Windings Of Distribution Transformers Research Articles

Creep failure analysis of crimp joints in aluminium and copper windings of distribution transformers

The present work investigates creep performance and contact resistance of Distribution Transformers (DTs) winding crimp joints under the influence of temperature and stress, aiming to reduce the failure rate of DTs. The integrity of crimp joints is crucial, as the field study revealed that a majority of crimp joints become unstable and show elevated contact resistance due to service exposure. These changes affect the steady-state creep rate of the crimp joint and the surrounding winding conductors, ultimately diminishing the operational lifespan of DTs. For experimentation, crimp joints with varying crimp points were fabricated for a 25 kVA DT using aluminium and copper conductors with diameters of 0.83 mm and 0.6 mm, respectively. For all types of joints i.e., Al-Al, Al-Cu and Cu-Cu, the 2-point crimp joint was identified as optimum in terms of tensile strength and contact resistance. The creep tests for the crimp joints were conducted under varying temperatures of 100 °C and 140 °C with stress levels of 40 MPa and 55 MPa as per ASTM E139-11. The steady-state creep rate and contact resistance of Al-Al crimp joints increased with an increase in temperature and stress. The creep life of DTs due to Al-Al and Al-Cu crimp joints is significantly less than the designed life of a typical DT. On the other hand, creep behaviour of copper winding crimp joints supports a significantly higher DT creep life even at a higher temperature of 140 °C and elevated stress of 55 MPa.

Creep Failure Analysis of Western Union Splice Joints in Distribution Transformer Winding

Creep Failure Analysis of Western Union Splice Joints in Distribution Transformer Winding

Novel power distribution short‐circuit testing technique based on supercapacitor and modular multi‐ports AC/DC conversion

AbstractWith the growing capacity of the distribution network, the probability of short circuit accidents gradually increases. Accidents such as deformation of distribution transformer windings, shift of laminations, fracture and even explosion of mechanical and insulation components caused by insufficient short‐circuit withstand ability seriously affect the safe and stable operation of the distribution network. The short‐circuit test proved to be an effective way to detect the performance of equipment under fault impact. A power source with supercapacitor is proposed here for short‐circuit test of 10 kV distribution transformers, which can realize the short‐circuit test application economically and flexibly with expandable capacity. First, the advantages and disadvantages of test power source by short‐circuit test generators, special power line, and the proposed method are compared. Second, the structure, working principle, design scheme, and control strategy of the novel short‐circuit testing device are introduced. Furthermore, a 14 MVA/5 MJ energy storage short‐circuit test power source is designed for the testing requirements of 10 kV/630 kVA distribution transformers. Finally, the simulations and experiments are used to verify its feasibility.

FINITE-ELEMENT CALCULATION OF ELECTROMAGNETIC FORCES IN THE DEFERENT SHAPES OF DISTRIBUTION TRANSFORMERS WINDING UNDER SHORT CIRCUIT CONDITION

This paper is concerned with calculating the electromagnetic forces in the windings of distribution transformers with different shapes of coils. The electromagnetic forces as well as the magnetic flux density and their distribution were analyzed and calculated using Finite Element Method (FEM). The Finite Element models of the distribution transformers with non-linear magnetic characteristics for the iron core are built using FEM software "ANSYS". In this paper, the static analysis method is based on two-dimensional models, and these models have been solved by using the formula for the magnetic vector voltage (A). Three types of three-phase distribution transformers were adopted, each with a capacity of 250 kVA and a voltage ratio is 11/0.416 kV. These types of transformers with different shapes of coils are stack core transformer with oval coil, wound core transformer with rectangular coil, and stack core transformer with cylindrical coil. The results obtained from the FEM analysis agreed with the design calculations which depend on the conventional design formulas. The most important contributions of this study are to building two-dimensional models for different types of distribution transformers. Calculating electromagnetic forces in transformers winding during short circuit conditions in different coil shapes. Studding the effect of the shape of the coils on the calculation of the electromagnetic forces in them. This work can save time, effort, and cost for transformer manufacturers in calculating the electromagnetic forces, also using this model for the virtual test leads to avoid the risk, and efforts spent to do the real short-circuit test.

Generalized regression neural network application for fault type detection in distribution transformer windings considering statistical indices

PurposeThis study aims to use frequency response analysis, a powerful tool to detect the location and types of transformer winding faults. Proposing an effective intelligent approach for interpreting the frequency responses is the most crucial problem of this method and has created many challenges.Design/methodology/approachHeat maps based on appropriate statistical indices have been supplied to depict the variations in the frequency responses associated with each fault type, fault location and fault extent along the windings. Also, after analyzing the results of artificial neural network (ANN) techniques, the generalized regression neural network method is introduced as the most effective solution for the classification of transformer winding faults.FindingsUsing a comparative approach, the performance of the used indices and ANN techniques are evaluated. The results showed the proper performance of Lin’s concordance coefficient (LCC) index and the amplitude (Amp) part of the frequency response. The proposed fitting percentage (FP) index can assist the intelligent classifiers in diagnosing the radial deformation (RD) fault with the highest accuracy considering all frequency response components in the classification procedure of winding faults.Practical implicationsVarious ANN techniques are used to detect and determine the type of four important faults of transformer winding, i.e. axial displacement, RD, disc space variation and short circuit. Various statistical indices, such as cross-correlation factor, LCC, standard difference area, sum of errors, normalized root-mean-square deviation and FP, are used to extract the features of the frequency responses to consider as the ANN inputs. In addition, different components of the frequency response, such as Amp, argument, real and imaginary parts are examined in this paper. To implement the proposed procedure, step by step, various types of winding faults with different locations and extents are applied on the 20 kV winding of a 1.6 MVA distribution transformer.Originality/valueContributions have been made in identifying and diagnosing transformer winding defects through the use of appropriate algorithms for future research.

Achieving the exact equivalent circuit of a large-scale transformer winding using an improved detailed model for partial discharge study

Investigating partial discharge (PD) is a critical issue in monitoring the condition of power transformers. For analyzing the transient state behavior of transformer winding, the availability of a model that accurately presents its behavior at high frequencies is necessary. The conventional detailed model is among the models used to perform PD studies. One of the underlying issues of the conventional detailed model is the low frequency validity range of the model, which is limited to about 1 MHz. In this paper, an improved detailed model is presented that overcomes the mentioned problem by adding stray capacitances to the conventional detailed model. A general state-space algorithm is proposed to calculate the reaction of the winding towards the application of the PD signal. In the present study, the frequency-dependent parameters of the improved detailed model have been calculated via the finite element method (FEM). The new methods and ideas are implemented and validated on a real 20 kV distribution transformer winding as well as a 63 kV, 30MV winding. Comparing the results of the proposed model with the waveforms obtained from using PD pulses to both windings in the laboratory environment shows the accuracy and the increased frequency validity range of this model.

A modified lumped parameter model of distribution transformer winding

The modelling of the distribution transformer winding is the starting point and serves as important basis for the transformer characteristics analysis and the lightning pulse response prediction. A distributed parameters model can depict the winding characteristics accurately, but it requires complex calculations. Lumped parameter model requires less calculations, but its applicable frequency range is not wide. This paper studies the amplitude-frequency characteristics of the lightning wave, compares the transformer modelling methods and finally proposes a modified lumped parameter model, based on the above comparison. The proposed model minimizes the errors provoked by the lumped parameter approximation, and the hyperbolic functions of the distributed parameter model. By this modification it becomes possible to accurately describe the winding characteristics and rapidly obtain the node voltage response. The proposed model can provide theoretical and experimental support to lightning protection of the distribution transformer.

Simulation modeling study on short circuit ability of distribution transformer

Under short circuit condition, the oil immersed distribution transformer will endure combined electro-thermal stress, eventually lead to the mechanical damage of the inner winding of distribution transformer, therefore it is necessary to study the short circuit ability of the oil immersed distribution transformer. In this paper, the typical current waveform of the distribution transformer under the short-circuit condition is analyzed theoretically. The short circuit fault simulation model of distribution transformer is built on the MATLAB/Simulink simulation platform, the short circuit waveform of the single phase, double phase and three phase short circuit is obtained. Further on the ANSYS simulation platform, three dimensional solid modeling of the three-phase five column transformer is carried out, the harmonic and transient field are applied to analyze the magnetic field distribution of the key components in distribution transformer under the short circuit condition, the internal cause of large short-circuit force in the winding of the distribution transformer under the short circuit condition is analyzed to certain extent. The results of the modeling and simulation show that the three phase short circuit current is larger than the single-phase and bi-phase short circuit current. Its value is 1.45 times the normal running current, which is the gradual damping asymmetrical short-circuit current. Magnetic induction intensity is in-homogeneous in the circumferential direction of the core column, and the degree of in-homogeneity varies along height. The magnetic density distribution under the short circuit condition is more than 6 times as much as the the normal condition. From the point of view of simulation modeling, short circuit ability of the oil immersed distribution transformer is analyzed, distribution characteristics of short circuit current and magnetic field of oil immersed distribution transformer under short circuit condition have been obtained, the theoretical reference value of the state characteristics and protection measures of oil immersed distribution transformer under the overload condition can be provided.

A Dimension Reduction Method Used in Detecting Errors of Distribution Transformer Connectivity

Many power utilities may have problems with the quality of data in records about which feeder a distribution transformer is connected to. This affects the operation and maintenance of smart grid infrastructure, outage management, line loss management, and workforce safety. The traditional manual way of verifying and updating the distribution transformer connectivity (DTC) is time-consuming and labor-intensive. Researchers have proposed the method which makes use of secondary side single-phase voltage of distribution transformers to verify DTC. However, when the windings of distribution transformers are yyn0 connected, the performance of the single-phase voltage method (SPVM) is unsatisfactory due to the unbalanced three-phase voltages. This paper proposed a dimension reduction method (DRM) that can convert the unbalanced three-phase voltage to a balanced voltage. The data which include 29 days' secondary side voltages of 3866 distribution transformers have been collected. The performances of the DRM and the SPVM have been compared. Results show that the DRM always has higher accuracy compared with SPVM. The influences of the maximum voltage difference and the reference distribution transformer have also been discussed. The DRM's results are more stable compared with SPVM.

Features of Testing Distribution Transformers for Short Circuit Withstand Strength

Short circuit (SC) withstand strength is one of the key characteristics that define the reliability of power transformers. The SC withstand strength test methods are stated in State Standards GOST (State Standard) 20243 and 55188 (IEC 60076-5). This work considers problems related to the SC withstand strength test of distribution transformers that are not indicated in these standards. The dependence of SC peak current on average temperature of windings is considered. The possibility of heating of the windings of distribution transformers during the SC withstand strength test and related to it change of peak factor are revealed. The requirements of standards related to the winding temperature when the SC withstand strength test are considered. It is revealed that during the test of distribution transformers, one can expect the deviation of actual peak factor from its designed value by about 5% that should be taken into account when testing and analyzing the test results. The specific of test of the transformers with δ-connected winding is that, as a rule, merely the line currents of this winding can be recorded during SC experiments, at that, there are differences in peak currents for line currents and phase currents of windings. The requirements presented in GOST (State Standard) 20243 and R 55188 (IEC 60076-5) for SC peak current were stated as applied to the phase currents of windings, as well as to the line currents of star-connected windings. The difference of the peak factor for the line currents when testing the transformers with star- and δ-connected windings was revealed in the work, and it was concluded that the abovementioned standards for the requirements and test methods to SC withstand strength of the transformers need to be refines.

Propagation of overvoltages transferred through distribution transformers in electric networks

The increasing requirements for improved reliability of electrical power devices and optimisation of dielectric insulation design means that the analysis of impact due to overvoltages is of high importance. This analysis should also consider propagation of overvoltages transferred through power transformer windings to other parts of the electrical network. Hazard levels of overvoltages affected the insulation systems of electrical devices are determined by wave propagation conditions of overvoltages as well as the protection level of applied overvoltages protection. The analysis of overvoltages transferred to low voltage networks is very important due to the increasing number of electrical devices and electronic apparatus with low resistance to overvoltages. This study presents a detailed analysis of the propagation of overvoltages transferred through distribution transformer windings, in typical medium and low-voltage electrical networks. The analysis is based on simulation results carried out by the use of electromagnetic transients program–alternative transients program (EMTP–ATP). A high-frequency model for transformers done on the basis of frequency characteristics of admittance measurements was used in simulations. Detailed analysis of influence of length of medium- and low-voltage lines connected to the distribution transformers and metal oxide surge arresters on switching overvoltages transferred to low-voltage networks are presented in the study.

Partial discharges location in transformer winding using wavelets and Kullback–Leibler divergence

A new algorithm to partial discharges (PD) location in transformer windings by means of the discrete wavelets transform (DWT) and the Kullback–Leibler (KL) divergence is presented. When the insulation system of transformers has considerable damage, high levels of PD appear. A PD analysis will help to prevent and to avoid catastrophic faults in the transformer insulation system, and is also useful to quantify the damage level into it. However, it is a complex task, because PD signals have a small magnitude and are presented during some microseconds. In this paper, a lumped parameter model RLC is used to model the transformer winding and to obtain PD reference signals. The DWT is used to process those signals. PD location is finally estimated by means of the minimum entropy concept that minimizes the Kullback–Leibler (KL) divergence between PD signals (test and reference signals). Different time durations and amplitudes to the PD signals were considered. The results of computer simulation confirm the accuracy of the proposed algorithm, with an error less than 5%. Also, the algorithm is validated in a distribution transformer winding.

Mitigation of lightning surge stresses in the high voltage windings of distribution transformers by introducing an electrostatic shield

The Medium Voltage (MV) Electricity Distribution Networks are frequently exposed to lightning and it is the main suspected reason for distribution transformer failures. Appreciable number of transformer failures occur due to insulation failures notably at points near line ends. The short rise time of a surge prompted by a lightning impulse can cause deterioration in the insulation and ultimately lead to a dielectric breakdown. The voltage distribution along transformer High Voltage (HV) winding becomes non-linear under surge conditions due to the capacitive-inductive currents. This research focuses on the possibility of neutralizing this effect with the aid of an electrostatic shield inserted in to the HV winding. Comparison of the behaviour of the winding under surge conditions with and without electrostatic shield requires appropriate transient models suitable for simulations. The necessary calculations required for this purpose were carried out and the proposed models are presented. The simulations were carried out with PSCAD software and the results are included in this paper. The oscillatory nature due to the combined effect of capacitors and inductors is observed in the wave tail region and some of the peaks created have potentials exceeding the peak potential of the surge waveform as well. The proposed transformer model have a definite positive effect compared with the available model as the results obtained by simulations clearly reveal the reduction of the stresses on the transformer HV windings when a shield is used. ENGINEER, Vol. 48, No.03, pp. 1-11 2015

Comparing Transformer Derating Under Harmonic Load and Unbalanced Supply Voltage Based on IEEEC57.110 Standard and TESFEM By FHL And K_Factor

Transformers are generally designed in order to deliver the required power to the connected loads with minimum losses at nominal fequency. Nowadays , power quality affected by non-linear loads and unbalanced condition and causes deflection from a perfect sinusoidal waveform. This condition leads to additional losses, increase temperature and reduction of the useful life of the transformers. In order to avoid of these difficulties, the nominal capacity of transformer should be reduced. This paper studies the increase of losses and Derating of Distribution transformer, working under Different harmonic load and unbalanced supply voltage conditions. In this paper, the three dimensional finite element method (3-D) is utilized as a Instruments for viewing magnetic flux density on core and windings of distribution transformers. The computer simulation according to the use of the FEM has been improved in Ansoft - Maxwell. In section 2, a 3-D time stepping finite element method (TSFEM) is used in order to modeling the transformer under different harmonic load with (THD I =12% and 23%). The maximum flux density in the transformer windings under sinusoidal and different harmonic load condition has been investigated in this section. In section 3, the effect of harmonic load on transformer load losses Has been examined. The result shows that under harmonic load current , copper loss an eddy current losses significantly increases. In section 4, derating of transformer under harmonic load based on IEEE c57.110 standard and TSFEM by using k-factor and harmonic loss factor (H FL ) Has been done. The result shows that IEEE standard is a Conservative method for transformer derating under high harmonic current (THD I ) and TSFEM is a Convenient method for transformer derating in this condition. In section 5, The Performance of the transformer when the supply voltage are balanced compared to the case in which supply voltage is unbalance , the result shows that , the unbalanced voltage Causes increase in the both copper loss and core loss. Afterwards, Based on the data obtained from the TSFEM, a distribution transformer under different unbalanced condition has been derated

An Estimation Method to Determine Equivalent Circuit Parameters for Harmonic Analysis

SUMMARYThis paper proposes a new estimation method to determine arbitrary circuit parameters for the harmonic analysis of a distribution network. Harmonic analysis is important in maintaining electric power quality. An interesting phenomenon, a temporary cut‐down of the harmonic voltage level at dawn, even if demand is slightly increasing at this time, is often observed in actual power systems. This paper proposes to utilize observations of this phenomenon to estimate circuit parameters. In addition, it is claimed that a detailed equivalent circuit is preferable to a conventional simplified equivalent circuit for explaining the phenomenon. This paper also discusses the usefulness of the Harmonic Characteristics Curve, which is proposed in this paper as a new concept, at the secondary winding of the distribution transformer, and the correlation of PQ, the monitored values of the active and reactive power at the secondary winding of distribution transformer. This correlation should determine the equivalent fundamental frequency admittance of the shunt capacitors installed for distribution lines. © 2014 Wiley Periodicals, Inc. Electr Eng Jpn, 187(1): 16–23, 2014; Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/eej.22451

Aplicación de un modelo de inventario con revisión periódica para la fabricación de transformadores de distribución

The paper proposes an Inventory Application Model with a periodic review under the policy (R, S), where the demand is not constant and varies widely. A proposed model with the historical critical materials used in the manufacture of low voltage windings of distribution transformers was proposed and implemented for the company studied by decreasing the inventory shortage. We found that the inventory holding costs for minimum levels of service was approximately 90% for each of the critical items. During the year the model was applied, penalty fine payments were reduced from $831,850 to $133,350 mx.

An Estimation Method to Determine Equivalent Circuit Parameters for the Harmonic Analysis

This paper proposes a new estimation method to determine any circuit parameters for the harmonic analysis of distribution network. The harmonic analysis is important to maintain quality of electric power. An interesting phenomenon, which is the cut-down of harmonic voltage level temporarily at dawn even if demand is a little bit increasing at this time, is often observed in the actual power system. This paper proposes utilizing the observation results of this phenomenon to estimate circuit parameters. In addition, this paper claim that detail equivalent circuit is more preferable for explaining that phenomenon than conventional simplified equivalent circuit. This paper also discusses the usefulness of the Harmonic Characteristics Curve, which is the proposed by this paper as new concept, at secondary winding of distribution transformer and the correlation of PQ, which are monitored values of active and reactive power at secondary winding of distribution transformer. This correlation should determine the equivalent fundamental frequency admittance of among of the shunt capacitors installed concerning distribution lines.

Signal Injection Strategies for Smart Metering Network Deployment in Multitransformer Secondary Substations

This paper analyzes the strategies for injecting power-line communications (PLC) metering signals in secondary substations with multiple distribution transformers. The study is based on real measurements of PLC connectivity across the low-voltage sections of transformers in a representative set of secondary substations. The results show that there is not a general rule indicating the quality of the PLC channel among secondary windings of distribution transformers in a secondary substation and, consequently, there is no general solution based in pre-existing information. Following this conclusion, a strategy is defined to maximize the performance of the PLC system, based on the PRIME PLC specification. This strategy is based in the use of a base node connected to one of the low-voltage panels, together with auxiliary nodes connected to the remaining low-voltage panels, and interconnecting all of the elements (base node and auxiliary devices) by means of non-PLC technologies.

Real-Time Techniques to Measure Winding Displacement in Transformers During Short-Circuit Tests

The ability of a transformer to withstand the dynamic effects of a short circuit test is conventionally evaluated by a measurement of short circuit reactance before and after the test. We propose new techniques for fault detection based on continual assessment during the test. The methods are based on the comparative null method for accuracy measurement in instrument transformers. They include voltage comparison, current comparison and real/reactive power measurements. High resolution sampling based acquisition systems are used to compute changes on a cycle-to-cycle basis. The proposed methods are validated on a number of models including a voltage transformer and a distribution transformer winding. They would be useful for designers in assessing events leading to failure in a more transparent manner.

Analysis of Very Fast Transients in Layer-Type Transformer Windings

This paper deals with the measurement, modeling, and simulation of very fast transient overvoltages in layer-type distribution transformer windings. Measurements were performed by applying a step impulse with 50-ns rise time on a single-phase test transformer equipped with measuring points along the winding. Voltages along the transformer windings were computed by applying multiconductor transmission-line theory for transformer layers and turns. Interturn voltage analysis has also been performed. Computations are performed by applying an inductance matrix determined in two different ways; by making use of the inverse capacitance matrix and by making use of the well known Maxwell formulas. The modeling of the transformer and the computations are verified by measurements