in performance, the pure cellulose model and polyoctaaminophenylsesquioxane and grafted 3-aminopropyltriethoxysilane (KH550) SiO2 and Al2O3 cellulose models were developed using molecular dynamics methods, and the mechanical properties and thermally stability were calculated and compared. The findings show that, in general, all three nano-modification models improved the thermal stability and mechanical properties, with nano-SiO2-KH550 cellulose having the best overall modification effect. In terms of mechanical properties, the nano-SiO2-KH550 cellulose showed the largest increase in compressive strength, with an 18.1% improvement. In terms of thermal stability, the KH550 grafted nano-silica/cellulose composite model showed the best modification effect, as evidenced by the smaller cohesive energy density and free volume region compressing the space between cellulose molecules, making the modified cellulose more compact internally, thus achieving the effect of inhibiting the movement of cellulose chains.

Dissolved Gas Analysis for Power Transformers within Distributed Renewable Generation-Based Systems

In this paper, a series of laboratory experiments are conducted to investigate the effect of momentary small variations in the transformer operating temperature on the dissolved gas analysis (DGA) measurement. With the increased penetration level of renewable energy sources of intermittent characteristics into electricity grids, operating power transformers are expected to experience frequent temperature variations. Sampling transformer oil during such temperature variation leads to inaccurate diagnosis. Experimental results reveal that gas evolution in transformer oil is greatly affected by the small variations in the operating temperature. Such small variation can be a result of the intermittent generation characteristics of renewable energy sources. Hence, false analysis may be reported if oil is sampled during generation or load fluctuation events. Experimental results are explained through chemical equilibrium constant theory, which indicates that dissolved gases reflect the change in aging rate of the transformer oil-paper insulation system. These results suggest a new paradigm for DGA process through correlating measurements with the transformer operating temperature through the generation and load profiles at the instant of oil sampling.

Aging evaluation for transformer oil-immersed cellulose insulation by using frequency dependent dielectric modulus technique

Frequency domain spectroscopy (FDS) technique is widely used for the aging evaluation of the transformer oil-paper insulation system. However, the typical ‘relaxation information’ of FDS data in low-frequency regions is seriously affected by both the electrode polarization effect and conductance effect, which often leads to an unsatisfactory aging condition evaluation result. Existing researches reveal that the Frequency Dependent Dielectric Modulus M*(ω) technique can be utilized for solving this issue. Since the relevant parameters, that aims to represent the aging condition of transformer oil-immersed cellulose insulation, extracted from M*(ω) has been rarely reported, and the feasibility demonstration of the field testing by exploiting the relevant parameters extracted from M*(ω) is also rare. Therefore, the novel attempt for obtaining the relevant parameters of M*(ω) were reported in this paper. Moreover, the accuracy and feasibility of the extracted parameters were later verified with the evaluation test in the lab and field conditions. In that respect, the present contribution demonstrated that the extracted parameters could be used as an available tool for the aging evaluation of transformer oil-immersed cellulose insulation.

An Efficient Approach With Application of Linear and Nonlinear Models for Evaluation of Power Transformer Health Index

In this paper, efficient and accurate linear and nonlinear models are proposed for indicating comprehensive health requirements of the transformer using health index (HI) concept. The models are established with 336 experimental datasets including oil characteristics and dissolved gas analysis (DGA) of various types of transformers placed in different areas. The significance of DGA parameters in transformer health condition is considered with the inclusive DGA factor ( DGAF ) parameter, which considers the weighting importance of seven dissolved gases. Nonlinear models used in this paper are artificial neural network (ANN) and adaptive neuro-fuzzy inference system (ANFIS), which represent the behavior of transformer insulation parameters. The nonlinear models are compared with multiple linear regression (MLR) which is a linear statistical model. The models are established with 80 percent of the experimental dataset. The other 20 percent of data are utilized for the efficiency assessment of the models. The results demonstrate that the models provide an assessment of the health condition of the transformers comparable to existing models with high accuracy. The contributions of this paper are: 1) Evaluating the overall HI of the transformer employing a complete set of 15 input parameters of transformer oil-paper insulation system. 2) Adding DGAF , %WaterPaper , IFT parameters and showing the importance of these parameters. 3) Regarding the condition of solid insulation of the transformer particularly. 4) Applying a diverse and large practical dataset composed of 336 different transformers located in different country areas. 5) Using the MLR method for three purposes. 6) Providing linear (MLR) and nonlinear (ANN, ANFIS) models for HI calculation of the dataset, simultaneously. 7) Verifying the applicability and efficiency of the ANFIS model for simulating HI value.

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.

Enhancing Service Life of Power Transformer through Inhibiting the Degradation of Insulating oil by New Approach

Oil reclamation is a transformer insulation reconditioning technique which may be used as on-line or off-line. However, there is a need of evidence showing the effect of this process on conditions of the paper insulation, which indeed affects the life span of a transformer. This research work focuses on oil reclamation experiment on an old retired distribution transformer. Electrical testing and post-mortem analysis of the transformer were conducted, aimed at investigating the design aspects and collecting information on the insulation conditions prior to the oil reclamation. Temperature and moisture sensors were installed to monitor the conditions within the transformer during the oil reclamation [2]. The experimental process of transformer Oil reclamation was performed into two phases, with regular oil sampling to analyze the changes in key oil parameters, namely acidity number (AN), moisture and breakdown voltage (BV). This was accompanied by paper sampling at the end of each reclamation cycle to study the effects of oil reclamation on properties, particularly moisture, LMA and degree of polymerization. The transformer which was used for the entire experimental process was about 45 years old, 200kVA, 1100 / 415-240V distribution transformer. In order to study the long-term effect of oil reclamation, oil samples were collected from an on-site reclamation exercise performed in a laboratory-accelerated thermal ageing experiment. Oil samples collected before and after the reclamation were aged alongside new oils for comparison [4]. Through the regular monitoring and measurement of oil parameters (AN, moisture and BD strength) over 144 hours and paper parameters (LMA, moisture and DP) at specific stages of phase 1, it was observed that the transformer oil-paper insulation system was significantly improved. The entire research work was performed into two phases (phase 1 and phase 2). The “phase 1” was aimed to improve and restore the oil parameters comparable to the parameters of new oil as specified in “IEC-60296” and its effect on the paper insulation. “Phase 2” was aimed to compare the life span of reclaimed oil filled transformer with the transformer in which aged oil has been replaced by new oil[6]. Effective study of oil reclamation was analyzed through laboratory accelerated aging experiment and real time application on a 45 years old transformer. Through the regular monitoring and measurement of oil parameters (AN, moisture and BD strength) over 144 hours and paper parameters (LMA, moisture and DP) at specific stages of phase1, it was observed that the transformer oil-paper insulation system was significantly improved [15].

Application of Time-Frequency Domain Combined Medium Response Technology in the Insulation Moisture Evaluation of Transformer Oil-Paper

In the current period, many transformers in domestic power supply departments are in the state of overdue service, there are many problems of insulation aging and increasing water in the body, and the rapid and accurate insulation moisture assessment is a technical problem that needs to be solved. The commonly used methods of medium response test for damp evaluation of transformer oil-paper insulation system include time-domain PDC and frequency-domain FDS. These two methods have their own advantages, but also have their own disadvantages. The purpose of this paper is to transform these two test methods into time-frequency combined medium response technology combining time-domain PDC method and frequency-domain FDS method, which can be used in transformer oil paper insulation moisture evaluation to improve the efficiency of transformer operation detection. In this paper, PDC method, FDS method and time-frequency combination method were used to obtain the response curves of insulating oils with different water contents, and the curves were analyzed. By comparing the results of measuring time and water content, the effectiveness of time-frequency combination method was verified. The experimental results of this paper further show that the time-frequency domain combined media response technology has a good application value in the evaluation of transformer oil paper insulation damp.

Hot Spot Temperature and Grey Target Theory-Based Dynamic Modelling for Reliability Assessment of Transformer Oil-Paper Insulation Systems: A Practical Case Study

This paper develops a novel dynamic correction method for the reliability assessment of large oil-immersed power transformers. First, with the transformer oil-paper insulation system (TOPIS) as the target of evaluation and the winding hot spot temperature (HST) as the core point, an HST-based static ageing failure model is built according to the Weibull distribution and Arrhenius reaction law, in order to describe the transformer ageing process and calculate the winding HST for obtaining the failure rate and life expectancy of TOPIS. A grey target theory based dynamic correction model is then developed, combined with the data of Dissolved Gas Analysis (DGA) in power transformer oil, in order to dynamically modify the life expectancy calculated by the built static model, such that the corresponding relationship between the state grade and life expectancy correction coefficient of TOPIS can be built. Furthermore, the life expectancy loss recovery factor is introduced to correct the life expectancy of TOPIS again. Lastly, a practical case study of an operating transformer has been undertaken, in which the failure rate curve after introducing dynamic corrections can be obtained for the reliability assessment of this transformer. The curve shows a better ability of tracking the actual reliability level of transformer, thus verifying the validity of the proposed method and providing a new way for transformer reliability assessment. This contribution presents a novel model for the reliability assessment of TOPIS, in which the DGA data, as a source of information for the dynamic correction, is processed based on the grey target theory, thus the internal faults of power transformer can be diagnosed accurately as well as its life expectancy updated in time, ensuring that the dynamic assessment values can commendably track and reflect the actual operation state of the power transformers.

Molecular dynamics simulation of diffusion behaviour of gas molecules within oil–paper insulation system

The diffusion behaviour of hydrogen, carbon monoxide, carbon dioxide, methane, acetylene, ethylene and ethane in oil and paper medium was examined using molecular dynamics to reveal the diffusion mechanism of gas molecules in transformer oil–paper insulation system at the microscopic level. These compounds are commonly used in the dissolved gas analysis of power transformers and produced during the ageing process of oil–paper composite insulating material. Two groups of models were constructed using molecular dynamics simulation software to simulate the diffusion behaviour of the aforementioned seven types of small gas molecules in oil and paper. The diffusion coefficients, displacement features, free volume characteristics and interaction energies of the gas molecules were investigated. In particular, the diffusion micro-mechanism of the gas molecules was observed. The differences in diffusion features among the gas molecules were discussed, and the factors influencing the diffusion of the gas molecules were compared. Simulation results indicate that the diffusion coefficients of gas molecules in cellulose is an order of magnitude lower than that in oil, and the diffusion coefficients of these gas molecules in the two types of insulation media have different orders. Free volume of gas molecules is the main factor that influences the diffusion behaviour in oil, whereas intermolecular interaction is the main influencing factor of diffusion behaviour in cellulose.

Investigation on dielectric response characteristics of thermally aged insulating pressboard in vacuum and oil-impregnated ambient

Dielectric response measurements are non-invasive and promising diagnostic methods that are being gradually used for assessing the insulation and aging condition of transformer oil-paper insulation system. It is known that moisture content in oil-impregnated insulation has significant effect on its dielectric response phenomena, and whereas at present there is few research about the characteristics of aged cellulose without the impact of water. In this work, a series of experiments are designed and carried out in well-controlled laboratory. Accelerated thermal aging is performed on oil-immersed pressboard samples with different time intervals (0, 120, 250 and 400 h). Then these pressboard samples with different aging states are put into petroleum ether to remove the residual oil inside, and dried in a vacuum oven to remove the petroleum ether and moisture. The degree of polymerization (DP) and scanning electron microscope (SEM) are implemented to characterize the aging status of pressboard samples. Two kinds of dielectric response tests, i.e., the polarization and depolarization currents (PDC) and frequency domain spectroscopy (FDS), are performed on each oil/water-removed pressboard sample in vacuum and new-oil-impregnated ambient, respectively. Samples with different aging states show quite different dielectric response characteristics in both ambients. With the aging status more serious, the pressboard sample exhibits higher values of polarization/depolarization currents and complex capacitances. It is confirmed that, besides the influence of water induced by aging, the deterioration of cellulose itself also causes the increasing of pressboard conductivity, and hence greatly affects the PDC and FDS test results. Dielectric response diagnostic techniques have a potential to reflect the aging information of solid insulation.

Investigations of Temperature Effects on the Dielectric Response Measurements of Transformer Oil-Paper Insulation System

Dielectric testing techniques, in both time and frequency domains, are currently widely used by power utilities for assessment of the condition of transformer oil-paper insulation systems. However, it has been reported that results of these tests are highly influenced by the operating temperature during measurements. The distribution, migration and equilibrium of moisture between oil and paper in a complicated insulation system is highly temperature dependent. It requires adequate experience and proper understanding to interpret the dielectric response results in the presence of temperature variations and thermal instability. Proper analysis of the dielectric test result is only possible with an understanding of the physical behavior of the insulation system in response to temperature. A circuit model, which describes the dielectric behavior of the transformers main insulation system, has been investigated in this paper. The values of the parameters of the model have been identified from the dielectric tests. A correlation has been observed between the operating temperature and the equivalent model parameters that can be used as additional information for better interpretation of the dielectric test results. This paper thus reports a detailed study on the effects of temperature on dielectric measurements of a transformer under controlled laboratory conditions. Some results of practical on-site testing are also presented to demonstrate the possibility of errors that may be introduced in dielectric test results analysis unless temperature effects are taken into consideration.