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Creep life assessment of distribution transformers
Abstract
The life of distribution transformers depends on creep phenomenon of winding conductors especially in power deficient areas and poor power distribution networks. The creep strain in winding conductors more than the specified clearance between adjacent windings causes fault hence failure of the distribution transformer. In this paper, the creep life of both aluminum and copper wound distribution transformers is calculated using experimental creep results of winding conductors. The effect of frequent switching is also calculated and observed experimentally on the creep life of the distribution transformers.
... This fact suggests that maximum stress owing to inrush current is produced on the inner side of the HV windings, which can lead to failure of the windings owing to creep and fatigue caused by excessive vibrations in loose winding conductors (Vasutinsky, 1962;Faiz et al., 2008;Dieter, 2001). Beniwal et al. (2010aBeniwal et al. ( , 2010b performed creep tests on aluminium and copper wires used in HV windings of 25 kVA DTs. The tests were conducted at the stress and temperature conditions as revealed by the calculations, which can be generated owing to inrush current in the HV windings during energisation of DTs ( Figure 5). ...
... The tests were conducted at the stress and temperature conditions as revealed by the calculations, which can be generated owing to inrush current in the HV windings during energisation of DTs ( Figure 5). Beniwal et al. (2010aBeniwal et al. ( , 2010b presented failure mechanism of DTs owing to creep phenomenon in conductors between radial spacers of adjacent windings ( Figure 6). And, an algorithm has also been proposed to assess the life of the DTs owing to creep by using the steady-state creep rate of aluminium and copper wire. ...
... Creep curves(Beniwal et al., 2010a(Beniwal et al., , 2010b: (a) aluminium wire of 0.8 mm diameter and (b) copper wire of 0. ...
In India, the failure rate of distribution transformers has been reported very high especially in the rural and remote areas. In these areas, frequent inrush current is produced in distribution transformers during their energisation. The generation of inrush current during each energisation may be dangerous for health of the distribution transformers. This has motivated the authors for extensive study on inrush current such as causes, modelling, effects and reduction techniques. A case study has also been given on role of creep in the windings due to inrush current as one of the reasons for failure of distribution transformers.
... 2 Literature survey Beniwal et al. (2010) devised a technique that takes into consideration creep in order to evaluate the switching resist capacity of distribution transformers as well as the life of the transformers. Because of this, they were able to design and manufacture reliable distribution transformers by following their instructions for selecting creep-resistant materials to use in power-deficient regions with haphazardly developed power distribution networks. ...
... In addition to this, they developed an algorithm for a computer that was based on fuzzy logic in order to determine the causes of transformer failures that are most likely to occur. Beniwal et al. (2010) suggested that the distribution transformers may now be regularly operated with little protection against cold load pickup and inrush current thanks to the author's solution. The algorithm makes it possible for this to take place. ...
Underfitting, model complexity, and resource optimization restrict machine learning's effectiveness on large datasets are hindered by these issues. Deep learning networks can be used to huge volumes of data to uncover new information, anticipate the future, and apply that knowledge. Many deep learning architectures have been created to outperform earlier machine learning approaches as data rises. In computer vision (CNN), the convolutional neural network is a popular deep neural network design. The convolutional layer, pooling layer, activation layer, and linked convolutional layer make up a CNN. To complete operations, a deep CNN uses several connected convolutional layers, 3 × 3 pixels of the input image are permitted past the filter at the convolutional layer.
... Earlier, the authors [8] have reported experimental results of creep behavior of EC grade aluminum and electrolytic copper winding conductors at some temperature and stress conditions which usually develop in HV windings of 25 kVA DTs due to CLPU and inrush current conditions. Further, the authors [9] extended their work by relating the steady state creep rates of EC grade aluminum and electrolytic copper HV winding conductors to the life and switchings capability of the DTs. ...
... The temperatures for creep tests were taken corresponding to the extreme hot spot temperature in HV windings. The authors [9] have presented calculation of hot spot temperature. ...
This paper presents an experimental study on electrical properties and creep behavior of thermo-mechanically processed 6060
aluminum alloy wire to assess its suitability as a winding conductor in place of electric conductor grade aluminum wire in
distribution transformers. Electrical resistivity and the stress exponent and activation energy for creep were measured for
both thermo-mechanically processed 6060 aluminum alloy and electric conductor grade aluminum wire. An algorithm is given for
estimate the life and switching capability of the distribution transformers considering creep. The results of this study can
be used as a guide to select the creep resistant winding material for designing and producing reliable distribution transformers
and is valuable especially in power deficient areas, poorly designed and haphazardly expanded power distribution networks
where repeatedly energized aluminum wound distribution transformers may fail due to creep in the high voltage winding conductors.
KeywordsElectric conductor grade Aluminum–Aluminum alloy 6060–Cold load pick up–Creep–Distribution transformer–Inrush current
... Finally, it was shown that the SPS system can also be used as a tool to measure, to some extent, the electro-plastic effect in conductive materials [13], where an applied electric current passing through conductive materials, such as metals and alloys, can have a significant effect on creep behavior and plastic deformation [15,16]. Such influence is important because it can affect the mechanical properties of the material and lead to undesired premature failure [17][18][19]. ...
Compressive creep properties of AlSi10Mg parts produced by additive manufacturing selective laser melting (AM-SLM) were studied using a spark plasma sintering (SPS) apparatus capable of performing uniaxial compressive creep tests. Stress relief-treated specimens were tested under an applied stress of 100-130 MPa in the 175–225 °C temperature range. Utilizing two different configurations, the creep tests were conducted either with or without a low-density electric current (˜2.63–3.26 A/mm2) flowing through the test specimens. The results revealed that the creep rate increased under the influence of an applied electric current. The creep parameters (i.e., stress exponent n and apparent activation energy Q), were empirically determined. The stress exponent values were found to be 19.6 ± 1.2 and 16.2 ± 1.4 with and without current, respectively, while apparent activation energy was found to be 142 ± 9 kJ/mol and 150 ± 13 kJ/mol with and without current, respectively. The experimental results, together with microstructural examination of specimens, indicate that plastic deformation was controlled by dislocation activity. Furthermore, it is suggested that the annihilation process of dislocations during creep was enhanced by the electric current.
... The start and end of a transformer winding is connected to the bushings at the top of the tank; very often it is observed that the connection at this point breaks. This is due to repetitive mechanical stresses that cause the forces to set the winding into vibrations and creep sets [3]. ...
In medium and high capacity transformers where current rating is high and the number of turns is low, the low voltage (LV) winding is generally of the helical type. These helical windings have very large magnitudes of electromagnetic forces during a short circuit. This is due to the inherent asymmetry of helical structure. The objective of this work is to use the finite element method to compute the radial and axial components of short circuit forces and identify areas of high stresses in the windings. This can be used to find the likely reason of transformer failure during a short circuit. For this work, a 3-phase power distribution transformer of 11kV/433V, 630kVA rating is considered. The effect on short circuit forces of the tapping in the center of HV winding is also studied.
... So, the fault classification sometimes gives no results. CIGRE method is combination of key gas ratio method and gas concentration method [28,29]. Duval's triangle method and Nomograph method are graphical methods. ...
This manuscript focuses the implementation of artificial neural network-based algorithms to classify dif-ferent types of faults in a power transformer, meant par-ticularly for NonDestructive Test for transformer fault classification. The performance analysis of Probabilistic Neural Network (PNN) and Backpropagation Network classifiers has been carried out using the database of dis-solved gases collected from Punjab State Electricity Board, Patiala, India. Features from the preprocessed data have been extracted using dimensionality reduction technique, i.e., principal component analysis. The selected features were used as inputs to the Backpropagation Network and PNN classifiers. A comparative study of the two intelligent classifiers has been carried out, which reveals that PNN classifier outperforms the Backpropagation Network classifier.
... Earlier, the authors [6] have identified and reported the elongation in the HV winding conductors owing to creep as one of the reasons for failure of repeatedly energized DTs having insignificant protection against inrush current and CLPU. Also, the authors [7] have reported the assessment of life and switchings withstand capability of frequently energized DTs considering the creep phenomenon in the HV winding conductors. The HV winding conductors may become loose due to creep or any manufacturing defect. ...
This paper presents an experimental study on axial fatigue behaviour of loose winding conductors of aluminum and copper wound distribution transformers. Algorithms are presented for estimating the life and switchings withstand capability of distribution transformers by considering axial fatigue in loose winding conductors. The life and switchings withstand capability of distribution transformers have been calculated by using experimentally determined results of fatigue life cycles of winding conductors corresponding to stress. The experiments were conducted considering wire sizes and other parameters of 25 kVA distribution transformers. The outcomes of this study can be used as a guide for designing and producing reliable distribution transformers by selecting suitable conductor diameter which may has better fatigue life cycles endurance capability. This study is useful especially for frequently energized distribution transformers having insignificant protection against cold load pick up and inrush current.
High-performance conductors are essential for economically and environmentally sustainable ways of electricity transfer in modern infrastructure, manufacturing and transportation, including electric vehicles. This report reviews the aluminum conductors, their fundamentals, classification and utilization markets, focusing on metallurgical characteristics of present commercial solutions and the strategy of future development directions. The inherent features of aluminum, both beneficial and detrimental, for electrical engineering are emphasized along with alloying concepts that provide the accelerated decomposition of matrix solid solution to minimize the electron scattering. Development activities are assessed of new generation of aluminum conductors that in addition to alloying utilize novel processing techniques such as ultra-fast crystallization, severe plastic deformation and complex thermomechanical treatments aiming at grain reduction to nanometer scale, crystallographic texture control and grain boundary engineering. Transition metals and rare earths are considered as the promising alloying candidates for high-strength conductors having superior thermal stability with extra importance given to immiscible systems of Al–Ce, Al–La and Al–Y along with multiply additions, combined to generate the synergy effects. The composites with cladding configuration and particulate reinforcement including via carbon-type strengtheners are discussed as the effective solutions of advanced conductors. A variety of strategies that aim at overcoming the strength–conductivity trade-off in conductor materials are presented throughout the report.
Graphical abstract
This study investigates creep behavior of distribution transformer (DT) winding Western Union splice (WUS) joints with and without the use of solder. The creep strain and contact resistance were measured during the creep test, and the life of aluminum and copper wound DTs were calculated. Improper WUS joint increases resistance and creates hot-spot and sparking, which accelerates the steady-state creep rate of WUS joint along with the winding conductor, ultimately reduces the life of DT. This research contributes to the DT manufacturers and electrical industries by addressing a knowledge gap regarding the optimal number of turns and creep performance of DT winding WUS joints. The aluminum and copper winding WUS joints of 25 kVA DT were prepared using 18 and 14 turns, respectively, which were optimized by performing the tensile test. Creep test of winding joints was performed as per ASTM E139-11. Results exhibited a higher steady-state creep rate and more contact resistance of the aluminum winding WUS joints for elevated temperature and stress conditions. Soldering of WUS joints improved the creep life of aluminum wound DTs and showed a lesser increase in contact resistance during creep. The copper winding WUS joint at 140 °C temperature and 55 MPa stress shows that the creep life for DT is much higher compared to the creep life of DTs due to aluminum winding WUS joints with and without solder.
Distribution transformer is a most important component of the distribution system to provide uninterrupted power supply to the consumers and it should be highly reliable. Distribution transformer failure affects the reliability and power quality of the supply system. This paper presents distribution transformer failure analysis to find out causes of failures responsible for very high rate of failure in a state owned electric utility Punjab State Power Corporation Limited (PSPCL) in India. Detailed failure analysis is conducted on 348 distribution transformers failed from 2010 to 2015 in a city based sub division area. Failure analysis is carried out in accordance with IEEE standards and summarized the results statistically in the first part. In second part, the failure modes, effect and criticality analysis (FMECA) is used to analyse distribution transformer failure modes, causes of failure and effects of failure. Further, criticality analysis is performed and a risk priority number is assigned based upon the severity classification, probability of occurrence and probability of detection to identify most critical parts of transformer. FMECA leads to identify preventive actions to be taken to reduce the risk of the transformer in future by eliminating the causes of failure, reducing severity and probability of occurrence.
The rural power networks play a very important role in the power network in China. The rural networks built in the old days are usually not standardized, and therefore, a lot of problems emerged these days. Such as, most of the transmission line is aged, the insulation ability of the transmission line degraded, the household voltage is relatively low, the distribution network line loss is very high, and so on. As the rural power network mainly distributes in the hilly and mountainous land, the altitude of the load points has an important impact to the location of distribution transformer. Meanwhile, because of the existence of the sag, the length of the transmission line is not a straight-line distance. Therefore, this paper modified the traditional optimization model used in the location of distribution transformer, introduced the line correction coefficient (α) which was connected with the sag, improved the line distance computational formula according to the altitude of the load points, and put forward the modified optimization model which is called M-TLOM. Then, an optimized location system of distribution transformer (DTLOS 1.0) based on the Visual Basic 6.0 was developed. The instance analysis indicated that, the modified model could reduce the loss of distribution network.
The rural power networks play a very important role in the power network in China. The rural networks built in the old days are usually not standardized, and therefore, a lot of problems emerged these days. Such as, most of the transmission line is aged, the insulation ability of the transmission line degraded, the household voltage is relatively low, the distribution network line loss is very high, and so on. As the rural power network mainly distributes in the hilly and mountainous land, the altitude of the load points has an important impact to the location of distribution transformer. Meanwhile, because of the existence of the sag, the length of the transmission line is not a straight-line distance. Therefore, this paper modified the traditional optimization model used in the location of distribution transformer, introduced the line correction coefficient (α) which was connected with the sag, improved the line distance computational formula according to the altitude of the load points, and put forward the modified optimization model which is called M-TLOM. Then, an optimized location system of distribution transformer (DTLOS 1.0) based on the Visual Basic 6.0 was developed. The instance analysis indicated that, the modified model could reduce the loss of distribution network.
Highlights
► A new model to extrapolate creep rupture data outside the test stress and temperature ranges is proposed. ► The model is a better representation of the creep process compared with the Larson–Miller method. ► The Larson–Miller constant C is a variable and the value of C depends on the material and the amount of stress.
Greater use of electric heating in residential areas in North America has led to the appearance of a new type of thermal stress resulting from service restoration following a power outage. This loading condition, known as cold-load pickup, was simulated on a sample of 18 distribution transformers by applying overloads up to three times the rated current at ambient temperatures as low as -40°C. The data gathered was used to develop a thermal model representing the transient behavior of distribution transformers. The results were then generalized to, produce a loading guide for evaluation of maximum hot-spot temperatures and loss of life under given cold-load pickup conditions.
The cold load pick up current during frequent energization of the distribution transformers produces high stress and hot spot temperature in their windings. This condition is responsible for elongation in the winding conductors due to creep. The winding conductors between spacers become loose due to elongation which in turn causes turn to turn fault in the winding. Thus, creep causes failure of the distribution transformers. The steady state creep rate is an important design parameter which specifies strain rate in the wire during steady state creep stage. This paper reports the steady state creep rate of the aluminum and the copper wires used in the windings of the 25 kVA distribution transformers.
There are three possible mechanisms occurring in high temperature creep at low stresses: diffusion creep, Harper-Dorn creep and grain boundary sliding. This paper examines the characteristics of these three processes and proposes two sets of measurements that may be used to give an unambiguous identification of the dominant creep mechanism. The approach is applied to published creep data and new measurements are reported for a Mg-0.55%Zr alloy which permit a clear distinction between diffusion creep and grain boundary sliding.
There are three possible mechanisms occurring in high temperature creep at low stresses: diffusion creep, Harper–Dorn creep and grain boundary sliding. This paper examines the characteristics of these three processes and proposes two sets of measurements that may be used to give an unambiguous identification of the dominant creep mechanism. The approach is applied to published creep data and new measurements are reported for a Mg–0.55%Zr alloy which permit a clear distinction between diffusion creep and grain boundary sliding.
The increased use of schemes of rotating load curtailment owing to limited power availability and high share of thermostatically controlled loads in total load demand has made the study of cold load pickup (CLPU) mandatory. This paper looks into the various aspects associated with the problem of CLPU and explains the phenomenon of CLPU. It reviews various modeling approaches and means of solving CLPU problems together with the affecting factors.
Tensile creep behaviour of OFHC copper in the temperature range 850 to 1074°C (0·83 to 0·99Tm) under low stress (0·1 to 0·6 MPa) has been investigated in tension for 0·4 and 0·6 mm thick foils with grain size ∼1 mm, in the plane of the foils. Increases in creep rate per unit stress at 0·99Tm were two orders of magnitude higher than predicted for Nabarro–Herring diffusional creep and were nearer to values expected from the operation of grain size independent Harper–Dorn creep, but the stress exponent n was closer to 2 than to the n=1 expected in this mechanism. Observations on specimen surfaces revealed some widely spaced slip bands, some small grain boundary movements and occasional cavitation on grain boundaries nearly perpendicular to the stress. Creep rates were comparable with predictions of the movement of dislocations, controlled by the rate of their generation at Bardeen–Herring sources at a spacing similar to that of the observed slip lines.
Over the past 15 years important advances have been made in the experimental study of the microstructural changes occurring during the non-linear steady-state creep of single phase crystalline matter at elevated temperatures. Curiously, although the results of these painstaking studies have gone a long way toward elucidating the mechanism of this phenomenon, they have been largely ignored in favour of some simple dislocation mechanisms that are not only inconsistent with these observations, but are also unable to describe correctly the known phenomenology. This review concentrates primarily on the recent experiments on microstructural alterations occurring during creep; however, it also surveys the many mechanistic models that attempt to describe this phenomenon, and finds them all deficient.
The creep rate (e˙) predicted by the boundary diffusion (Db) model is e˙≃150σDbWΩ/(GS)3kT , where σ is the stress, W is the boundary width, (GS) is the average grain size, and Ω is vacancy volume. The stress dependence is the same as the lattice diffusion model, given by C. Herring, while the grain size dependence and the numerical constant are greater for boundary diffusion. Discussion of the mechanism of creep in polycrystalline alumina is based on the differences between the lattice and boundary diffusion models.
According to a suggestion of Nabarro, any crystal can change its shape by self‐diffusion in such way as to yield to an applied shearing stress, and this can cause the macroscopic behavior of a polycrystalline solid to be like that of a viscous fluid. It is possible that this phenomenon is the predominant cause of creep at very high temperatures and very low stresses, though not under more usual conditions. The theory underlying it is developed quantitatively, and calculations of rate of creep, or equivalently of effective viscosity, are given for aggregates of quasi‐spherical grains and for wires composed of cylindrical grains. Allowance is made for the effect of tangential stress relaxation at the grain boundaries. It is suggested that mosaic boundaries and boundaries between grains of nearly the same orientation may be unable to serve as sources or sinks of the diffusion currents, in which case the creep rate will depend only on the configuration of grain boundaries having a sizable orientation difference. Numerical comparison of the theory with experiments on the high temperature creep of wires favors this view, but is not entirely satisfactory. Suggestions for further experiments are made.
Inrush currents are frequently encountered during the switching
process of transformers. The purpose of this paper is to present an
investigation of forces that appear as a result of inrush currents.
While both short circuit and inrush transformer situations might seem
identical, they vary significantly from the core magnetization
viewpoint. Three-dimensional computations have been carried out on a
typical single-phase power transformer using the Hopfield neural network
energy minimization technique. Results are given in the paper as well as
comparison with short circuit forces
The creep of pure polycrystalline aluminum was studied at low stresses, and temperatures near the melting point. It was found that at stresses between 3 and 13 lb/in² the secondary creep rate varied linearly with the stress, whereas at slightly higher stresses, the creep rate increased with the fourth power of the stress. The activation energy for creep was determined to be about 35,500 calories per mole over the entire stress range. The low stress creep results in the range of "viscous" creep were analysed from the viewpoint of the Nabarro-Herring model for stress directed self-diffusion of vacancies, and it was found that the experimental creep rates were about one thousandfold greater than those predicted theoretically. This factor plus additional experimental observations on the displacements of markers at grain boundaries, and on creep recovery upon removal of the stress led to the conclusion that the Nabarro-Herring model does not apply to the case of aluminum, and that creep occurs by a dislocation climb mechanism at all stresses considered.
The load recovery due to an aggregation of thermal loads
controlled with thermostats after voltage variations of any size and
duration is studied by using a physically-based model. Analytical
expressions for the load recovery are deduced in the limit case of an
aggregation of infinite structurally identical infinitesimal loads with
a given total mean power. The analytical results are validated by
comparison to stochastic simulations carried out on an aggregation of
10000 thermostatic loads. The results are used to explain some
properties of the load recovery in the general case with random load
parameters selected within a given range for a given probability
distribution
The primary purpose of this paper is to provide a quantitative method for electric power utilities to predict the magnitude and duration of the peak demand following a power outage in cold weather. The study is limited strictly to electrically heated homes. A model has been developed based on experimental data that predicts the power demand of any number of homes following a power outage. The authors believe that this information should influence both the design of the distribution system and the method of power restoration.
The results of simulation of the cold load pickup characteristics of electric resistance space heating loads, electric water heating, heat pump and air conditioning loads as well as other residential customer appliances are presented. The effect of cold load pickup from various saturations of the electric space heating loads on the distribution substation transformer, its protective equipment and on protective equipment on distribution feeders is discussed. Critical saturation levels for heating load appliances are suggested based on maximum loading and thermal criteria. A computer program listing for the simulation is furnished.
From failure experience on power transformers, it was very often
suspected that inrush currents, occurring when energizing unloaded
transformers, were reason for damage. In this paper, it was investigated
how mechanical forces within the transformer coils build up under inrush
compared to those occurring at short circuit. Two-dimensional and
three-dimensional computer modeling for a real 268 MVA, 525/17.75 kV
three-legged step up transformer was employed. The results show that
inrush current peaks of 70% of the rated short circuit current cause
local forces in the same order of magnitude as those at short circuit.
The resulting force summed up over the high voltage coil is even three
times higher. Although inrush currents normally are smaller, the forces
can have similar amplitudes as those at short circuit however with
longer exposure time. Therefore, care has to be taken to avoid such high
inrush currents. Today controlled switching offers an elegant and
practical solution
Greater use of electric heating in residential areas in North
America has led to the appearance of a new type of thermal stress
resulting from service restoration following a power outage. This
loading condition, known as cold-load pickup, is simulated on a sample
of 18 distribution transformers by applying overloads up to three times
the rated current at ambient temperatures as low as -40°C. The data
gathered is used to develop a thermal model representing the transient
behavior of distribution transformers. The results are then generalized
to produce a loading guide for evaluation of maximum hot-spot
temperatures and loss of life under given cold-load pickup conditions
From failure experience on power transformers very often it was suspected that inrush currents, occurring when energizing unloaded transformers, were the reason for damage. In this paper it was investigated how mechanical forces within the transformer coils build up under inrush compared to those occurring at short circuit. 2D and 3D computer modeling for a real 268 MVA, 525/17.75 kV three-legged step up transformer were employed. The results show that inrush current peaks of 70% of the rated short circuit current cause local forces in the same order of magnitude as those at short circuit. The resulting force summed up over the high voltage coil is even three times higher. Although inrush currents are normally smaller, the forces can have similar amplitudes as those at short circuit, with longer exposure time, however. Therefore, care has to be taken to avoid such high inrush currents. Today controlled switching offers an elegant and practical solution.
Guide for loading of oil immersed transformers
- Indian Standard
Indian Standard. Guide for loading of oil immersed transformers, IS:6600:1972.
Transformer reliability
- Gupta
Gupta HO. Transformer reliability. Electr India 2006;47:118-23.
Loss of life calculation of distribution transformers
- Rani R Gupta
- Ho
- Dwivedi
- Dk
Rani R, Gupta HO, Dwivedi DK. Loss of life calculation of distribution transformers. In: 10th Int Con Transf, TRAFOSEM-2008 IEEMA, New Delhi; 2008. p. 35–7.
Mechanical metallurgy. SI metric ed. London: McGraw-Hill Book Co Ltd
- G E Dieter
Dieter GE. Mechanical metallurgy. SI metric ed. London: McGraw-Hill Book Co Ltd; 2001. p. 432-70.
Loss of life calculation of distribution transformers
- R Rani
- H O Gupta
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Rani R, Gupta HO, Dwivedi DK. Loss of life calculation of distribution transformers. In: 10th Int Con Transf, TRAFOSEM-2008 IEEMA, New Delhi; 2008.
p. 35-7.
Theory of dislocations
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