A power line impedance spectrum analyzer using real-time digital signal processing

Abstract

Power distribution system impedance as seen by power converters and other non-linear loads is important for the determination of harmonic current injection and propagation caused by these loads. This thesis presents a real-time power line impedance measuring system developed with a digital signal processor (DSP) and a personal computer which will detect impedance resonance conditions. At any frequency where the impedance appears resonant, i.e., it has a very large value, amplification of the harmonic currents venerated by non-linear loads becomes a concern. The impedance is measured by first injecting, into the line, a current signal that contains each frequency to be measured. This signal is generated by driving an 8-bit digital-to-analog converter (DAC) with waveform data stored in an EPROM and conditioned by appropriate analog amplifiers. A DSP board located inside a personal computer digitizes this waveform and the corresponding voltage drop. A program running on the DSP computes the spectrum of each signal by taking several FFTs, averaging them together, and computing the magnitude. Meanwhile, a program running concurrently on the PC prompts the DSP to send the data when it has finished computing, and the PC graphically displays the results and computes the impedance at each harmonic. Any resonance conditions detected are flagged for investigation and/or correction.