J. C. Olivares-Galvan

A Review of Transformer Losses

Abstract This article presents an extensive survey of current research on the transformer loss problem, particularly from the view of practical engineering applications. It reveals that the transformer loss problem remains an active research area. This article classified the transformer loss problem into three main groups: (a) tank losses due to high-current bushings, (b) losses in transformer core joints, and (c) stray losses in the transformer tank. It is based on over 50 published works, which are all systematically classified. The methods, the size of transformers, and other relevant aspects in the different works are discussed and presented.

A Novel Octagonal Wound Core for Distribution Transformers Validated by Electromagnetic Field Analysis and Comparison With Conventional Wound Core

This paper analyzes a novel configuration of transformer core, called octagonal wound core (OWC), and shows the minimization of the excitation current and the reduction of the eddy-current losses. The OWC is compared with the conventional wound core (CWC) configuration. The comparison is based on two-dimensional and three-dimensional finite-element method (FEM) simulations, taking into account the nonlinear properties of the magnetic material of the core. The results show that the OWC reduces the excitation current and the eddy-current losses when compared with CWC. Moreover, several combinations of grades of the grain-oriented silicon steel (GOSS) were investigated so as to further reduce the eddy-current losses and the excitation current.

Reduction of Stray Losses in Flange–Bolt Regions of Large Power Transformer Tanks

In large power transformers, the presence of stray currents in the structural elements near the high current bushings can be considerable, and this leads to hot spots. This work presents a practical analysis of overheating in the bolts that join the tank and the cover, which are near the high current bushings of the transformer. Overheating results are analyzed and discussed for the case of a 420-MVA transformer. The hot spots in the flange-bolt regions are discovered by thermal maps that are obtained during power transformer operation as a part of a preventive maintenance program. In this paper, we use copper links to ensure the connection of both the cover and tank body, significantly reducing the overheating of the flange-bolt region. The copper link solution has been validated by measurements. We have used calibrated measurement instruments in all the experiments. Moreover, a 3-D finite-element analysis of the geometry of interest has been used to verify the copper link solution.

New Analytical Formulas for Electromagnetic Field and Eddy Current Losses in Bushing Regions of Transformers

This paper presents a new and rigorous analytical calculation of electromagnetic field and eddy current losses in the zones of transformer tanks where bushings are mounted. This is done by solving Maxwells equations in the regions surrounding bushings, with the corresponding boundary conditions and considering linear permeability. Then, by solving the modified Bessels equation, the analytical formulas to calculate the magnetic field and eddy current losses in these regions are obtained and several cases are studied. The results are compared with 3-D finite element simulations and show very close correspondence. The obtained formulas allow straightforward calculations that can help designers to select proper parameters to optimize the design of transformers. This paper can be taken as the basis for the analysis of the nonlinear permeability case.

Techno-economic Evaluation of Reduction of Low-voltage Bushings Diameter in Single-phase Distribution Transformers

Abstract The contributions of this article are the analysis and economic evaluation of the impact on tank wall losses of a diameter reduction of low-voltage bushings of pole-mounted single-phase distribution transformers. Finite element simulations of 5- to 167-kVA transformers were performed. The study was motivated when bushing manufacturers reduced diameter from 4.6 to 3.6 cm. Results show that when the diameter of low-voltage bushings is reduced, (i) load losses increase and (ii) total owning cost decreases for transformers up to 15 kVA and increases for transformers of 25–167 kVA. The insertion of non-magnetic material between bushing holes is also evaluated.

Asymmetry During Load-Loss Measurement of Three-Phase Three-Limb Transformers

When the load-loss measurement test is conducted on three-phase transformers, an appreciable asymmetry is observed among the power readings of the three phases. This asymmetry is the result of two causes, viz. asymmetrical disposition of phases in space with respect to each other and unequal stray losses produced by phases. The disposition of phases leads to asymmetrical mutual impedances between phases and this is the principal contributor to the phenomenon. Another factor that may have an important contribution to the phenomenon is the deviation of the phase angle difference between the voltages of the three phase source (used during the test) from 120deg. The causes are analyzed using a detailed three-dimensional (3-D) finite-element (FE) simulation of a 31.5MVA, 132/33 kV transformer. In addition, a six-port network impedance model is deduced from open-circuit 3-D FE simulations. The impedance model is able to reproduce any condition of the transformer (e.g., open-circuit, short-circuit or on-load conditions) since it captures all the transformer electromagnetic phenomena. The six-port network results are discussed in order to elaborately clarify the intriguing problem of asymmetrical load-loss distribution, which is important for both transformer manufacturers and users. The results are further explained through sequence components of currents.

Comparative studies of the stabilities to oxidation and electrical discharge between ester fluids and transformer oils

The growing demands for improved fire safety, source material sustainability, environment friendliness, and asset life extension have driven the research and development efforts of natural/synthetic esters, less-flammable fluids. This contribution reports some investigations on commercially available ester fluids. Many comparisons are made to the quality test results of mineral oil, as this is something we are all familiar with. The stability under electrical stress and the stability to aging of the insulating fluids were investigated. The gassing performance characteristics of natural ester fluids are far superior to those of conventional mineral oil. A significant reduction in insulation aging rate was observed with synthetic ester fluids.

Separation of core losses in distribution transformers using experimental methods

The separation of eddy current and hysteresis losses in transformer cores is obtained using the two-temperatures and the two-frequency methods. Loss calculations for six ratings using the voltage test waveform ratio are included to compare and analyze results. A brief description of the test methodologies, that are easy to apply, is given. An example of the application of the methodologies, the obtained measurements and obtained results are included. In some cases, the results show that eddy current losses for the analyzed ratings are greater than 60% of no-load losses. Results include the impact of no-load losses in total owning cost of transformers.

Calculation of Nonlinear Electromagnetic Fields in the Steel Wall Vicinity of Transformer Bushings

Successful analytical formulas have been previously proposed to calculate the losses in tank regions of transformers assuming linear permeabilities in the analyzed boundary-valued problem. This has resulted in easy-to-implement and low-cost computational design procedures from a transformer factory economical point of view. However, designers and analysts of transformers are constantly seeking new ways of reducing transformer losses in actual power networks with thousands of transformers. As a result, this paper has focused on proposing new analytical formulas to determine the electromagnetic field in bushing regions of transformers, taking account of the true nature of the nonlinear permeability behavior of the tank wall. This way, the nonlinear Maxwells equations in the regions surrounding the bushings are solved using an integral equation formulation that properly includes boundary conditions. A practical iterative procedure is thus proposed to solve the resulting nonlinear equation. The iterative scheme shows excellent numerical convergence properties with a very low computational demand as compared with finite-element (FE) nonlinear models. A comparison between our analytical results and those of 3-D FE simulations reveals a close match for a wide range of conductor currents. Hence, our new formulas can be used to improve the design of transformers, increasing their efficiency.

Stability of environmental friendly fluids under electrical and thermal stresses

Knowing that insulating fluid is a weak link in a composite or impregnated dielectric system with reference to both dielectric strength and ease of contamination, many investigators have devoted great efforts to investigate various fluids. Increasing demand for environmentally friendly materials in the industry have driven the research and development efforts of natural/synthetic esters and less-flammable fluids. It is the object of this study to investigate the stability under electrical and thermal stresses of various insulating fluids. Comparisons are made to the quality test results of mineral oil, as this is something we are all familiar with. The gassing performance characteristics of natural ester fluids are far superior to those of conventional mineral oils.