3phase to single phase

You are quite right James. With three balanced phases then the theoretical neutral current is zero as the neutral currents arrive at the star point 120 degrees out of phase and cancel each other out. Once you add harmonic currents to the mix things get very different. Standard dimmers, normally being single phase devices connected between a phase and neutral cause lots of third harmonic distortion at 150 Hz. There are also higher multiples, the greatest in magnitude being typically 9th, 15th etc. These multiples of three are known as triplens. They are unique in the harmonics scenario in that they add arithmetically in the neutral. It is not uncommon to see neutral currents higher than phase currents on networks with a lot of harmonic distortion produced by single phase devices. Just to illustrate the point i had to resolve these issues at the Royal Opera House with their 3MW of dimmers a while back and tackle these things every day.

Edited by oldskool - 16 July 2014 at 9:12am

Have sorted similar problems for the last 25 years at Shepperton Studios, Marks & Spencer etc etc etc. These phenomena are nothing new. The crap spewed out by all those switch mode power supplies found in everything these days is just making the problem worse nationally. Funny, in some parts of the grid you can clearly see the harmonic nasties due to everybody switching their televisons on about 5.00 to 6.00 PM and then disappear again about 11.00 PM. The last time I measured that was on an 11kV feed to a big hospital surrounded by housing estates.