Power cables are inherently capacitive and, as capacitor banks, their capacitance can produce a risk of resonance with the inductive parts of the network.
These resonance risks and the harmonics themselves can produce the following problems for cable systems:
- Cables involved in system resonance may be subjected to voltage stress and corona.
- Increased heating due to higher rms current, skin effect, and proximity effect. The skin effect will vary with the frequency and conductor size.
Power cable conductors commonly lie very close to one another, and therefore the high-frequency currents in the outer skin of one conductor influence the spread and behaviour of high-frequency currents in the skin of the adjoining conductors, giving rise to a “proximity effect.”
"The skin effect and proximity effect are proportional to the square of a frequency. Cables therefore have to be derated if there is significant harmonic distortion, particularly if iTHD is greater than 10%."
Feeder cable to the drive cubicle
The power cable feeder to any variable-frequency drive (VFD) drive carries 60 Hz fundamental or sinusoidal current plus the harmonic currents produced by the drive. The selected feeder size needs to be based on the heating from the total rms current (fundamental plus harmonics) and the skin effect of the higher order harmonics.
🔺 The cable therefore has to be derated to compensate for additional heat caused by the harmonic currents and the associated skin effect.
Skin effect also depends on the conductor size and, hence, large conductor sizes should be avoided.
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| Figure 3 – Cable derating vs. harmonics with six-pulse harmonic current distribution |
Figure 3 shows the cable derating factors plotted against percentage harmonic current with the harmonic mix associated with a typical 6-pulse VFD. Due to the skin effect, more derating is required for large conductors.
In terms of the application of cables for variable speed drives, or other signicant harmonic sources, the following recommendations can be made:
Use three-conductor and not single-conductor cables.
Avoid large conductors to minimize the losses due to skin effect. The conductor size should not exceed 350 kcmil.
Use shielded cables for equipment rated above 600 V.
