New Trends in Noise Reduction of Power Transformers

In the electrical network power transformers play a key role in transmission and distribution of electrical energy. Noise emissions on power transformers will be supervised in Germany strictly according to the German Federal Immission Control Act (Bundes-Immissionsschutzgesetz) [1] and are a challenge for the utilities to fulfill the therein stated requirements. When in operation power transformers make noise, produced by an overlapping of different sources. On the one hand, no-load noises are produced by vibrations caused by the magnetostrictive deformation of the core. On the other hand, electromagnetic forces in the windings, tank walls and magnetic shielding predominantly influence the load noise. In addition, cooling systems using fans have a large influence on sound emissions. The determination of sound levels of transformers is regulated in the standard DIN EN 60076-10 "power transformers part 10: determination of sound levels" [2]. This standard describes sound pressure and sound intensity measurement methods by which sound power levels of transformers, reactors and their associated cooling auxiliaries may be determined. It aims to apply to measurements made at the factory. Besides this measurements can also be performed at site. In this article the reasons of noise of transformers will be investigated. Various activities to reduce noise will be described. Optimization of the cooling system, particularly for large power transformers, is necessary due to high customer expectations and holds an ever increasing status. This development can also be observed in the relevant standards. As an example the maximum permissible value for sound emissions of 91 dB(A) of a 40 MVA transformer had been indicated in the issue of May 1983 of standard DIN 42508 [3]. In the revision of August 2009 this value was lowered to 83 dB(A). A difference of 10 dB(A) is considered as twice as loud by human beings. Thus, the sounds of power transformers have almost halved in value during the last 20 years. Furthermore new trends for controlling the fans using ECtechnology and the results of preliminary investigations will be presented. Primary focus of these studies is the optimization of losses and noises as well as achieving a reduction in wiring expenditures. INTRODUCTION The German Federal Immission Control Act [1] is a law which provides protection against damaging environmental influences by way of air pollution, noise, vibrations and similar processes. The purpose of this law is to protect humans, animals, plants, ground, water, the atmosphere as well as cultural and any other assets from damaging environmental effects, and to prevent the emergence of damaging environmental effects. The provisions of this law also apply to the erection and operation of high voltage substations. In the 16th, 18th and 22nd Federal Immission Protection Directives on the German Federal Immission Control Act, threshold values for sound emissions of different sources of sound are prescribed. The sound emissions of power transformers are strictly monitored according to the German Federal Immission Control Act. An overview of everyday sounds is depicted in Fig. 1. Ever greater demands are placed on the noise levels of power transformers by standards and consequently by the utilities. The factors which lead to the production of noise as well as new trends to optimize noise emissions will be explained in more detail in the following section. Fig 1: Everyday sounds in dB(A) TRANSFORMER NOISE EMISSIONS No-load and load noise The creation of no-load noise is dependent on the existing magnetic flux density and is influenced considerably by the operating voltage and induction with which the transformer is operated. Frequencies caused by a multiplication of the power line frequency, in particular the 200 Hz and 300 Hz components of the sound spectrum cause the core to vibrate considerably where the induction is increasing. This effect predominates under no-load conditions. C I R E D 21st International Conference on Electricity Distribution Frankfurt, 6-9 June 2011