Engineering Controls Database

Transformer (1) – Noise Case Study

Problem:	Overview: The National Institute for Occupational Safety and Health (NIOSH) published an industrial noise control manual in 1978 [NIOSH 1979]. As part of this manual, a number of general noise control methods were provided. These methods are meant to reduce the amount of sound energy released by the noise source, or divert the flow of (sound) energy away from the receivers, or protect the receivers from (sound) energy reaching them. The key to noise control is finding the control that is both and economical. One of the general noise control methods provided in the NIOSH noise control manual is summarized in this write-up. Case study: This case history discusses noise control treatments that were included in the design of a new electric station and evaluates their effectiveness. A 345/115-kilowatt substation, designed for an 11-acre site located in a mixed commercial/residential area in New England, was to include two 300 MVA OA/FOA/FOA autotransformers and an oil-to-air heat exchanger for the underground 345-kilowatt line. Standard National Electrical Manufacturers' Association (NEMA) sound levels for transformers of this class are 84/86/87 dBA. The heat exchanger contains two 8-ft-diameter, 4-bladed, propeller-type fans, driven at 364 rpm by one 1-hp motor per fan. The fans are rated at 0.135 in. of water static pressure and 51,700 actual cfm air flow.
What this means to you:	The power company wanted to avoid (1) noise complaints from its new neighbors and (2) noise-related delays during the application hearings pending before various regulatory agencies. A study by Bolt Beranek and Newman Inc., submitted by the power company to the regulatory agencies in the form of a report, established appropriate sound level criteria, provided detailed noise control design, and estimated the community noise impact from station operation.
Description of control:	Various acoustic criteria were established for the station to meet the city and state sound level regulations. However, the power company's own criterion was the most stringent: A nuisance or probable-complaint condition must not be created by noise from the operating facility. From this criterion, an engineering design goal was chosen to limit the transformer tonal noise to within about 5 dB of the nighttime ambient residual sound levels measured in octave bands at nearby noise-sensitive locations. Residual ambient sound pressure level measurements were made at nearby noise-sensitive areas during the day, evening, and night-time periods. The late-night ambient sound levels were used to establish the transformer noise design goal. Several alternative noise control treatments can be considered for transformers. These include: • Specification of sound levels lower than those set by NEMA • Barrier walls or partial enclosures • Complete enclosures • Purchase of additional real estate or noise easements as buffer zones • Relocation to an area without noise-sensitive neighbors. Both transformers were purchased from the manufacturer with sound levels specified to be 9 dB less than the NEMA standard. The lower-than-standard sound levels for this transformer were 75/77/78 dBA. This reduction is accomplished in the design of the transformer by providing a large core reducing the magnetostrictive forces, which, in turn, reduce the noise radiated by the tank wall. A partial enclosure was also provided along three sides of the transformer. Noise-sensitive areas were positioned in three directions from the site. There were no noise-sensitive land uses in the remaining direction, and therefore an increase in noise level could be tolerated. The open side of the enclosure was, of course, aligned toward the direction that was not noise-sensitive. The size and location of the partial enclosure relative to the transformer was designed to provide adequate insertion loss without restricting ventilation or maintenance. The enclosure walls were constructed from patented concrete blocks with sound absorption on the transformer side of the walls provided by slots leading into the interior cavities of the blocks. Sound absorption on the interior surfaces of the walls was necessary to minimize the build-up of sound within the enclosure. The masonry walls also served as fire protection between the two transformers.
References:	NIOSH [1979]. Industrial noise control manual – revised edition. Cincinnati, OH: U.S. Department of Health Education and Welfare, Public Health Service, Center for Disease Control, National Institute for Occupational Safety and Health, DHEW (NIOSH) Publication No. 79-117.
Keywords:	community noise control community noise control electrical transformer electrical transformer noise control noise control
How well does this control work?	Measurements made after the station was operating show the sound level design goal was achieved. The transformer tonal noise is usually masked by ambient sounds and is therefore seldom audible at nearby sensitive areas. (The late-night ambient sound levels are occasionally lower than those used in the design goal and, hence, the transformer noise can occasionally be heard in the community. If it were appropriate to eliminate completely the possibility of a noise source from being heard, even more stringent design goals could be established (e.g., 5 to 10 dB lower than the expected sound level of the masking ambient. In this case, such extreme measures were inappropriate.) Figure 1 shows the results of sound pressure level measurements before and after the trans-formers were energized. These measurements were obtained during the late nighttime hours, when the potential for station audibility was greatest. It should also be noted that no complaints have been received after three years of operation. [img=1]