Orono_Route_Permit_Application - Docket # W002/TL-11-223

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Table 11 Calculated Audible Noise (dBA) for Proposed Transmission Line Design (3.28 feet above ground) Note: Noise calculations done using the EPRI Enviro software and the BPA standard method of calculation. The noise generated from the transmission line is not expected to exceed background noise levels and will, therefore, not be audible at any receptor location. Transmission conductors and transformers at substations can produce noise when it is foggy, damp, or rainy, including a subtle cracking or humming noise. Any audible noise will be well below the MPCA noise standards established for NAC 1, as shown in Tables 10 and 11 above. Transformer Substation Noise Transformer "hum" is the dominant noise source at substations. Transformer hum is caused by magnetostrictive forces within the core of the transformer. These magnetic forces cause the core laminations to expand and contract, creating vibration and sound at a frequency of 100Hz (twice the a.c. mains frequency), and at multiples of 100Hz (harmonics). Typically, the noise level does not vary with transformer load, as the core is magnetically saturated and cannot produce any more noise. Generally, activity -related noise levels during the operation and maintenance of substations and transmission lines are minimal. The nearest occupied structures to the proposed Orono Substation and related transmission facilities associated with the Project include two residences located approximately 220 feet to the south and approximately 290 feet west (see Appendix B, Figure B-2). It is unlikely that substation noise will be audible at these structures. The proposed substation will be designed and constructed to comply with state noise standards established by the MPCA Mitigative Measures The transmission lines and substation are designed and constructed to comply with state noise standards established by the MPCA. Transmission line noise levels are not expected to exceed the MPCA noise standards outside the right-of-way for all NACs. Likewise, substation noise will not exceed applicable limits, including the MPCA noise limits. Therefore, no mitigation is proposed for the audible noise generated by the proposed Project. Orono Project June 7, 2011 MPUC Docket No. E002/TL-11-223 44