Environmental Assis for Orono Substation Expansion

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Table 6: Noise Area Classifications NAC Day (7 a.m. —10 p.m.) Night (10 p.m. — 7 a.m.) LSO 1,10 L50 L10 1 60 65 50 55 2 65 70 65 70 3 75 80 75 80 Notes: Minnesota Rules part 7030 uses the decibel A -weighting network and applies statistical sound levels (L -Level Descriptors) to account for changes in sound levels over a period time as shown. The 1_10 is defined as the noise level exceeded 10 percent of the time, or for six minutes in an hour. The L50 is the noise level exceeded 50 percent of the time, or for thirty minutes in an hour. The L5 is the noise level exceeded five percent of the time, or for three minutes in an hour. The Project would result in short-term noise from the construction phase of the Project as well as noise from the conductors and substation once the Project is in operation. Construction Noise Short-term exceedance of daytime noise standards would be intermittent and temporary in nature. Noise from general construction noise are expected to occur during daytime hours as the result of heavy equipment operation and increased vehicle traffic associated with the transport of equipment and construction personnel to and from the work area. Conductor Noise Transmission conductors can produce noise under certain conditions. The level of noise depends on conductor conditions, voltage level, and weather conditions. In foggy, damp, or rainy weather, transmission lines can create a crackling sound due to the small amount of electricity ionizing the moist air near the conductors, or corona discharge. In practice, noise levels produced by 115 kV transmission lines are generally less than outdoor background levels and, therefore, are not usually audible. The worst-case scenario is when the conductor is exposed to heavy rain conditions (one inch per hour). However, during heavy rain the background noise level of the rain is typically greater than the noise from the transmission line. As a result, people do not normally hear noise from a transmission line during heavy rain. Sound emanating from conductors would typically be noticed during light rain, dense fog, snow, and other times when there is moisture in the air; transmission lines would produce audible noise approximately equal to household background levels. Xcel Energy calculated the estimated audible noise that may be produced from the proposed transmission line using the Bonneville Power Administration (BPA) CFI8X model. To ensure that the noise was not under -predicted the worst-case scenario was used as the benchmark. The anticipated noise levels derived from the modeling are presented in Table 7. Environmental Assessment PUC Docket E002/TL-11-223 Page 27