Measuring Instrument for High-Frequency Currents Flowing on Power Lines

摘要

Measurement instruments are discussed for the common-mode current and the differential-mode current of signals and interference at frequencies in the range of 1 to 30 MHz propagating on power lines. An instrument is fabricated for trial. The present instrument basically consists of a pair of current probes and a 180° hybrid circuit. Voltage outputs proportional to the currents of both modes are obtained. In this paper, first the basic properties are theoretically discussed and a design guideline is provided for improvement of the sensitivity and the mode separation characteristic. In particular, it is necessary to assure a common-mode separation characteristic of -40 dB in order to measure the common-mode current that is a cause of electromagnetic interference in power line communications. It is found that for this purpose the errors in the dimensions of two probes and in the spacing of the probe and the power supply line must be kept below 1 to 2%. Further, the disturbance currents in a microwave oven and a notebook PC are actually measured. It is shown that the common-mode current and the differential-mode current can be measured simultaneously. Thus, it is demonstrated that the present instrument is effective for investigation of interference measurement methods and for setting allowable values, and is extremely useful for prediction and reduction of electromagnetic disturbances due to interference waves propagating on power lines.