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LOAD NOISE INCREASE OF TRANSFORMERS BY LOAD HARMONICS

Michael Ertl ; SIEMENS Transformers, Nuremberg
Stephan Voss ; SIEMENS Transformers, Nuremberg

Sažetak

Electrical power and distribution transformers in field operation regularly show significant
deviation in the load noise level in comparison to the sound measurements in the test field of the
manufacturer where controlled and standardised measurement conditions apply. The main reason
therefore are (a) the changed acoustical ambient conditions on-site caused by sound reflection and
diffraction effects of nearby obstacles; (b) changed structural and acoustical resonances, and (c) the
different principal operation conditions on-site. Whereas at the factory tests a pure sinusoidal current with
short-circuited secondary winding is applied for load noise determination, transformers on-site operate at
changed load power factors (phase angle between voltage and load-current), loading beyond the rated
power (overload, partial load) as well as harmonics in the load-current.
Harmonic components in the power grid are caused primarily by nonlinear loads and by the
nonlinear transfer characteristic of the power grid. Any electrical device which shows a nonlinear
consumption of active power or a nonlinear change of the source impedance draws a distorted current
waveform even if the supply voltage is sinusoidal. With the growing use of electrical and electronic
devices, distorted waveforms in power supply grid by harmonics have been largely increased within the
past years. Devices like variable speed drives, six-pulse bridge rectifiers used in power electronics, and
discharge lamps draw a non-sinusodial but pulsating current. Whereas most electric devices generate
solely odd harmonics, some devices with fluctuating power consumption generate odd, even and also
interharmonic currents [1].
Load noise is generated by electromagnetic forces acting on transformer windings. The forces
are proportional to the square of the load-current (Section 2.2). This quadratic response behaviour
generates winding vibrations at frequencies beyond the power frequency. In case of harmonics in the
load-current, the resulting magnetic force and noise components have harmonics as well- with a more
complex frequency spectrum. The related increase of the load noise level must be considered in the
estimation of the total noise level of transformers under real load conditions. The question arise how to predict the resulting noise level at these loading conditions to guarantee the maximum allowed sound
level at the transformer site as requested by legislative regulations or the owners specification.
Starting from basic principles, we describe and quantify the generation of harmonics in winding
forces, winding vibrations and load noise of electrical transformers caused by load harmonics. A calculation
scheme for a fast and practically accurate estimation of the increase in the load noise level for a given load
spectrum presented. The effects of A-weighting and frequency-dependent sound radiation efficiency of
transformers to the total noise level are discussed. Finally, three practical examples are given.

Ključne riječi

Hrčak ID:

198911

URI

https://hrcak.srce.hr/198911

Datum izdavanja:

15.7.2017.

Posjeta: 1.004 *