Ramsis S. Girgis

Calculation of core hot-spot temperature in power and distribution transformers

Accurate calculation of the value of the core hot-spot temperature at the design stage is becoming very critical in order to ensure that hydrogen generation that can be caused by oil-film degradation at core hot-spot temperatures as low as 110-120 ºC is limited. Presented are two calculation methods developed for this purpose. Results of the FEM calculation of the location and value of the hot-spot temperature in cores of three-phase, three-limb cores of power and distribution transformers show excellent agreement with tested values on a number of transformers. Insight learned from this analysis was used to enhance a previously available empirical method. This simplified calculation method is appropriate for everyday use and, hence, is suitable for design calculations. This method was shown to have very good accuracy in the range of 2 ºC for most of the cases. The simplified calculation method was also shown to provide a much greater accuracy than the original empirical method.

Performance Parameters of Power Transformers Using 3D Magnetic Field Calculations

Todays challenges in the design of power transformers necessitate, among other things, very accurate calculation of the performance parameters of the transformer (such as losses, impedance, shielding, s.c. forces, etc.), reduction of load losses, and preventing overheating and local hot spots. Presented in this paper are theoretical and experimental studies conducted to precisely assess the need and projected benefits of these calculations based on the 3D determination of the leakage field of the transformer. Also presented are results of the already achieved objectives in winding loss calculation and magnetic shielding design.

Other Factors Contributing to the Core Loss Performance of Power and Distribution Transformers

This paper presents results of analytical and experimental studies that give insight into the nature and mechanism of the generation of localized losses in the core joints, losses caused by core stacking holes, and the increase of iron loss of the core material due to slitting. These three loss components alone have been found to contribute about 3-4% to the total no-load loss of a large power transformer and as much as 10% in a small distribution transformer. The results show how the magnitude of these loss components varies with operating flux density, core material, excitation frequency, and geometrical parameters of the core.

Measured Variability of Performance Parameters of Power & Distribution Transformers

This paper presents measured variability between values of performance parameters of three multi-unit orders of power and distribution transformers. The performance parameters studied are no-load loss, exciting current, load loss, impedance, windings temperature rise, and sound level. Factors contributing to the magnitudes of the variability of these different performance parameters are discussed in detail. These factors include material variability, production process variability, and measuring process variability. As demonstrated in the paper, each of these contributes to the variability of the different performance parameters of the transformer in different proportions. Finally, the paper discusses how the results presented can be used to determine the appropriate design margin as well as tolerances to be allowed in industry standards. The results of this work have already been used to arrive at the new tolerances implemented in IEEE C57.12.00 for no load and load losses (Ref. 1)

Calculation of Winding Losses in Shell-Form Transformers for Improved Accuracy and Reliability Part-1: Calculation Procedure and Program Description

This paper presents a 3D program developed by Westinghouse Electric Corporation for accurate calculation of eddy and circulating current losses in windings of shell-form power transformers. Comparisons made between the 3D results, test results, and other analytical results on a number of commercial transformers prove the validity-of the program and the superior accuracy it provides. As described in the paper, the 3D program calculates, in addition, the relative contributions of the individual coils to the total winding losses. Such information is used to arrive at effective winding design methods for reducing load losses. The program also calculates magnitudes of the highest circulating currents and locations of the most heavily loaded strands. Such information is very important in designing reliable transformers by avoiding winding hot spots. This paper is divided into two parts. In this part of the paper the 3D loss program is described. First, the front-end of the program and the computer communication system used to submit, execute, and returns the results of, the program are described. This is followed by a concise description of the calculation procedure used in the program and the output of the program.

Experimental Verification Of Three-Dimensional Analysis Of Leakage Magnetic Fields In Large Power Transformers

In a previous paper by the first author, a three-dimensional analysis for the calculation of the leakage magnetic field in power transformers was presented. The experimental verification of this analysis is the subject of this paper. Extensive comparisons between calculated and measured values of flux densities in the winding of an experimental three-phase shell form transformer are presented. The results as well as the agreements and deviations obtained are discussed in light of the ultimate objective of accurate determination of various transformer performance parameters. The close agreements obtained verify the accuracy of the method of calculation as well as the validity of the experimental and measuring techniques. The results also demonstrate that accurate determination of losses, circulating currents, short circuit forces, impedance, shielding requirements and other performance parameters is possible only through three-dimensional calculations of the leakage magnetic field in the transformer.

Impact of GICs on Power Transformers: Overheating is not the real issue.

There has been some misconception in the electric power industry that geomagnetically induced currents (GICs) can cause significant overheating damage to the majority of medium and large power transformers. Because of the nature of GICs, the majority of power transformers would not experience damaging overheating because of them. This article presents the real issue with GICs, which is that the combination of the increase of reactive power absorption and injected current harmonics into the power system could result in compromised grid stability.

Hydrogen gas generation due to moderately overheated transformer cores

This paper first presents an overview of this phenomenon from its initial discovery to the stage of the factory/laboratory investigations performed to confirm this new mechanism of gas generat ion in power transformers. This is followed by an overview of the increased significance of developing accurate calculation of the core hot spot temperature. The paper then presents the detailed accurate diagnosis performed on a 600 MVA transformer that had this issue in the field, the improvement made, and the calculation of the predicted gas generation performance of its loading cycle throughout a year of operation. The paper then presents the basis for the additions/changes proposed to be implemented in the IEEE Standards as a result of the discovery of this new gas generation mechanism. The paper also provides the basis for recommending to the IEEE Standards the selection of 130°C as the allowed maximum core hot spot temperature under sustained worst conditions of load, core — excitation and ambient temperature.

Appropriate test conditions proposed for Industry Standards of measuring transformer noise

The subject of deciding on the most appropriate method and conditions for accurately measuring the noise level of a transformer has been discussed through the transformer industry for decades. It is the objective of this Paper to contribute to this effort by identifying more appropriate conditions for both the Sound Pressure Method and the Sound Intensity Method. The Paper presents results of comprehensive indoor measurements made of the frequency spectrum and total noise of a large number of power transformers using the two measuring methods. First, the paper presents data used to develop appropriate conditions for more accurate measurement of transformer noise using the Sound Intensity Method. The paper then presents data used to develop a more accurate and reliable Sound Wall - Reflection Correction to be used with the Sound Pressure Method. Lastly, the Paper presents appropriate method of measuring Load noise of power transformers. Sound measuring conditions developed and presented in this paper are being proposed to replace existing conditions in the IEC and IEEE Industry Standards of transformer noise testing.

Proposed standards for frequency conversion factors of transformer performance parameters

This paper presents the development of appropriate frequency conversion factors for performance parameters of transformers. These factors are needed in order to allow manufacturers to convert measured values from 60 to 50 Hz and vice versa. Conversion factors are necessary when equipment at a manufacturers test facility allows direct measurement at one frequency and not the other. These factors are developed in this paper for no load loss, exciting current, load loss, and noise level using both analytical and actual tested data. The measured data, provided by several manufacturers, were used to confirm the theoretical results. This paper can be used as a basis for proposing standard frequency conversion factors in the IEEE/ANSI and IEC test standards for power and distribution transformers. Such a standard makes it possible for manufacturers to use the same frequency conversion factors, and hence, have a more uniform accuracy of the reported test data at both 50 and 60 Hz.