The role of load harmonics in audible noise of electrical transformers

Harmonic components in load currents have a larger impact on the load noise level of transformers than might be expected from their amplitude. There are several reasons for this larger impact: (a) the interaction of higher harmonics with the large fundamental load current at power frequency, (b) the increasing sound radiation efficiency with increasing frequency, and (c) the greater sensitivity of the human ear to higher frequencies, which is considered in sound measurements by applying the A-weighting filter. This paper describes the process of generation, transmission, and emission of load noise in the presence of load harmonics. A calculation scheme is presented that is able to estimate the noise increase and the noise spectrum of electrical transformers under non-sinusoidal load conditions. The proposed calculation scheme is applied to three practical examples.