Design of a Current-Transformer Based Inductive Coupler for Power-Line Communications

In this paper, the design of a current-transformer based inductive coupler for power-line communications is documented. A worst-case design is presented in a step-by-step fashion and experimentally confirmed by doing verification measurements in the laboratory as well as live, 240-V tests in a controlled environment. Although field deployment of inductive couplers usually relies on parasitic capacitance and/or surge protection capacitors to complete the current loop, in this controlled experimental setup various discrete loop capacitors were utilized to characterize the impact on transmission. A band-pass filtering response was achieved without any external filter circuitry, as internal coupler parasitic parameters were utilized for this. For the single-coupler laboratory measurements, both the lower and upper cut-off point results were approximately within 10% of predicted values. These results are quite encouraging, seeing that coupler core permeability is specified with a ± 20% tolerance. Live tests showed that (i) cable length (series inductance) impacts on the upper cut-off point of the system passband, while (ii) loop capacitance affects the lower cut-off point of the system passband.