Reduction of Conducted Interference by Steel Armor in Buried Cables: Measurements and Modeling

Switched-mode power supplies in variable-frequency drives often combine good electrical efficiency with generous production of conducted emission. The common-mode (CM) current through the output-to-motor cable may perturb nearby systems via cable-to-cable crosstalk. Parameters relevant for the coupling are the rise--fall times and amplitude of the output current and voltage, and types and lengths of the cables. Of the many techniques to reduce the crosstalk, we investigated a particular one: reduction of the CM current by the armor of the motor cable. The armor is intended for mechanical protection, but may also substantially reduce the crosstalk. In an actual installation, we measured the transient current through the three phase leads of a motor cable. The cable length was 85 m, and it was buried in wet soil for most of its length. We also measured the transfer impedance of the armor up to 10 MHz, and used this to calculate the overall CM current in a coupled cable model using assumed reasonable values for the cable and soil parameters. Measurements and model results agreed well on the amplitude of the dominant resonance at about 300 kHz. The ratio of the inside transients to CM current was a factor of 20.