Reduction of Line Current Harmonics in 3-Phase Off-Line Rectifier Systems by Use of an Efficient 3-Phase to 9-Phase Autotransformer Conversion Technique

摘要

A technique is described which provides a substantial reduction (50 dB or more) of the low frequency harmonics of a 3-phase off-line rectifier system. In a conventional off-line rectifier system, the line current waveform has a total harmonic distortion (THD) of approximately 78 percent. With the technique described herein, the line current is modified to a nearly sinusoidal waveshape having less than 5 percent THD, predominantly 17th and 19th harmonic. The technique employs autotransformers to achieve a high degree of volumetric and energy efficiency at a substantially lower cost, and with reduced acoustic hum and stray magnetic fields as compared to conventional transformer configurations. A power density of approximately 12 Watts/in 3 or better is expected to be achieved with this technique for applications requiring several kilowatts of input power, when the 20 degree system is employed. For the 40 degree system, a power density of approximately 6 Watts/in 3 or better is expected. This technique also has the advantage of reducing the line current harmonics without the use of reactive filter elements. It is therefore inherently insensitive to the effects of the negative input impedance characteristics of switching regulator power supplies, which can result in instability and/or a high degree of susceptibility to conducted interference. (rh)