3 Phase Neutral Current with Harmonics

Message Topic Search Topic Options Post Reply Create New Topic Printable Version Translate Topic

   Shaggers has often cited how the Neutral on a 3 phase supply, can end up carrying more than 1 phase of current, due to Harmonics of the load, when theory suggests, that it should never exceed the current from one phase, if you ignore harmonics.
Found a nice article proving point, with real world figures. https://eadepower.wordpress.com/2014/03/26/harmony-rampant-discord-more-like/

Written by the bloke who literally wrote the book on BS7909 (17th Ed, but for temporary power supplies), James Eade. Indeed he is re-writing the book to sort out the BS7909:2011 citing IET PAT regs, pre the 4th Ed PAT 2012, and how 4th Ed PAT does not cover hired equipment.....

In summary, 2 off 72 Way dimmers, running off a 3 phase supply. Load is mostly incandescent, and pretty well balanced over the 3 phases. Resolving the three phases, should imply around 14A of load on the neutral, with around 160A load per phase, so not bad balance. Indeed the measured current on N, at 50Hz, is indeed 14A ish.

However, measured total N current is shown at 231A, and did peak at over 300A!! Despite the largest current draw on any one phase being 170A!!

Just food for thought really, given how nasty cheap chinese SMPSU can generate nasty harmonics....