Low-frequency and Tonal Characteristics of Transformer Noise

Electricity substations and their associated transformers are necessary for community power needs. In NSW these substations are often located in rural areas where background noise is low. Noise ‘hum’ from the transformers can be an issue for residents located nearby to the substation, and the tonal and low-frequency nature of the noise can add to the annoyance. The NSW Industrial Noise Policy applies penalties for these noise characteristics when assessing such sources. This paper analyses noise measurements of substations in rural areas. One-third octave band noise measurements were conducted both close up to the transformers, and also in the residential community. The measurement results show that although tonal and low frequency characteristics might be evident close up to the transformers, these characteristics dissipate over distance and the INP penalties do not necessarily apply at the residential receiver. INTRODUCTION The electricity used by households and businesses in NSW is generated at power stations then transmitted through high voltage transmission lines to substations. The substation performs the task of stepping down the voltage suitable for domestic and commercial distribution. There are approximately 91 large zone substations in NSW and many more smaller distribution substations. Since substations are often located in rural areas, the nearest residential receivers are commonly isolated dwellings on large lots. Background noise is therefore usually very low due to the lack of other industrial development and low levels of traffic noise. Occasionally medium density residential areas on the edge of towns abut the substation land holding. Since transformers generate noise emissions with a distinct tone at a relatively low frequency, it is sometimes assumed that these characteristics will also be evident at the residential receiver, to the point that a tonality penalty and/or a lowfrequency penalty is automatically applied during an assessment. Particularly when utilising noise modelling say during the planning phase of a substation, the tones and harmonics the substation source are strongly evident in the predicted results at the receiver. However, since tonality is assessed by determining the emergence of a particular tone above the noise level of the adjacent frequency bands, the background noise in an area plays a significant part in determining whether a noise source will be tonal or not. Even where the tone of a substation may still be somewhat audible, it may be that the tone has ‘blended’ enough with the background noise so that the tone is no longer inherently annoying. This paper analyses the results of noise measurements conducted in rural areas, in the vicinity of substations, with the aim of determining typical distances from substations beyond which tonal and low-frequency penalties would no longer apply according to the INP. ELECTRICITY SUBSTATION TRANSFORMERS Zone substations typically contain at least two step-down transformers. These transformers operate with a distinct ‘hum’ which is generated by the periodic mechanical deformation of the transformer core and the winding coils, under the influence of fluctuating electromagnetic flux. The vibration of the core is at twice the frequency of the alternating magnetic flux. Since the line supply frequency is 50Hz, the noise emissions from these transformers are typically characterised by tonal spikes at 100Hz, and also at the 200Hz and 400Hz harmonic frequencies. Figure 1 Typical substation transformer 2-4 November 2011, Gold Coast, Australia Proceedings of ACOUSTICS 2011 2 Acoustics 2011 During a transformer’s operation, the vibrations from its core and windings get transmitted to the transformer tank surface through air-borne transmission (the air surrounding the core), and by structure-borne transmission at points where the mounting of the core structure is attached to the tank. The vibrating tank surface eventually radiates noise into the exterior air. Some of the other sources of noise associated with transformer operation, such as cooling fans and pumps, are usually negligible contributors to the far-field noise. NSW INDUSTRIAL NOISE POLICY General Criteria The NSW Industrial Noise Policy (INP) is generally used for setting noise goals in substation noise assessments in NSW. The assessment procedure for the Industrial Noise Policy has two components: • Controlling intrusive noise impacts in the short term for residences • Maintaining noise level amenity for particular land uses for residences and other land uses. In setting the project specific noise levels, the more stringent of the intrusive and amenity criteria is adopted. In general, the intrusive criterion tends to be the controlling criterion for substations as there is often little other industrial development in the regional areas where the substation is located. Penalties for Low-frequency and Tonal Sources Where a noise source contains certain characteristics (i.e. if it has an inherently tonal, low frequency, impulsive or intermittent character), then it can cause greater annoyance than other noise at the same level. According to the INP an adjustment of 5dBA for each annoyance aspect, up to a total of 10dBA, is to be added to the measured value to penalise the noise for the additional annoyance caused. The INP provides definitive procedures for determining whether a penalty or adjustment should be applied for increased annoyance. Tonal noise is defined as noise containing a prominent frequency and characterised by a definite pitch. A one-third octave (or narrow band analysis) is required and a 5dBA penalty is applied to the measured or predicted level when the level of one-third octave band exceeds the level of the adjacent bands on both sides by:  5dB or more if the centre frequency of the band containing the tone is above 400 Hz  8dB or more if the centre frequency of the band containing the tone is 160 to 400 Hz inclusive  15dB or more if the centre frequency of the band containing the tone is below 160 Hz. Low frequency noise is defined as noise containing major components within the low frequency range of 20Hz – 250Hz. Low frequency noise is considered to be evident where the difference between the measured A and C weighted levels is 15dB or more, and another 5dBA penalty may be applied. MEASURED TRANSFORMER NOISE LEVELS Near Field Noise Levels Australian Standard 60076.10.1-2009 “Power transformers – Determination of sound levels” is the current standard for quantifying noise levels from transformers. The procedure in this standard calls for noise levels to be measured at a series of points around the tank of the transformer. One-third octave band noise measurements have been conducted in accordance with the standard for the near field noise levels presented in this paper. A typical noise spectrum for these near field measurements is presented in Figure 2. The tone at 100Hz is distinct, being two times the electrical supply line frequency. There is also significant energy at the 200Hz and 400Hz harmonic frequencies. Figure 2. Typical Transformer Noise Spectrum Noise Levels in the Community The following three case studies are examples of where residences are affected by audible substation noise. Noise measurements have been conducted late at night at the residential boundaries in a low background noise environment to quantify noise emission levels and to determine whether tonal or low frequency characteristics exist. In each case, noise measurements have also been conducted close up to the transformers in the switchyard to quantify the character of the source. Proceedings of ACOUSTICS 2011 2-4 November 2011, Gold Coast, Australia Acoustics 2011 3 Case 1 The substation was located in a rural area surrounded by grassy fields and farmland, and is approximately 2km from the main highway. Background noise levels were around 30dBA, although slightly less than 30dBA at some residential locations. The graph in Figure 3 presents a comparison of measured noise levels both close up to the transformer in the substation, at the substation boundary, and at the residential boundary. The results are in one-third octave bands to enable analysis of tonality. Figure 3. Measured Noise Levels (Case 1) As shown by the arrowed notations, the following conclusions can be drawn for Case 1 according to the annoyance test procedures outlined in the INP • Close up to the transformers, noise emissions are clearly tonal, with the obvious tones being at 100Hz, and at the 200Hz harmonic. • Noise is still considered tonal at both the boundary locations (35m and 60m away), however the tones are only evident at 200Hz, but no longer at 100Hz. • At the residential boundary 440m away, tonality was determined at 400Hz, but not at 100Hz or 200Hz • At the residential boundary 570m away, noise from the substation was not considered tonal according to the INP procedures Comparison of the A-weighted and C-weighted overall noise levels confirmed that a penalty for low frequency noise would not need to be applied at any residence.