Proposed Transformer Model Research Articles

Improved low‐frequency transformer model based on Jiles–Atherton hysteresis theory

Transformers are the most widely used apparatus in power systems to transfer electrical energy from one side to another. Despite remarkable achievements in transformer modelling over the past few years, three-phase transformer modelling techniques have not progressed to that of their single-phase counterparts, mainly because of topological correctness of core and coil structure, complicated interaction between fluxes of the three iron core legs, and non-linear saturation of ferromagnetic material. An improved three-phase transformer model with extended inverse Jiles-Atherton (JA) hysteresis theory is proposed in this study. The extended inverse JA model with anisotropic anhysteretic magnetisation function is addressed, and the classical eddy-current-loss term and anomalous-loss term are directly incorporated into the original energy balance equation to represent the non-linearity and iron core losses rather than representing them with a parallel combination of resistances and saturable inductances (R//L model) as most previous studies have done. The results of inrush current case indicate that the proposed transformer model can predict the residual flux, include eddy-current-loss and anomalous-loss, and predict the inrush current peak values with relative error less than 6%.

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

Enhanced Transformer Model for Low- and Mid-Frequency Transients—Part II: Validation and Simulation Results

The transformer model developed in Part I is validated based on the zero-sequence test data, a ferroresonance event, and the geomagnetically induced current (GIC) measurements. Various transformer core constructions, including single phase, three limb, five limb, conventional shell type, and seven-limb shell type are represented, based on the proposed transformer model. The simulation results show high accuracy of the proposed transformer model in representing the studied cases. The study reveals that the saturation of the transformers at a given GIC level is more severe than that predicted by the existing transformer models. Furthermore, the proposed transformer model clearly explains the reasons for the previously reported discrepancies between the GIC experimental and simulation results. The study also concludes that for an accurate transformer model, particularly for the three-limb core type, the detailed representation of the zero-sequence characteristic is an essential requirement. Such a characteristic can be obtained based on either the proposed approach of the paper or a zero-sequence dc excitation test in addition to the test data at the power frequency.

A method for transformation of engineering bill of materials to maintenance bill of materials

In order to solve the transformation problem of Bill of Materials (BOM) from engineering BOM to maintenance BOM for Maintenance, Repair and Overhaul (MRO) systems, a formal transformation model of BOM view is proposed. In this model, the intermediate component, inherited component, virtual component are defined in the specific maintenance management domain, and the transformation process from engineering BOM to maintenance BOM is discussed through feature recognition methods and rules. The proposed transformation model has been developed and deployed in an MRO system for a steel manufacturing enterprise.

Implementation of three-phase transformer model in radial load-flow analysis

This paper presents an efficient approach for developing three-phase transformer admittance matrices in the radial power-flow analysis. The proposed transformer model overcomes the singularity problem of the nodal admittance submatrices of ungrounded transformer configurations. This has been achieved by applying symmetrical components modeling. The classical (6×6) transformer nodal admittance matrix written in phase components is converted to sequence components instead of the (3×3) admittance submatrices. In this model, the phase shifts accompanied with special transformer connections are included in the radial power-flow solution process without any convergence problems. The final model of the transformer is represented by a generalized power-flow equation written in phase components. The developed equation is applicable for all transformer connections. The transformer model is integrated into the radial power-flow and tested using the IEEE radial feeders. The results have shown that the developed transformer model is very efficient and the radial power-flow has robust convergence characteristics.

A Class of Discrete Transformation Survival Models With Application to Default Probability Prediction

Corporate bankruptcy prediction plays a central role in academic finance research, business practice, and government regulation. Consequently, accurate default probability prediction is extremely important. We propose to apply a discrete transformation family of survival models to corporate default risk predictions. A class of Box-Cox transformations and logarithmic transformations is naturally adopted. The proposed transformation model family is shown to include the popular Shumway model and the grouped relative risk model. We show that a transformation parameter different from those two models is needed for default prediction using a bankruptcy dataset. In addition, we show using out-of-sample validation statistics that our model improves performance. We use the estimated default probability to examine a popular asset pricing question and determine whether default risk has carried a premium. Due to some distinct features of the bankruptcy application, the proposed class of discrete transformation survival models with time-varying covariates is different from the continuous survival models in the survival analysis literature. Their similarities and differences are discussed.

Lightning-Induced Overvoltages Transmitted Over Distribution Transformer With MV Spark-Gap Operation—Part I: High-Frequency Transformer Model

In Finland, distribution transformers are frequently subjected to lightning strokes. Therefore, it is worthy to study the voltage transferred to the secondary terminal of the transformer due to lightning strokes hitting the primary terminals. In order to do conduct this study, the high-frequency model of the distribution transformer is to be investigated. In this paper, an accurate and simplified high-frequency model of the distribution transformer under lightning strokes is presented. The proposed model is introduced based on two-port network theory where its parameters are calculated at two resonance frequencies experimentally measured. The model is suitable for unloaded and loaded conditions and it is experimentally verified under different balanced loads considering two different practical distribution transformers. Then, the model verification is extended to study the impact of medium-voltage spark-gap operation on the transmitted voltage. The experimental results have validated the proposed transformer model.

Modeling and Analysis of a Single-Phase Distribution Transformer With Midtap on the Secondary Side

This paper presents an equivalent mathematical model for a typical single-phase distribution transformer with midtap on the secondary side and constructs its function blocks using Matlab/Simulink to verify its correctness under normal loading and short-circuit conditions. Both the copper and core losses can be taken into account in the proposed model. Furthermore, a sample distribution system is built to examine the correctness and performance of the proposed mathematical model. The field test and simulation results indicate that the proposed transformer model can correctly portray the characteristics of a physical single-phase distribution transformer with midtap on the secondary side under normal loading and various short-circuit conditions.