The Sound of Silence: ‘Silent Transformers’ to Help Consolidated Edison Meet New York City’s Ultrastrict Noise Ordinances
Silence is a source of great strength. Although these words were coined by the ancient Chinese philosopher Lao Tzu, they apply to modern transformers. These devices ensure our power supply, and it is important not to ignore the negative sides of their operation. Noise is a major issue.
Transformers exhibit vibrations while operating, which generate a characteristic hum classified as noise. This noise is characterized mainly by four pure tones, the frequencies of which are in the range of human speech. The noise causes irritation and discomfort, so it is important to ensure residential areas are unaffected by noise of nearby transformer installations.
Probably the strictest noise ordinance in the world is in New York City. To fulfill the restrictions, it is necessary to understand the total process of sound generation, transmission and radiation. Such knowledge has enabled ABB to design and build quiet transformers for customers throughout the world. Most recently, ABB delivered ultralow-noise power transformers, referred to in this article as “silent transformers,” to the Consolidated Edison (ConEd) utility in the Manhattan borough of New York City.
Transformer Noise, Characteristics
Three sources of sound and noise exist in power transformers: core noise, load noise and cooling system noise.
Core noise is caused by the magnetostriction property of core steel. Magnetostriction is a term for the small, mechanical deformations of core laminations in response to the application of a magnetic field. The change in dimension is independent of the direction of the flux, and hence it occurs at twice the supply frequency. Because the magnetostriction property is nonlinear, however, higher frequency harmonics of an even order are introduced at higher flux densities. Therefore, core noise has components at multiples of 100 or 120 Hz (for 50-Hz and 60-Hz transformers, respectively). The relative magnitudes of the noise at these different frequency components depend on core material, core type and operating flux density.
Load noise mainly is generated by windings vibrations caused by the electromagnetic forces as a result of the interaction of load current and leakage flux produced by this current. Another source of load noise is tank vibrations caused by the magnetic-pull forces exerted by the leakage flux. The main frequency of this sound, therefore, is twice the supply frequency. The level of load noise is determined by load current, winding design and the type of tank shielding used.
Cooling system noise is generated by the operation of the cooling equipment, fans and pumps. This noise has a broad frequency spectrum with a peak at the blade passage frequency, the frequency at which the fan impeller blades pass some rigid disturbance in the air flow, and sometimes twice that frequency. Pumps also produce noise of a broad band nature and contribute to the total noise of the transformer.
Sound Level Units
Sound-level limits typically are determined by the perception of the human ear to sound. For example, doubling the magnitude of the sound pressure is felt by the human ear as a small increase in sound level. The human ear is also 10 times more sensitive to a sound of 1,000-Hz frequency than it is to a sound of 100-Hz frequency. Sound, therefore, is measured in decibels (dB), and a decibel is 10 × Log10 (sound pressure). Sound levels typically are presented and specified in A-weighted decibels; dB(A), where the sound levels are attenuated according to their frequencies per an “A-filter,” which depicts the frequency response of the human ear.
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