Three-phase Core-type Transformer Research Articles

МАТЕМАТИЧНЕ МОДЕЛЮВАННЯ ТРИФАЗНОГО ТРАНСФОРМАТОРА З УРАХУВАННЯМ АСИМЕТРІЇ МАГНІТОПРОВОДУ

The paper analyses the aspects of modelling the operation of a three-phase core-type transformer. The features of the magnetic system construction of the transformer, which generate asymmetry, are disclosed. The fundamentals and methods for modelling a transformer in dynamic modes are considered. The main purpose of the paper is to research the influence of taking into account the asymmetry of the transformer magnetic core on its dynamic model. The paper proposes the mathematical model of a three-phase planar core-type transformer, which takes into account the asymmetry of the magnetic core. The mathematical model is based on the model of a generalized electric machine in the ABC axes, which is widely recognized and characterized by a high level of adequacy. A refined model, which takes into account the power losses in the magnetic core, has used. As is well known, such a model is characterized by higher accuracy and adequacy in the calculation of transient processes. For the proposed mathematical model, a comparative analysis of the calculated curves of transient processes for a 63 kVA transformer with a voltage of 6 / 0.4 kV is made. To disclose the effect of taking into account the magnetic core asymmetry of the transformer in the mathematical model, the current differences were calculated in pu for the modes of switching on at full power and three-phase short circuit. These modes are the most common and indicative in the analysis of the operability and stability of transformers. It was found that the most significant difference, which is obtained by taking into account the magnetic system asymmetry of the transformer, is 5…6% when it is switched on at full load. Thus, the relatively simple transformations and updating of the mathematical model of the transformer allow significantly increase its adequacy. The proposed mathematical model can be more effectively used when the non-typical design of three-phase transformers or reactors with significant asymmetry of the magnetic core is considered.

Performing Structural Design and Modeling of Transformers Using ANSYS-Maxwell

Transformers have attracted great interest since they have been used due to their robustness and application in power systems. Therefore, the nominal values of transformers grow even more in larger power systems due to the constantly increasing power demand. Many types of research are carried out to increase the performance characteristics of transformers and their compatibility with power systems. There are different methods and analysis tools for these studies. One of them is ANSYS@Maxwell, which performs analysis based on the Finite Element Method (FEM). With this program, the design, modeling, analysis, and performance evaluation of the transformer in a high-performance simulation environment can be achieved through effective strategic modeling. In this study, the design and modeling of a three-phase core-type transformer with coils and terminals are explained in detail in ANSYS @ Maxwell simulation platform. Besides, the transformer models examined were adapted using ANSYS@MAXWELL software based on the finite element method. Analyzes are performed to estimate the core-losses, leak-losses, DC-losses, and winding-eddy current losses-of transformers with this program. A large number of meshes were used in FEM analysis of 2D and 3D models to examine the losses in detail.

Duality-Derived Transformer Models for Low-Frequency Electromagnetic Transients—Part II: Complementary Modeling Guidelines

This two-part paper is intended to clarify definitions in dual transformer modeling that are vague, provide accurate modeling guidelines, clarify misconceptions about numerical instability, provide a unified dual model for a specific type of transformer, and introduce new paths of research to the power systems/electrical machinery community for low-frequency transients. Part I discussed the topology of duality-based models and some important issues, such as the most common approaches to derive dual models and their variety, the equivalence of negative inductance and mutual couplings to represent the leakage inductance of three-winding transformers, and the numerical oscillations caused by the use of nontopological models. This part of the paper discusses and compares white-, gray-, and black-box models. The paper also reviews hysteresis models (static and dynamic) and highlights the differences between the air-core inductance and saturation inductance. The available dual models for the representation of the transformer tank are then presented. A unified and accurate model of a three-phase core-type transformer adequate for all low-frequency transients is presented. Finally, concrete guidelines are presented for the appropriate selection of the model topology and parameters for different low-frequency transient studies.

A Novel Offline to Online Approach to Detect Transformer Interturn Fault

This paper presents an approach based on the use of transformer no-load and light load current harmonic analysis to detect the presence of an interturn fault at the incipient stage. The presence of fault is detected by performing a trend analysis of specific harmonic components and primary current magnitude. Transformer testing methods suffer from a common drawback of offline nature and require an expert opinion for accurate condition assessment. This paper presents a method that transforms no-load current harmonic analysis, which is a special test for transformer testing, into an online method at light load conditions. Thus, the necessity of complete shutdown and dependency on expert opinion is eliminated since it only requires one reference signature per phase. The proposed approach is verified for different vector groups of the three-phase core-type transformer and three independent units connected in star-star and for a fault on the low-voltage or high-voltage winding of the transformer. The presented experimentation shows that the proposed approach can detect the presence of a fault at an incipient stage involving less than 2% of turns and fault current up to 1 p.u. A case study is presented that validates the method for the transformer of a higher power rating.

Comparison of four methods for computing positive‐sequence reactances of three‐phase core‐type transformers

This study compares four methods for computing the positive-sequence reactances of three-phase core-type transformers: traditional basic formulae (TBF); two-dimensional (2D) finite difference method (FDM); and 2D and 3D finite element methods (FEM). A computer program was coded for the FDM, and a commercial software was applied for the FEM. The four applied methods are magnetostatic, and they were applied for computing the three positive-sequence short-circuit reactances of a three-winding transformer. The results of these methods were compared among themselves and with the measured values. In comparison with the measured values: (a) the TBF are relatively accurate, although they are based on an approximate representation of a simple 2D cross-section for the transformer window; (b) the results with different 2D cross-sections are also relatively accurate; (c) the results with the 3D modelling are more accurate than the TBF. These facts confirm that the space between the windings determines the value of the transformer positive-sequence reactance. Therefore the tank-winding distance and the tank magnetic permeability have little effect on the results. A weighted average of the results of different 2D cross-sections was computed with the FDM and with the FEM, and the obtained values are very close to the result with the 3D modelling. Although the results are relatively accurate, the main simplifications related to the performed magnetostatic analysis are highlighted.

The effects of linear and non-linear loads on three-phase transformers supplied from different sources

In this study a new approach for the calculation of harmonic transformer parameters by making use of equivalent circuit model of one phase transformers is put forth. With this aim in mind, a three-phase core type transformer from 4 kVA, 400/230 V, 50 Hz, Yy0 connection group was used. To supply transformers voltage sources with sinusoidal and non-sinusoidal wave shapes were used. The transformers supplied on these sources were connected two different loads: one linear and one non-linear. The interactions of these transformers, under different load and supplies, with harmonics were measured simultaneously with two harmonic analyzing devices connected to primer and secondary and the measurements up to the 15th harmonics were assessed and equivalent circuit components were calculated for each harmonic component separately. From the values measured and calculated, copper losses of primer and secondary circuits of three-phase transformers, iron losses and the efficiency of the transformer was calculated harmonically. Measurement and calculations were made separately for two different supplying and two different loads and results were compared. Besides, for each condition, total harmonic distortion (THD) of voltage and current and K factors were calculated, their effects on transformers performance were revealed. Key words: Harmonics, linear loads, non-linear load, transformer loses, equivalent circuit, k-factor.

A Three-Phase Core-Type Transformer Iron Core Model With Included Magnetic Cross Saturation

A Three-Phase Core-Type Transformer Iron Core Model With Included Magnetic Cross Saturation

Ferroresonance Phenomenon in Power Transformers - Experimental Assessment

This paper reports on the results of recent research and development efforts to study and assess various impacts of ferroresonance phenomenon on operating performance of power transformers and to investigate ways to mitigate the effects of such phenomenon. Three-phase core-type transformers are used in the study. The results of the technical investigation and the associated ferroresonance experimental work performed showed that ferroresonance can cause dangerous overvoltages and overcurrents in three-phase coretype transformers. Sensitivity analysis results are also presented to determine the impact of various design and operating parameters on the resulting overvoltages and overcurrents caused by ferroresonance.

Topology based magnetic model for steady-state and transient studies for three-phase core type transformers

Existing matrix models for transformers reproduce specified short-circuit tests, portraying leakage flux in air, but do not explicitly model the core. Therefore, a precise relation between core flux and air flux is not clearly established. Core representation is defined simply on the basis of a specified excitation current (positive- and zero-sequence), permitting linear magnetizing branches to be incorporated in the matrix. Nonlinear inductances are connected separately at winding terminals to introduce core nonlinearities, however these are not identified physically with individual core limbs, making precise accommodation of hysteresis, saturation and eddy current effects difficult. Published models based on the duality existing between magnetic and electric circuits identify a means for correctly interfacing core flux with air flux, permitting core nonlinearities to be incorporated on a physical basis. However practical application of these models has been hampered by a difficulty in deriving the required model parameters. A formulation is presented here to build a topological model based on normally available test data. Results of short-circuit tests are presented demonstrating that existing matrix models can introduce errors. >

Exciting Inrush Current of Three-Phase Core-Type Transformer with Three Legs

Exciting Inrush Current of Three-Phase Core-Type Transformer with Three Legs