Modeling Harmonic Loads on Neutrals - Neher McGrath

[AJM32]

Has anyone seen any research on how to properly model the heating affect of neutral current due to harmonics in Neher McGrath ductbank heat analysis? I have a data center project, where the load could have a good bit of harmonic distortion, and I've been modeling the neutral loads in ductbanks as just single phase power sources (with loads equal to 1.73 times the phase load for maximum harmonic distortion). Is the heating effect the same for currents induced by harmonics as it would be for 60Hz power? References would be greatly appreciated.
Thanks,
Andrew

 

[dpc]

Just compute the effective rms current including harmonics. It's the square root of the sum of the squares fora all harmonic currents in the Fourier series of your total current. There is an increased ac resistance due to skin effect for higher order harmonics, but if you're looking at 3rd harmonics, it isn't enough to matter.

 

[AJM32]

Thanks dpc. Thoughts on how I should model it when I actually don't know the harmonic currents? This is an installation where the client wants to have the most flexibility possible for the space (which is a shell data center), and wants it engineered as robustly as possible to take into account any possible harmonics for any type of load.

 

[dpc]

I don't have any published references. I think you will need to make some assumptions. If this is below 1000 V in the US, the NEC ampacity tables are very conservative, and do not actually require any de-rating due to mutual heating in ductbanks. This is only required for higher voltages. Increased separation between circuits in the ductbank reduces mutual heating.

 

[electricuwe]

Unless there is old-fashioned single-phase power conversion equipment without PFC intended to be used, harmonics in the neutral will not be an issue.

Triplen harmonics in the neutral has been a big issue in the time of single-phase rectififers with capacitive DC-link.

Now a lot of standards make power-factor control mandatory for nearly every power electronics device connected single phase. A more smart approach is to use equpment with three-phase input where possible.