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The subject of deciding on the most appropriate method of determining the true sound power level of a power transformer has been discussed through the transformer industry for decades. It is the objective of this 2 -Part paper to identify magnitudes of the contributions to the inherent difference in power transformer noise level measurements when using the Sound Pressure method versus when using the Sound Intensity method. The paper presents results of comprehensive outdoors and indoors measurements made of the frequency spectrum and total noise of a large number of power transformers using the two measuring methods. In Part -I of the paper, results of the outdoor measurements made on the total noise produced by three power transformers in low ambient / no sound reflections locations are presented. Results of the indoors measurements made on 48 power transformers of 12 -336 MVA power ratings are presented in Part -II of the paper. Appropriate conditions for accurate indoors measurement of transformer noise when using the Sound Intensity method have been derived. Modifications to the existing IEC and IEEE Industry Standards of transformer noise testing are proposed.

This paper presents the following aspects of Load noise in power Transformers: (1) Characteristics of Load Noise, (2) Contribution of load noise to the total noise of a transformer and why it has not been well -recognized by the industry in USA and some other parts of the world, (3) Accurate calculation of load noise and the need for the accurate calculation, (4) Accurate measurement of load noise, (5) Magnitude of variability of load noise, (6) Parameters affecting load noise level and means to reduce load noise, and finally (7) Need for the inclusion of load noise in the IEEE Standards and customer specifications.

Over the years, there have been a number of cases where users experienced higher noise levels of transformers on-site versus what was measured in the factory acceptance tests. In some of these cases, measurements have shown that a power transformer can have significantly different noise characteristics (both noise level and frequency spectrum) on-site compared to what was measured in the factory. This difference can range from a few decibels to more than 10 dB in some cases. Some of these differences are found to be caused by local environmental factors that influenced the on-site measurements. Others were caused by operating conditions on-site that are different from those of the factory tests. This paper presents results of investigations and measurements performed, over the past several years, on the main contributors to this difference. For each of these contributors, data are presented that demonstrate the impact of this contributor as well as quantify typical magnitudes of this impact. It is hoped that the material presented in this paper will assist users in: 1) understanding some of the pitfalls of on-site measurement of transformer noise; 2) how operating conditions contribute to the noise performance of transformers on-site; and 3) how significantly higher on-site noise levels can, in some cases, indicate an issue with the transformer or any of its components/accessories. It is also hoped that the material in this paper will contribute to industry standards/application guides on noise determination and abatement.

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