Design of a digital signal current detector

The single-phase earthed protection based on the S´ signal injection method has been successfully applied for more than ten years in non-effective earthed power system in China. Detecting and processing the injected weaker diagnostic current has always been implemented by analog integrated circuits. But its measuring accuracy and interference rejection are limited. So, a digital signal current detector (DSCD) is designed in this paper. According to characteristics of the injected diagnostic current and requirements for detecting it, the design solution for the DSCD is proposed. High performance digital filters are designed. And the amplitude of the injected current is calculated by Fourier algorithm. The DSCD samples the injected diagnostic current by the high-accuracy Analog-to-Digital converter ADS8325 and processes it by the mixed signal microcontroller MSP430 family. Field applications verified the effectiveness of the project.