IEEE C57.136-2000 pdf download IEEE Guide for Sound Level Abatement and Determination for Liquid-Immersed Power Transformers and Shunt Reactors Rated Over 500 kVA
3. Sources of sound
3.1 General characteristics Sound coming from the transformer itself, as opposed to cooling fan or pump sound, is tonal in nature, consisting of even harmonics of the power frequency. The predominant source of transformer sound is the core. The low-frequency, tonal nature of this sound makes it harder to mitigate than the broad-band, higher frequency sound that is produced by cooling fans, pumps, and other sources. Not only do low frequencies propagate farther with less attenuation, but also tonal sound is more annoying to the public even in a high sound level ambient. This combination of low attenuation and high perception makes tonal sound the dominant problem in the neighboring community around the transformer. To address this problem, most community sound ordinances impose penalties or stricter requirements for tonal sound. A site may be in compliance with overall sound levels, but out of compliance with tonal levels.
In siting a transformer it is, therefore, critical to consider the dominant tones in the sound spectra, and not just the overall sound levels. These tones will not only dictate the severity of the problem, but also the type of sound mitigation approach that is necessary. For example, if low-frequency tones such as the second and fourth harmonics of the power frequency are present, these are difficult to attenuate with simple sound abatement methods such as a single side wall. In this case an active control system or a full enclosure may be necessary. If very high frequency tones, such as the tenth or twelfth harmonics of the power frequency, are present in the near field, there is a chance that they will attenuate naturally, and no treatment may be necessary if the nearest neighbor is far enough away.
Tones are best measured on a narrow-band spectrum (1-2 Hz bandwidth). Although strong tones are notice- able on one-third or octave band spectra, at higher frequencies two or more tones will always be added together in the same band, making their relative contributions indistinguishable.
IEEE C57.136-2000 pdf download
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