Common mode current and radiations mechanisms in PLC networks

This paper presents the results of a three-year, in-depth study of the mechanisms of EMC emissions generated by an in-house PLC network. The study was led in close collaboration with the Institute for Information and Communication Technologies, the Swiss Federal Office of Communication and, in the framework of the OPERA project, with the EMC team of the Swiss Federal Institute of Technology, Lausanne. Numerical simulations and experimental measurements of a simplified PLC setup were used to carry out an initial study on a number of aspects of the relation between the radiated field and the common-mode current and the impact of the resonances present in the PLC network. A detailed analysis of a 1:1 scale representative room built on our open area test site, which includes several appliances, power outlets, switched outlets and lighting fixtures in its layout, allowed us to elaborate a simplified electrical model describing the generation mechanisms and the nature of the common-mode current flowing in the power line cables. Several results show that the magnetic near field, close to the wire line, exhibits large spatial variations over the whole frequency range from 1 to 30 MHz, which can be as high as 45 dB due to the resonances taking place in the setup. In addition, variations of the common-mode current as high as 35 dB have been observed, engendering a spread of the K factor of 20 dB. A detailed analysis also revealed large variations in the wave impedance, which ranged from some 50 ohms to more than 1000 ohms depending on the proximity to the standing waves along the line, implying that emission levels can be greatly underestimated or overestimated if their analysis is limited to the magnetic field. Our results indicate that the common-mode current generated along the power-line cables is mainly due to electrical asymmetries stemming mainly from switched loads or switched outlets. Perhaps one of the most important results of this study is the fact that the high v- ariability of the common-mode current along the line makes difficult to evaluate the disturbance potential of a setup from measurements of the CM current at a single point.