Ed G. teNyenhuis

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.

Power Transformer Characteristics and Their Effects on Protective Relays

There are a variety of protective relays using different measuring techniques to provide reliable and secure transformer protection. This includes electro-mechanical, solid state and numerical relays. Within each group, various algorithms exist. Advancements made to transformer technology and design over the past 3 decades, have changed the characteristics of the transformer inrush current, and have often introduced incorrect operations in the existing harmonic restraint relays during energization

Flux distribution and core loss calculation for single phase and five limb three phase transformer core designs

This paper presents results of an analytical core loss calculation method developed for single phase and five limb three phase core types. Shown are calculation results of flux distributions, flux density waveshapes, spatial flux density curves and loss distributions in cores other than the 3 phase-3 limb core. The analytical results show that the flux distribution is essentially uniform in the single-phase 2 limb and 3 limb cores. The flux waveforms are also shown to be essentially sinusoidal in different regions of the core. However, for the single-phase 4 limb core and the three-phase 5 limb core, the flux distribution is nonuniform and flux waves are very distorted. The calculated values of core losses were, generally, in excellent agreement with tested core losses of actual commercial transformers. The results on the very large 4 and 5 limb cores show that the production complexity of these cores cause a slightly higher tested core loss which needs to be included in the development of the core loss calculation for these cores.

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)

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.

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 Hz 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. Measured data, provided by several manufacturers, was used to confirm the theoretical results. This paper will 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 will make 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.

Closure on "Proposed standards for frequency conversion factors of transformer performance parameters"

The author replies to comments by J.C. Olivares et al. (see ibid., vol.18, no.4, p.1599, 2003) on the original paper by R.S. Girgis et al. (see Proc. IEEE Power Eng. Soc. Transm. Dist. Conf. Expo., vol. 1, p. 153-8, 2001). They address the issue of conversion factors for %Z, the magnetic flux density and the noise conversion factor.

Other factors contributing to the core loss performance of power and distribution transformers

Summary form only given, as follows. 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.