Power Transformer Insulation Research Articles

The influence of ultrasound signal parameters on sonoluminescence light intensity

Abstract Topics of this article concern the study of the fundamental nature of the sonoluminescence phenomenon occurring in liquids. At the Institute of Electrical Power Engineering at Opole University of Technology the interest in that phenomenon known as secondary phenomenon of cavitation caused by ultrasound became the genesis of a research project concerning acoustic cavitation in mineral insulation oils in which a number of additional experiments performed in the laboratory aimed to determine the influence of a number of acoustic parameters on the process of the studied phenomenona. The main purpose of scientific research subject undertaken was to determine the relationship between the generation of partial discharges in high-voltage power transformer insulation systems, the issue of gas bubbles in transformer oils and the generated acoustic emission signals. It should be noted that currently in the standard approach, the phenomenon of generation of acoustic waves accompanying the occurrence of partial discharges is generally treated as a secondary phenomenon, but it can also be a source of many other related phenomena. Based on our review of the literature data on those referred subjects taken, it must be noted, that this problem has not been clearly resolved, and the description of the relationship between these phenomena is still an open question. This study doesn’t prove all in line with the objective of the study, but can be an inspiration for new research project in the future in this topic. Solution of this problem could be a step forward in the diagnostics of insulation systems for electrical power devices based on non-invasive acoustic emission method.

Thermal lifetime of cellulose insulation material evaluated by an activation energy based method

A rapid method, based on a logarithmic degradation model of insulation material, is proposed to reduce the test duration in lifetime assessment of cellulose paper insulating materials. This method proposes the determination of the activation energy from a non-isothermal measurement made by differential scanning calorimetry or another thermal analysis technique and an aging test at a single elevated temperature. The use of the onset temperature of the exothermal peak at ca. 300 °C is proposed for evaluation of the activation energy of degradation. For comparison, the thermal aging of Kraft cellulose paper for power transformer insulation was performed according to the general standard IEC 60216-1/2001 at three different temperatures: 155, 135 and 115 °C, and subsequently, the lifetimes at different service temperatures were estimated. The experimental data proved to have good agreement between the applied methods, the differences being <10 % in terms of the estimated lifetime across the range of service temperatures. The novel proposed method is effective in terms of both energy and manpower costs as compared to the current method: a factor of around 10 in the case of reducing the aging time, a factor of 3 for the time needed for measurements, and a factor of 10 for the reduction of power intake.

Accelerating Dielectric Response Measurements on Power Transformers—Part I: A Frequency-Domain Approach

Dielectric frequency response, also known as frequency-domain spectroscopy (FDS), was introduced about a decade ago as the preferred method for measuring moisture content in the cellulose insulation of power transformers. A conventional FDS measurement is carried out as a frequency sweep down until 100 μHz, thus causing unavoidable large measuring time due to the very low-frequency oscillations. Nevertheless, FDS is the favored choice for onsite measurements due to its robustness against noise. This contribution proposes a newly developed modification to the FDS measurement technique. In Part I of this two-part series, we propose reducing the measuring time by measuring multiple sinusoidal oscillations simultaneously. Discrete Fourier transformation is used to separate the individual oscillations in the frequency domain. The capacitance and dissipation factor (tan delta) are then calculated for each individual oscillation. The proposed advancement described in part I reduces the measuring time up to 73%.

Moisture in cellulose insulation of power transformers - statistics

The article presents the results of extensive investigations aimed at determining the moisture content in a large population of power transformers (161 units). The project was carried out during the period of 2003-2011 in Poland, Germany, and Sweden. All results were obtained by means of the Frequency Response Spectroscopy (FDS) method. Among the transformers tested were grid transformers, generator step-up (GSU) transformers, and distribution transformers. According to the classification rules established by IEEE, 30% among the investigated transformers could be classified as “wet” (2-4% moisture content). The observed moisture variation with age was found to depend on transformer design and operating conditions. It is suggested that, based on the presented statistical results and knowledge concerning transformer type and age, one may discriminate between probably dry transformers and transformers that require actual moisture analysis and therefore prioritize further condition assessment.

Degradation of cellulosic insulation in power transformers: a SEC–MALLS study of artificially aged transformer papers

The molecular weight distributions of artificially aged transformer papers were studied using SEC–MALLS with 0.5 % DMAc/LiCl as solvent, providing chain length distributions and averages. The slow dissolution and presence of non-cellulosic contaminants tend in some cases to introduce errors in the DPw estimates, and a modified data processing procedure was developed to correct for extraneous scattering. Data were compared to the intrinsic viscosities obtained in 0.5 M Cuen and the ‘viscosimetric DP’ calculated thereof according to ISO 5351. DPvisc was 2–3 times lower than the weight average DP (DPw), but closer to or slightly above the number average DP (DPn) obtained by SEC–MALLS in the case of pure cellulose. Ageing of transformer papers were in some cases associated with changes in the polydispersity (DPw/DPn). The apparent degradation rate (defined as δ(1/DP)/δt) gradually decreased with time, resulting in a tendency for a ‘level-off’ DP in the range 200–300 for DPvisc and DPn, and roughly 1,000 for DPw.

Comparative Study on Effect of Temperature &amp; Moisture on Transformer Insulation Conductivity

It has been acknowledged that the insulation of the power transformers is the main cause of their failures. This can easily be attributed to the fact that all power transformers connected to electrical power networks bears loads and the load causes temperature rise and degradation of the cellulose paper. The by-product of the degradation includes moisture. The high temperature and moisture combine to accelerate the rate of insulation breakdown by increasing the material conductivity, σ and also increase the aging rate of the transformer. This paper has reviewed some transformer assessments and tests as sample data to study the effects of temperature and moisture on power transformer insulation conductivity. The results from the tests and measurements were analyzed and presented accordingly. The analyses of results found that irrespective of the assessment technique used, be it FDS, PDC or RVM, conductivity of the transformer insulating materials increases with any increase in both temperature and moisture content.

Early Failure Detection in Power Transformers

This paper presents equipment for early detection of failures in the insulation of power transformers, checking existing partial discharges inside. The equipment involves hardware, control and signal acquisition software, and signal analysis software. This equipment has a set of algorithms that were made with intelligent extraction techniques and interpretation of data. The degradation diagnosis of the equipment insulation is based on digital signal processing algorithms for extraction of features and also in artificial intelligence algorithms that allies a mining involving all the data linked to the equipment throughout its operating life can make an assessment of the operating conditions of the equipment and suggest interventions and provide an estimated time so that they have to be made.

Evaluation of Partial Discharge in Power Transformers by Acoustic Emission Method and Propagation Modeling of Acoustic Signal

Transformers are one of the most important and costly components of any power system. Proper monitoring of the health of a transformer is thus inevitable. Online monitoring of partial discharges (PD) is one of the ways by which the risk of catastrophic failure of a power apparatus can be reduced. Common insulation in power transformers are pressboard and oil. Unconventional PD measurement was developed for several decades ago as a form of PD detection system. In these PD measurement systems that are named unconventional method PD is analyzed by indirect features of this phenomenon, which includes electrical, acoustic, UHF, optical, and chemical methods. The main advantage of unconventional method is the ability of this method in order to decrease the amount of signal to noise ratio for on-site or on-line measurement. A study of the correlation between acoustic emission signals generated by PD and electrical PD charges is inevitable. By developing the speed of measuring devices like digital signal processor (DSP) the possibility of evaluation and calculation of acoustic signals in very short time have been increased. The aim of this paper is evaluation of acoustic signals for two different kind of pressboard (new and aged) with two types of sensors (75 kHz and 150 kHz peak frequency) and simulation of Acoustic Emission (AE) signals propagation which are produced due to PD inside a sample transformer tank by using Finite Element Method (FEM). 

Power transformer insulation – history, technology and design

This paper reviews briefly the history of paper and pressboard insulation being used for electric insulation of oil immersed power transformers. An introduction to today's insulation technique is given. Important details of materials selection, as well as drying, clamping and oil impregnation procedures required to achieve short-circuit resistant windings are explained. Aging effects are described, as they gradually weaken the solid insulation mechanically and electrically. The value of post-mortem analysis of transformers before scrapping is shown with examples. A list of factors important for the manufacturing and operation of reliable transformers, yet often neglected, completes this article.

Can the bubble effect occur in an oil impregnated paper bushing?

This paper elucidates the question whether bubble effect can possibly take place in the insulation of OIP (Oil Impregnated Paper) type of bushings. The investigations were performed on a bushing insulation model and indicated to a possibility that the effect may occur. Presence of metallic screens in the insulation does not limit the development of gas bubbling; they only force the motion of water vapor bubbles along the model axis. The temperature dependence of bubble initiation on the water content in paper part of the bushing insulation is similar to the presented earlier in literature characteristics for bubbling in main insulation of power transformers. The possibility of bubble effect occurrence may explain the relatively frequent cases of transformer bushing failures.

A new proposed moisture diffusion coefficient for transformer paper

In a previous work, a moisture diffusion coefficient for non-impregnated transformer insulation paper was proposed, which is useful for analysing the moisture dynamics in electrical power transformer insulation during its construction stage. This diffusion coefficient was determined using a set of experimental drying curves obtained in a thermogravimetric analyzer and an optimization process that employs genetic algorithms. The expression for the moisture diffusion coefficient proposed in that previous work is dependent on the temperature, insulation thickness and average moisture concentration. The use of average moisture concentration is not common in diffusion models. In this paper, a new expression for the moisture diffusion coefficient is proposed in which the dependence on average moisture has been substituted by a dependence on the local concentration. This new expression was obtained that is more suitable for diffusion models. To obtain that expression, the same experimental data used in the previous study have been used, but the genetic algorithm’s optimization process was modified. In this paper, a description of the applied procedure is detailed, and the new proposed expression is presented. Finally, a comparison between both equations, the one dependent on average moisture and the one dependent on the local moisture, is performed using a drying diffusion model implemented in a finite element commercial software package.

Electro-acoustic detection, identification and location of partial discharge sources in oil-paper insulation systems

The detection of Partial Discharges (PD) is a reliable technique to analyze the status of electrical insulation in power transformers. Phase resolved partial discharge patterns are being complemented with high frequency pulse waveform analysis, in order to identify discharge sources. In addition to this, acoustic techniques are being implemented trying to locate PD sites in large power transformers. In this work, an inductive loop sensor will be used to identify two different PD sources by means of the energy distribution of the detected waveforms. Additionally, these PD sources will be located by means of acoustic measurements with an electrical reference, through the analysis of the acoustic activity detected for each sensor individually.

Lifetime estimation methods in power transformer insulation

Mineral oil in the power transformer has an important role in the cooling, insulation aging and chemical reactions such as oxidation. Oil temperature increases will cause quality loss. The oil should be regularly control in necessary time. Studies have been done on power transformers oils that are used in different age in Iranian power grid to identify the true relationship between age and other characteristics of power transformer oil. In this paper the first method to estimate the life of power transformer insulation (oil) is based on Arrhenius law. The Arrhenius law can provide loss of power transformer oil quality and estimates remaining life. The second method that is studies to estimate the life of power transformer is the paper insulation life prediction at temperature160 ° C.

Adaptive neuro-fuzzy inference system approach for simultaneous diagnosis of the type and location of faults in power transformers

Electrical, mechanical, and thermal stresses can degrade the quality of the insulation in power transformers, causing faults [1]. Several methods are used for fault diagnosis in transformers, e.g., dissolved gas analysis (DGA), measurement of breakdown voltage, and tan ��, pollution, sludge, and interfacial tension tests [2]. Of these, DGA is the most frequently used. Thermal and electrical stresses result in fracture of the insulating materials and the release of several gases. Analysis of these gases may provide information on the type of fault. Various standards have been suggested for the identification of transformer faults based on the ratio of dissolved gases in the transformer oil, e.g., International Electrotechnical Commission (IEC) standards [3]���[7], and these standards has been quoted in many papers, e.g., [8]���[15]. However, they are incomplete in the sense that, in some cases, the fault cannot be diagnosed or located accurately. Intelligent algorithms, e.g., wavelet networks [16], neuro-fuzzy networks [17], [18], fuzzy logic [8], [12], and artificial neural networks (ANN) [2], [9], [10], [19], [20] have been used to improve the reliability of the diagnosis. In these algorithms, the type of fault is diagnosed first, and the fault is then located using the ratio of the concentrations of CO2 and CO dissolved in the transformer oil [21], [22]. The algorithms are not entirely satisfactory. The wavelet network has high efficiency but low convergence, the fuzzy logic method has a limited number of inputs and, in some cases, it is very difficult to derive the logic rules, and the ANN need reliable training patterns to improve their fault diagnosis performance. In this paper, we present a new method for simultaneous diagnosis of fault type and fault location. It uses an adaptive neuro- fuzzy inference system (ANFIS) [23]���[27], based on DGA. The ANFIS is first ���trained��� in accordance with IEC 599 [3], so that it acquires some fault determination ability. The CO2/CO ratios are then considered additional input data, enabling simultaneous diagnosis of the type and location of the fault. The results obtained by applying it to six transformers are presented and compared with the corresponding results obtained using ANN and some other standards and methods.

Power Transformer Insulation Lifetime Estimation Methods

Mineral oil in the power transformer has an important role in the cooling, insulation aging and chemical reactions such as oxidation. Oil temperature increases will cause quality loss. The oil should be regularly control in necessary time. Studies have been done on power transformers oils that are used in different age in Iranian power grid to identify the true relationship between age and other characteristics of power transformer oil. In this paper the first method to estimate the life of power transformer insulation (oil) is based on Arrhenius law. The Arrhenius law can provide loss of power transformer oil quality and estimates remaining life. The second method that is studies to estimate the life of power transformer is the paper insulation life prediction at temperature160°C.

Comparative Study on Conductivity Using Polarization and Depolarization Current (PDC) Test

Polarization and Depolarization Current (PDC) testing is a non-destructive dielectric testing method to determine the conductivity of insulations. It is one of the dielectric diagnostic techniques based on time domain measurement. PDC measurement technique has gained immense popularity due to its ability to assess the condition of HV insulation. PDC measurement can provide information about the conductivity within the initial periods (seconds) after a DC step voltage application. This paper present the review and comparison results from several published papers on application of PDC method in finding the conductivity of the various types of insulators. The scope of the review covered solid and liquid insulations types. In this paper, for solid insulation the studied was focused on cables insulations, electric machine stator insulation and paper insulator in power transformer insulation. For liquid insulation, the review and comparison was done on the biodegradable and mineral transformer oils. MATLAB software was used to simulate the conductivity level of the several types of HV insulation material. The conductivity level of insulation was found dependent on difference between the polarization and depolarization current values of the insulation material. The review results show that the PDC technique successfully give indication on level of conductivity of the HV insulation materials.

Methanol: A Novel Approach to Power Transformer Asset Management

All electrical utilities deal with the task of determining the residual life of their in-service power transformers. Given the difficulties experienced with the use of first and second generations of markers (carbon oxides and 2-furfuraldehyde), several organizations are now considering the use of methanol for this purpose. Hydro-Québec, which discovered this approach, uses this molecule on a regular basis to evaluate the state of the cellulose insulation of in-service power transformers and applies it to Électricité de France's nuclear power plant transformers. In this paper, some examples of the application of methanol in the field are presented against the information received from the early marker generations.

Analysis of Transformer Insulation by Tan Delta Testing Method

This paper presents analysis of power transformer insulation by one fundamental insulation power factor test, also known as Tan Delta. It is a routine test conducted at site to know the healthiness of insulation in transformers. Out of 108 transformers tested for research work, case studies were chosen for analysis purpose. Experimental data shows our experience on the measurement of Tan Delta techniques of earthing systems and dryness of insulation in transformers. Result shows that the Tan Delta testing method is very effi cient method.

Comparasion between Oil Immersed and SF6 Gas Power Transformers Ratings

Power transformers present the largest portion of the capital investment in addition the power transformer remains oil immersed which presents a fire hazard that is particularly objectionable in urban and metropolitan environment. After careful studies of various alternatives to conventional transformer technology to produce a gas insulated power transformer with increased mega volt-ampere ratings, gas insulated power transformer has been developed with a use with non flammable SF6 gas. For changing the insulating material from oil to SF6 gas a comparative study between these types of transformers should be made. In this paper two mathematical models are suggested and simulated by computer programs to calculate the temperature of the cooling mediums and transformer windings. The obtained results are in agreement wit the measured values in the field.

Comparison between Oil Immersed and SF6 Gas Power Transformers Ratings

Power transformers present the largest portion of the capital investment in addition the power transformer remains oil immersed which presents a fire hazard that is particularly objectionable in urban and metropolitan environment. After careful studies of various alternatives to conventional transformer technology to produce a gas insulated power transformer with increased mega volt-ampere ratings, gas insulated power transformer has been developed with a use with non flammable SF6 gas. For changing the insulating material from oil to Sf6 gas a comparative study between these types of transformers should be made. In this paper two mathematical models are suggested and simulated by computer programs to calculate the temperature of the cooling mediums and transformer windings. The obtained results are in agreement wit the measured values in the field.