The limit of the substation noise level over which it begins to irritate the community

Noise sources in power substations

This article will try to shed some light on the most common noise-related concerns associated with existing substations and provide mitigation methods to address these concerns. The continuous noise created by the operation of power transformers and reactors, as well as the noise produced by the operation of high-voltage circuit breakers or load interrupters, are the two main sources of noise in substations.

Other noise sources in substations worth mentioning are corona discharges (buzzing), arcing during the operation of switches, etc. Power transformers and reactors generate the most important and the most significant noise. Continuous humming noise these equipment generate is very often disturbing for communities living near the substation.

Existing substations are often equipped with older, vintage power transformers and reactors. The technology used to manufacture such equipment typically results in relatively high levels of noise emission. Most of the existing substations were initially built far from residential areas to limit the effect of this noise on the public and the surrounding neighbourhoods.

However, back in earlier years, there were fewer guidelines and regulations regarding acceptable noise levels available during the construction of substations. In the last couple of decades, the expansion of urban and suburban areas has resulted in many substations being located within or in direct proximity to residential areas.

Furthermore, throughout the last few decades, public concern about industrial noise has grown, and new, more strict regulations and bylaws have been enacted to control noise levels in residential areas.

All these occurrences highlight the importance of audible noise as a problem, one that the utility industry must address fully during the planning and retrofitting of existing substations.

Table of Contents:

1. The Physics Behind Substation Noise
   1. Characteristics of Transformer Noise
   2. Spreading of Sound
2. What does the Community Say About the Noise?
3. Substation Requirements
4. How to Measure the Noise?
   1. Analytical Method for Noise Calculation
5. Methods of Substation Noise Control

1. The Physics Behind Substation Noise

1.1 Characteristics of Transformer Noise

The magnetostriction of the iron core is the principal cause of noise in transformers. The electromagnetic forces between the individual turns of the windings provide a secondary but much lower source.

The principal frequency of the resulting vibration is twice that of the supply frequency (120 Hz) and, because the magnetostriction characteristic of iron is nonlinear, harmonics (240, 360 and 480 Hz) are also generated. These harmonics of noise present a major role in contributing to the annoyance of the noise as perceived by individuals.

The load has little effect on the noise emission.

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1.2 Spreading of Sound

The energy from a point source of sound propagates as the “inverse-square law” in an open outdoor environment, meaning that as the sound moves outwards from the source, the energy drops as the square of the distance. As a result, the sound energy reduces by a factor of four, or 6 dB, for every doubling of the distance.

For distances of up to 150 m and beyond, this theoretical attenuation usually holds true. It is sensitive to the effects of ground and atmospheric absorption beyond this distance, as well as inhomogeneities associated with turbulence, moderate and high winds, and temperature gradients.

Furthermore, due to changing atmospheric conditions, the rise in noise level spread by temperature inversion rarely remains constant and usually changes with time (a few seconds to a few minutes). Another aspect that raises the subjective reaction when compared to a constant noise is the modulation of the noise, particularly the type generated by transformers.

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2. What does the Community Say About the Noise?

There are a wide variety of regulations and laws in use in different countries worldwide to control audible noise levels in the community. In some countries, many noise level regulations have been developed to suit local conditions. The majority of these are qualitative, although a number of the larger communities and some cities have had quantitative regulations in various forms.

Nowadays, in most industrialized countries qualitative regulations and/or laws are commonly used. Many large cities and megacities have quantitative regulations depending on the situation complexity.

The above general rule has been found to apply particularly in the design of new installations. It is also applicable to old installations, but there will be some existing stations where noise treatments will be difficult to install due to space and clearance limitations.

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3. Substation Requirements

Existing substations have to comply with newer, more restrictive noise regulations sooner or later, especially when complaints are being received about the elevated noise level of a certain substation. While in some countries the new regulations might not apply to existing installations in general, utilities are concerned with community acceptance and as “good corporate citizens” will modernize their installations to meet the requirements of the latest regulations and laws.

Let’s highlight the factors that influence the methods used to mitigate noise problems in existing substations:

Factor #1 – The level of noise exceeding the approved level for the area in which the substations is located.

Factor #2 – The economics of various solutions: the analysis has to take into account the life cycle cost of the installation, as well as, the benefits the utility may acquire by addressing the concerns of the community.

Factor #3 – Operational and maintenance implications of installing a certain sound mitigation solution around transformers and/or reactors.

Factor #4 – Issues related to the constructability of the sound mitigating solution (can it be done with the equipment live, is there a need for rerouting of power and/or control cables, etc.).

Table 1 gives typical noise limits specific to particular settings: