Unlike cooling-fan or pump noise, the sound radiated from a transformer is tonal in nature, consisting of even harmonics of the power frequency. It is generally recognized that the predominant source of transformer noise is the core. The low frequency, tonal nature of this noise or buzzing makes it harder to mitigate than the broadband higher frequency noise that comes from the other sources.
This is because low frequencies propagate farther with less attenuation. Also, tonal noise can be perceived more acutely than broadband levels, even with high background noise levels. This combination of low attenuation and high perception makes tonal noise the dominant problem in the neighboring communities around transformers. To address this problem, most noise ordinances impose penalties or stricter requirements for tonal noise.
Even though the core is the principal noise source in transformers, the load noise, which is primarily caused by the electromagnetic forces in the windings, can also be a signiﬁcant inﬂuence in low-sound-level transformers. The cooling equipment (fans and pumps) noise typically dominates the very low-and very high-frequency ends of the sound spectrum, whereas the core noise dominates in the interme-diate range of frequencies between 100 and 600 Hz.
These sound-producing mechanisms can be further characterized as follows.
When a strip of iron is magnetized, it undergoes a very small change in its dimensions (usually only a few parts in a million). This phenomenon is called magnetostriction. The change in dimension is independent of the direction of magnetic ﬂux; therefore, it occurs at twice the line frequency. Because the magnetostriction curve is nonlinear, higher harmonics of even order also appear in the resulting core vibration at higher induction levels (above 1.4 T).
Flux density, core material, core geometry, and the wave form of the excitation voltage are the factors that inﬂuence the magnitude and frequency components of the transformer core sound levels. The mechanical resonance in transformer mounting structure as well as in core and tank walls can also have a signiﬁcant inﬂuence on the magnitude of transformer vibrations and, consequently, on the acoustic noise generated.
Load noise is caused by vibrations in tank walls, magnetic shields, and transformer windings due to the electromagnetic forces resulting from leakage ﬁelds produced by load currents. These electromagnetic forces are proportional to the square of the load currents.
The load noise is predominantly produced by axial and radial vibration of transformer windings. However, marginally designed magnetic shielding can also be a signiﬁcant source of sound in transform-ers. A rigid design for laminated magnetic shields with ﬁrm anchoring to the tank walls can greatly reduce their inﬂuence on the overall load sound levels.
The frequency of load noise is usually twice the power frequency. An appropriate mechanical design for laminated magnetic shields can be helpful in avoiding resonance in the tank walls. The design of the magnetic shields should take into account the effects of overloads to avoid saturation, which would cause higher sound levels during such operating conditions.
Studies have shown that except in very large coils, radial vibrations do not make any signiﬁcant contribution to the winding noise. The compressive electromagnetic forces produce axial vibrations and thus can be a major source of sound in poorly supported windings. In some cases, the natural mechanical frequency of winding clamping systems may tend to resonate with electromagnetic forces, thereby severely intensifying the load noise. In such cases, damping of the winding system may be required to minimize this effect. The presence of harmonics in load current and voltage, most especially in rectiﬁer transformers, can produce vibrations at twice the harmonic frequencies and thus a sizeable increase in the overall sound level of a transformer.
Through several decades, the contribution of the load noise to the total transformer noise has remained moderate. However, in transformers designed with low induction levels and improved core designs for complying with low sound-level speciﬁcations, the load-dependent winding noise of electromagnetic origin can become a signiﬁcant contributor to the overall sound level of the transformer.
In many such cases, the sound power of the winding noise is only a few dB below that of the core noise.
Fan and Pump Sound
Power transformers generate considerable heat because of the losses in the core, coils, and other metallic structural components of the transformer. This heat is removed by fans that blow air over radiators or coolers. Noise produced by the cooling fans is usually broadband in nature. Cooling fans usually con-tribute more to the total noise for transformers of smaller ratings and for transformers that are operated at lower levels of core induction.
Factors that affect the total fan noise output include tip speed, blade design, number of fans, and the arrangement of the radiators.
SOURCE: Jeewan Puri – Causes and Effects of Transformer Sound Levels