Transformer Winding Insulation Research Articles

Increasing the operating life of power transformer winding insulation when upgrading 25-kV alternating current traction substations

Upgrading the traction power supply is an important task in developing railway power engineering. The Russian railway transport development strategy envisions updating power supply equipment, including the reconstruction of 763 traction substations, which will be very expensive. Power transformers are the main, and most expensive, apparatuses in traction substations. They account for more than 50% of the cost of substation equipment. Steps in increasing the operating life of power traction substations during upgrading the ac traction substations are described in the present article. Substation upgrading involves up-to-date equipment and technologies. The decision to replace or further use existing substation power transformers is primarily based on data on the real condition of transformers. The 25 kV alternating current power transformers traction substations operate with the high unbalanced current of windings. The current of the most loaded winding can possess double magnitude of currents in other windings, while the losses of windings differ by four times. A qualitative analysis of transformer operating life based on insulation wear for three traction substations is presented. The winding with maximum insulation wear is assessed on the basis of power consumption. In order to extend the operating life, windings need to be connected to neutral links of a traction substation.

Electric stresses on transformer winding insulation under standard and non-standard impulse voltages

This work presents an extension of our previous work on voltage stress analysis of a 3MVA, 33/11kV, 3-phase, 50Hz, Dyn 11 transformer against the application of standard and non-standard impulse voltages. A preliminary comparative study on the variation of maximum voltage to ground and voltage across the coils under different impulse voltages indicated that non-standard impulse voltage waveforms develop higher voltage stress in the windings. They pose high risk and are critical to transformer insulation system. The obtained test results provide the basis for further study of non-standard impulse voltage waveforms and make necessary correction in the existing impulse testing standards. However, to take care of the stringent effect of actual lightning and other impulse voltages occurring in practical field a more detailed study considering a wide variety of non-standard impulse voltage waveforms is aimed to be done in future.

Study on the Methodology of Diagnosis for Transformer Winding Insulation Defects

Power transformer is the important equipment in power system, once the occurrence of faults may bring serious harm to power grid, and even cause great losses. This paper proposes a transformer winding insulation defect detection method. The method only uses the impulse voltage test. It can obtain the diagnosis signal source which contains a lot of harmonic. This paper can judge the insulation defects in the location according to compare the extreme value distribution of every winding voltage and currents correlation coefficient .and can determine the coordinate values. The method does not need large or complex additional equipment, can realize the detection and diagnosis of transformer winding insulation defect location. It is image, intuitive, convenient and practical. This paper studies the insulation defects and the severity of transformer winding, puts forward the method for changing winding current and voltage detection winding insulation defects. And it uses a large number of experiments to verify the insulated and reliability of transformer winding according to different position, different types of insulation defects.

Study on the Methodology of Detection for Transformer Winding Insulation Defects Based on Impulse Test

A methodology of diagnosing the winding insulation defect according to data of transient voltage and current from the impulse voltage test is put forward in this paper. Lightning impulse test for transformer is divided into half-wave and full-wave at the moment. According to the full voltage and reduced voltage at two pressure processes. The transient voltage and current from the impulse voltage test can be divided into Series of harmonics. The insulation defect is diagnosed by dividing impulse voltage and current into series of harmonic and structuring discriminant function according to the longitudinal ratio method and cross ratio method for the corresponding period of harmonic voltage and current. The methodology is also applicable to diagnose whether there are insulation damage in the windings those have passed the lightning impulse test.

Study on Detection Method of Transformer Winding Insulation Defects Based on Lightning Impulse Test

Lightning impulse test for power transformer is a highly theoretical and empirical analysis tests for insulation assessment attaches great importance to the power transformer. Facing the problems of detection and diagnostic method based on lightning impulse test for power transformers, this paper proposes a new method of diagnosing the winding insulation defects using two groups of voltage and current with the half-wave and full-wave. This method treats parts of the lightning impulse voltage and current as the signal source, constructs recognition criterion which is closely related to the voltage and current, and diagnose transformer winding insulation defects. Whether the insulation damage exists in the transformer winding which has passed the lightning impulse test can also be diagnosed by the method.

Research on the Effect of Lightning Oscillatory Wave to Transformer Winding Turn-to-Turn Insulation Fault

This paper has studied the effect of lightning oscillatory wave on transformer winding insulation fault and finds that lightning oscillatory wave can not directly cause the breakdown of turn-to-turn insulation. Even 200 times lightning oscillatory wave applied on turn-to-turn insulation, the breakdown voltage of turn-to-turn winding almost does not change. On the basis of analysis and research about the distribution and function of lightning oscillatory wave, this paper presents that lightning oscillatory wave does not directly cause the breakdown of turn-to-turn insulation, but acts on turn-to-turn insulation in the form of strong electromagnetic force. Due to the vibration and deformation of the winding, it will cause damage to the insulation and lead to inter-turn short circuit or insulation breakdown. Lightning oscillatory wave also has cumulative effect on transformer winding inter-turn insulation.

Life prediction of a full-scale transformer winding insulation through statistical analysis of AC voltage endurance test data

It is now acknowledged that for many solid dielectrics below a threshold electrical stress, electrical aging can either be neglected or considered to progress very slowly. Hence, it is recommended to exclude from life prediction data obtained at high electrical fields. In general, it is common to be faced with an insufficient number of life data obtained at low fields. Therefore, we first introduce the Inverse Power - Weibull model, which can estimate statistically the confidence bounds on the lifetime at a given reliability at operating voltage. Then, as the main contribution of this paper, the existing models pertaining to the size enlargement effect are extended to a non-uniform stress situation, and two methods are developed to calculate the confidence bounds of the lifetime of a full-scale high voltage component. Finally, the Inverse Power - Weibull model and the calculation method we derived are applied to the life data collected from transformer winding insulation specimens. The voltage versus lifetime characteristics of the epoxy resin insulation of the windings comply with earlier research. After completion of the statistical analysis, a warranty life is suggested for the winding of a full-size transformer, with a high reliability and confidence level.

Identification of simultaneously occurring dynamic disc-to-disc insulation failures in transformer winding under impulse excitation

A method for identification and localization of isolated as well as simultaneous dynamic series insulation failures in transformer winding has been proposed. Trained wavelet network has been employed to identify the fault characteristics, viz. location(s), nature(s) and type of unknown impulse insulation failures. The network identifies the simultaneous dynamic insulation failures using the cross-wavelet transform based features extracted from recorded winding currents resulting impulse excitation. Four different fault emulators have been used to emulate different combinations of simultaneous dynamic disc-to-disc insulation failures in transformer winding. It has been found that the developed network with extracted features has identified the fault characteristics of simultaneous dynamic series insulation failures of transformer winding insulation within ±9% of winding length with good accuracy. The characteristics as well as experimentation details of simultaneously occurring dynamic disc-to-disc insulation failure(s), cross-wavelet transform based feature extraction and wavelet based fault characteristics identification are explained.

Approach for an integral power transformer reliability model

SUMMARYIn electrical power transmission and distribution networks power transformers represent a crucial group of assets both in terms of reliability and investments. In order to safeguard the required quality at acceptable costs, decisions must be based on a reliable forecast of future behaviour. The aim of the present study is to develop an integral transformer lifetime model which involves degradation mechanisms for most relevant subsystems, applicable to individual power transformers and transformer populations. In this paper, we present a predictive model for power transformer reliability which involves three essential ingredients: failure statistics, physical understanding of the degradation process, and actual knowledge of the present condition. The model is based on evaluation of existing literature and past experience on degradation mechanisms, failure modes and diagnostic techniques. The model is illustrated for integral reliability of three transformer failure modes, related to the degradation of the transformer winding insulation, of bushings and of tap‐changers. Copyright © 2011 John Wiley & Sons, Ltd.

Cross-correlation-aided Fuzzy c-Means for Classification of Dynamic Faults in Transformer Winding During Impulse Testing

An approach for the classification of dynamic faults in transformer winding insulation during impulse testing based on the features extracted from the cross-correlation sequence is proposed. The features for fault classification are extracted from the cross-correlation sequence of impulse fault currents acquired from healthy, as well as faulted, conditions of transformer winding. The efficacy of the extracted features for identification of dynamic insulation failures are assessed by using two classification tools—the fuzzy c-means algorithm and the artificial neural network. The tools have successfully identified dynamic faults with different fault establishment times along with the location of the failure in the winding. Impulse fault currents for various faults having different fault establishment times are acquired by simulating static and dynamic insulation failure in an analog model of 33-kV winding of a 3-MVA transformer using the developed non-linear, bilateral fault simulator. Results show that the extracted features are capable of identifying the impulse fault patterns with acceptable accuracy. The concepts of cross-correlation feature extraction, simulation of dynamic faults, and fault classification methods are explained.

Comparison of Transformer Detailed Models for Fast and Very Fast Transient Studies

The fast and very fast transient overvoltages can endanger the insulation of transformer windings. Therefore, it is necessary to develop models, to study these transients. This paper compares the two well-known multiconductor transmission line (MTL) and lumped parameter detailed (RLC ladder network) models. The high-voltage (HV) winding of a 220/35-kV, 50-MVA transformer has been modeled using these models. The simulation results of the MTL and the RLC ladder network models have been compared with the measurements results in the frequency domain, by application of the Pearson's correlation coefficients. It is shown that the RLC Ladder Network model should be used for fast transient overvoltage (FTO) studies, i.e. frequency range of 10 kHz < / < 1 MHz and the MTL model with frequency dependent loss factor, demonstrates a better agreement with the measurement results for very fast transient overvoltage (VFTO) studies, i.e., in the frequency range of 1 MHz les / les 5 MHz.

Non-destructive measurement of the degradation of transformer insulating paper

Knowledge of the condition of power transformer winding insulation paper is fundamental to making optimum asset replacement decisions in the power industry. The ability to assess the aged condition of Kraft paper quickly and non-destructively using portable instrumentation would significantly increase the opportunities for gaining this knowledge. Insulation paper degrades over time in-service and its degree of polymerization (DP) reduces, eventually affecting its mechanical strength. At low DP levels the insulation may start to disintegrate and the risk of electrical breakdown increases. Currently-used methods of estimating DP are either approximate or destructive. The use of spectroscopy together with multivariate statistical analysis (MVSA) provides a powerful non-destructive evaluation of the condition of paper. From initial feasibility studies, we have developed a simple, portable system (TRANSPEC) using fiber-optics and broad-band spectroscopy that can measure the degree of polymerization of various aged transformer papers to a precision of approximately 30 DP units with a spatial resolution of 14 mm. The system can also measure the chemical composition and condition of the insulating mineral oil. MVSA regression models were constructed from library spectral data, and these models are used to predict the DP of other papers with parameters that fall within the range spanned by the set of calibration samples. Separating oil and moisture information from wetted paper is possible and will be reported in a separate publication. With a single TRANSPEC system, non-destructive in-situ analysis of the DP of insulating paper is possible, providing a rapid cost-effective method for transformer insulation condition assessment and monitoring, which correlates well with current destructive methods.

Infrared detection of transformer insulation degradation due to accelerated thermal aging

Regular monitoring of the paper insulation of electrical windings is necessary to ensure safe and reliable operation of oil-paper insulated electrical transformers. The degradation of the paper insulation due to thermal aging yields furanic derivatives which are specific to the paper and cannot be produced by general oxidation of the oil. The concentration of such furan compounds in the oil gives an accurate indication of the condition of the paper insulation. Normally, off-line methods such as HPLC (high performance liquid chromatography) are applied to periodically assess the furan concentration within oil samples which have been extracted from the transformer. We report experimental findings on the production of the furan compound FFA (furfuraldehyde) in an oil-paper system subject to accelerated thermal aging. Our approach is based on IR spectroscopy. This technique could lay the basis for the construction of direct online monitoring equipment suitable for monitoring of transformer winding insulation.

An Approach to be Prediction of the Lifetime of Electrical Insulations

The problem of predicting the lifetimes of solid electrical insulations is approached statistically, using a method of prediction based on the procedure developed by Englehardt and Bain. The method has been used successfully to predict the lifetimes for accelerated tests on polyethylene sheet insulation and for transformer winding insulation in service. The predicted lifetimes are consistent and close to the observed ones. The accuracy of the prediction seems to depend strongly on the number of samples.

Field power-factor testing of transformer insulation and operating experience

One requirement of a field test on transformers is that weaknesses which develop in bushing or winding insulation be readily located and failures prevented. This has been effectively accomplished in the American Gas and Electric Company system by a power-factor testing program of transformer insulation which was initiated in the fall of 1933. For some 18 months prior to this time, a field testing program for oil circuit-breaker insulation had been in progress. As pointed out in previous publications, <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1,4</sup> these tests had proved satisfactory particularly in locating faulty bushings and bad oil circuit-breaker insulation. It is the purpose of this paper to discuss the tests we have made on transformer winding insulation, the methods used in locause of progressive faults has been detected and eliminated without need of major repairs. This procedure has helped materially in keeping equipment in first-class operating condition, in preventing failures, and in realizing substantial monetary savings.

Field Power-Factor Testing of Transformer Insulation and Operating Experience

One requirement of a field test on transformers is that weaknesses which develop in bushing or winding insulation be readily located and failures prevented. This has been effectively accomplished in the American Gas and Electric Company system by a power-factor testing program of transformer insulation which was initiated in the fall of 1933. For some 18 months prior to this time, a field testing program for oil circuit-breaker insulation had been in progress. As pointed out in previous publications,1,4 these tests had proved satisfactory particularly in locating faulty bushings and bad oil circuit-breaker insulation. It is the purpose of this paper to discuss the tests we have made on transformer winding insulation, the methods used in locause of progressive faults has been detected and eliminated without need of major repairs. This procedure has helped materially in keeping equipment in first-class operating condition, in preventing failures, and in realizing substantial monetary savings.