Salvador Magdaleno-Adame

Comparative studies of the stability of various fluids under electrical discharge and thermal stresses

It is a well-known fact that the service reliability of power transformers largely depends upon the condition of the dielectric fluid. The steady deterioration (under the influence of the multiple stresses) of the insulation has an important impact on the condition of the transformers themselves. This contribution reports some investigations on some commercially available ester, silicone and mineral oil fluids. Some of their physicochemical properties are studied by submitting them to various stresses: electrical, thermal and oxidation. Many comparisons are made according to the quality test results of mineral oil, as it is very familiar to the transformer industry. A new technique using a reactive free radical reagent, 2,2-diphenyl-1- picrylhydrazyl (DPPH), added to both new oil for reference purposes and different aged oil to assess free radical concentration is presented. The gassing tendency under either thermal or electrical stress, along with the physicochemical properties of the fluids, is affected; it is assessed with the production of oxidative aging by-products. 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.

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.

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.

3D Finite Element analysis of magnetic shunts and aluminum shields in clamping frames of distribution transformers

This paper presents a 3D Finite Element (FE) analysis of stray losses in core-type distribution transformers, where a magnetic shunt and a C-shape electromagnetic shield are utilized to protect a clamping frame of stray fluxes produced by high current leads (HCLs) of low voltage bushings. A real clamping frame of a three-phase core-type oil distribution transformer is utilized to study the electromagnetic behaviour of the shields and magnetic shunts. The electrical steel of the magnetic shunt is modelled utilizing anisotropic properties and aluminum is employed for the electromagnetic shield. Impedance boundaries were utilized in the frame and in the aluminum shield to compute the stray losses. Stray losses in the clamping frame are computed for three cases: 1) frame without any shielding method, 2) frame with a magnetic shunt, and 3) frame with a C-shape aluminum shield. The current in the HCLs were varied from 2 kA to 20 kA. The results obtained between the use of electromagnetic shield and magnetic shunts in frames of distribution transformers are also discussed in this paper.

Impact of low voltage bushings diameter on single-phase distribution transformers losses

This paper analyses steel tank wall losses of pole-mounted single-phase distribution transformers due to high currents crossing through the bushing holes of the low voltage terminals, and evaluates the impact of the holes diameter on load losses. The study also includes a description and comparison of analytical and empirical approaches used by other authors. The analysis was performed using simulations obtained by finite element method. The finite element model was validated reproducing results found in literature and simulations obtained for a 37.5kVA single-phase transformer model. The study was motivated by a reduction of diameter (from 4.6cm to 3.6cm) in the bushings supplied by manufacturers that represented a cost reduction. Results show no appreciable change of load losses with 1cm diameter reduction and consequently the decrease in manufacturing cost means a beneficial impact.

Reduction of stray losses in Tertiary Voltage Bushings in power transformer tanks

This paper presents an analysis and computation of stray losses in the tank cover of a 75 MVA three-phase core-type transformer. Stray losses in the region surrounding high current bushings are estimated using 3D Finite Element (FE) simulations. In the considered region the stray losses are high and its reduction is important to avoid the presence of hot spots in the tank cover of power transformer. In this paper, an L-shape non-magnetic Stainless Steel Insert (SSI) is utilized to reduce the stray losses in the region of the Tertiary Voltage Bushings (TVBs) of the transformer. Stray losses in the tank cover are estimated for a level of overload of 30% considering two cases: 1) When there is no SSI and 2) When the SSI is considered. The reduction of stray losses in the tank cover of power transformers helps to avoid the presence of dangerous high temperature spots. Hot spots can degrade the transformer oil and they can produce a potential failure of the equipment during operation.

Wireless Power Transfer: Literature survey

This paper analyzes the bibliography on Wireless Power Transfer research covering the period of 2001-2013 using two databases (Web of Science and Scopus). We will report some differences found in the databases used in this research. Due to the large number of publications in peer review journals, conferences and symposia contributions were not included. Four tens of peer review journals were investigated, in which 90 and 122 papers including the words Wireless Power Transfer in their title have been published in the period 2001-2013, respectively. The most productive and high-impact authors and countries are identified. The four most productive countries are USA, South Korea, China, Japan and Singapore in both databases. 127 average citations were received by each one of the five most cited papers in Scopus. The bibliographic research presented in this paper is important because it includes and analyzes the best research papers on Wireless Power Transfer coming from more than two tens of countries all over the world and published in top rated science technology journals.

Experimental procedure to obtain electromagnetic properties of A-36 low carbon steel plates utilized in transformers

Electromagnetic properties of low carbon steel, used for structural elements (tank, flitch plates and frames) of distribution and power transformers, are unknown to designers and manufacturers. Normally, steel manufacturers do not provide information on electromagnetic properties for low carbon steels. These properties are important to determine the power losses in the structural elements which have an impact in the temperature of these elements. Generally, designers use some estimations of low carbon steel properties. Hence, knowledge of the low carbon steel properties that are close to the real ones will definitely help to have better designed transformers, as it will give designers the opportunity to choose the steel that produces lower losses and consequently lower temperature in structural parts. This paper presents an experimental procedure to obtain electromagnetic properties for A-36 low carbon steel used to manufacture structural elements of distribution and power transformers. The Epstein frame method is applied to obtain the electromagnetic properties for A-36 carbon steel plates. Magnetization and power loss curves were obtained in laboratory tests. This paper provides electromagnetic properties for this low carbon steel. This is valuable information for transformer designers, as the real magnetic saturation and real stray losses produced in structural elements that use this material can be calculated.