Low Frequency Conducted Emissions of Grid Connected Static Converters

Pulse-width modulated rectifiers are nowadays commonly used for AC to DC power conversion. PWM rectification technology is very effective and allows for bidirectional power flow with the possibility of power factor improvement and low-order harmonic emission limitation. Unfortunately, employed PWM boost topology results with generation of input current harmonic distortions in a frequency range tightly correlated to modulation frequency. PWM carrier frequency and its harmonic components in typical applications are usually located in frequency range from single kHz up to hundreds of kHz. Increase of harmonic emission in this particular frequency range can be disturbing for other sensitive systems, therefore can cause lack of electromagnetic compatibility. Interferences in low frequency range are increasingly frequent EMC issue because of extensive use of low power PWM rectifiers and significant increase of rated power of individual converters, which already reach several MW. Presented comparison of current harmonic emission spectra of PWM rectifiers with relation to classic diode rectifiers is focused on frequency range of 2 to 9 kHz, for which emission limitation rules are currently under development. New class of EMC problems arising in low frequency range in power system are underlined as a result of increased harmonic emission in frequency range of 2 to 9 kHz.