Transformer noise

Transformer noise comes from the core due to the magnetostriction effect, from the current flowing through windings, as well as from the fans or other equipment

byLokesh Solanki

Transformer noise
Image source: Virginia Transformer

Introduction

  • What is sound?

Sound is an air pressure disturbance that human ear can ‘hear’. Speech produces sound, for example. However, sound disturbances are produced by practically anything that moves. The frequency of a sound wave is perceived as pitch, and amplitude is perceived as loudness.

When a sound is unwanted, it becomes “noise”. Transformers emit noise during operation, where the magnitude of noise increases with their MVA capacity and voltage class.

Sound level of transformers is specified by NEMA TR1.

  • How is it produced?

Transformer core noise is caused by a phenomenon called magnetostriction. In very simple terms, this means that if a piece of magnetic sheet steel is magnetized, it will extend itself. A transformer is magnetically excited by an alternating voltage and current, so it becomes extended and contracted twice during a full cycle of magnetization. This means that the extension or contraction mechanical frequency is two times the electrical frequency, i.e. the frequency of the voltage.

 

Lead: Transformer noise comes from the core due to the magnetostriction effect, from the current flowing through windings, as well as from the fans or other equipment

A transformer core is made from many thin sheets of special electrical steel. It is made this way to reduce losses due to circulating eddy currents, and the consequent heating effect. If the extensions and contractions described above are taking place in various directions depending upon the clamping of the laminations, each sheet can behave non-uniformly over its length and width. This ‘writhing’ and twisting motion produces harmonics of the fundamental frequency up to the 16th harmonic. These extensions are a few microinches in dimension, however, enough to cause a vibration such as noise. This is the core noise.

Apart from the core noise, transformer windings contract and expand with the current. Frequency of the winding noise is produced by the current flowing through windings in the leakage magnetic field. The noise generated by the core and winding is transmitted to the tank wall via the mechanical structure and through the oil. Magnetic shields, if used, will also vibrate with the magnetic flux and create noise.

During operation, the transformer generates heat, which is dissipated by radiators, while fans are used to enhance cooling. The fan motor and blades cause additional noise. The fan noise is added to the transformer noise. Thus, the total noise of the transformer during operation is increased.

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