Harmonic Distortion of Current Consumed by Frequency Converters at Low Loads

The use of frequency converters in group electric-drive systems and marine electric-power systems with a power plant based on internal combustion engines with variable rotation frequency makes it possible to reduce the cost of electrical equipment and to improve the weight-and-size characteristics. However, the harmonic distortions generated by frequency converters negatively affect the quality of electrical energy and lead to additional losses in the stator windings of synchronous generators and to worsening in the switching conditions of semiconductor switches, as well as to other negative effects. This paper is devoted to examining the harmonic composition of the current consumed by frequency converters with a dc link at low power loads. Six frequency converters of different grades with different topologies and power capacities have been studied. The spectral composition of current consumed by frequency converters upon changing output voltage frequency is presented. The effect of a network choke and of the loading power exerted on the harmonic composition of the consumed current is considered. It is shown that the spectral composition of the generated distortions is individual for each frequency converter, but the 5th, 7th, 11th, 13th, 17th, and 19th harmonics remain dominant, whereas the current consumed by the converters exhibits a significant harmonic distortion in a fractional load mode up to 30%.