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

Abstract

Abstract-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.

A Case Study on Lightning Protection, Current Injection Measurements, and Model

“…We also measured the transfer impedances Z t in the laboratory on 1 m long cable samples, as detailed in [13]. A vector network analyzer has been used for frequencies of 300 kHz and higher, and a combination of a signal generator and lock-in detector for frequencies below 1 MHz.…”

“…with R dc is the dc resistance per meter length, J n is the Bessel function of order n, x = −jf /f δ , and f δ is the characteristic frequency, where the skin effect sets in (see also [13]). The measured Z t of cables b and c is more complex than (3); at high frequencies the fit overestimates the measured |Z t | (see Fig 5).…”

A Node-to-Node Admittance Functions Implementation of an Improved Frequency Dependent Multiconductor Transmission Line Model

“…For the case of frequency independent PUL parameters, (2) and (3) in time domain can be numerically obtained by simulating enough cascaded electrically small sections. On the contrary, for the case of frequency dependent PUL parameters, the time domain responses of an MTLs can be obtained by using the IFFT approach [52]. Although it is only appliable for linear terminations and can be time consuming for a wide frequency band and a long MTLs, it provides a reference solution for the validation purposes in this paper.…”