Cable capacity reduction due to harmonics 1

[JPetersen]

Hello,
We have a cable installation (separate conductors) feeding a VFD - 5.5 MVA. Converter is "6-pulse" type.
The feeding cables will run 98% full load (24-7), and we are not sure if the cable capacity should be upgraded due to the harmonics. Right now we have not the exact numbers for THD and frequency content, but can anyone give us general design hints and common practices. Also litterature recommendations are highly appreciated.
Best Regards
Petersen

 

[GordS]

I have a paper by D Rice (IEEE Transations on Industry Applications, Jan 1986 - "Adjustable Speed Drive and Power Rectifier Harmonics - Their Effects on Power System Components" that calculates a derating factor based on the assumption there is NO resonance, a 6 pulse waveform, and ignoring the proximity effects of surrounding conduit, armour, etc. It is based on HLF (Harmonic Loading Factor) which is the ratio of harmonic current fundamantal current / cable rated 60 Hz current. As an example, a graph is shown that indicates that with an HDF of 100%, a 1000 MCM would need to be derated to 94%. This is the worst case on the provided graph.

If you can get hold of a text by G. Anders on Cable Rating, I think you will find more detailed methods that account for some of the assumptions mentioned above.

 

[wareagle]

When you say the cable is 98% full load what do you mean?
Is that 98% of the actual capacity? I hope not.

 

[JPetersen]

Thanks for your tips on papers.

And yes, the current in the cables will be 98% of the rated cable capacity (if we do not change the cables).

wereagle "hope not" why?

 

[wareagle]

JP
You post "The feeding cables will run 98% full load (24-7)"
is the reason I asked the question. I may not understand you situation correctly but the NEC requires cable to have an ampacity of 125% of the continuous load which makes the cable 80% loaded.