High 3rd harmonic on the neutral

Denis McMahon:.

Higher harmonics can also add, but they are less of an issue because they are much smaller.

I gave this more thought as I was clipping my yew hedge this afternoon.


A flute produces something near a sine wave, but a clarinet does not. However, if the magnitude of any harmonic were greater than the fundamental, would that frequency not be the fundamental?


I then wondered what would happen if all the current were at the third harmonic. It seems fairly clear that now we simply have a supply at 150 Hz.


So now I wonder whether the triplen harmonic(s) can ever be more than 50% of the current.


Let's have a balanced load of 100 A per phase and 50% 3rd harmonic. 3 x 50 A at 50 Hz cancel out; and 3 x 50 A add up so we have 150 A in the neutral.


Now let's take a supply with 100 A on 2 phases and none on the third. The third harmonic accounts for 50 % of the current. So at 50 Hz, 50 A goes down the neutral.* At 150 Hz 2 x 50 A goes down the neutral. So now we have 100 A in two of the line conductors and 150 A in the neutral.


Doubtless the likes of Mike or OMS will comment if I am talking drivel. ?


* see Annex A722.

Denis McMahon:.

Higher harmonics can also add, but they are less of an issue because they are much smaller.

I gave this more thought as I was clipping my yew hedge this afternoon.


A flute produces something near a sine wave, but a clarinet does not. However, if the magnitude of any harmonic were greater than the fundamental, would that frequency not be the fundamental?


I then wondered what would happen if all the current were at the third harmonic. It seems fairly clear that now we simply have a supply at 150 Hz.


So now I wonder whether the triplen harmonic(s) can ever be more than 50% of the current.


Let's have a balanced load of 100 A per phase and 50% 3rd harmonic. 3 x 50 A at 50 Hz cancel out; and 3 x 50 A add up so we have 150 A in the neutral.


Now let's take a supply with 100 A on 2 phases and none on the third. The third harmonic accounts for 50 % of the current. So at 50 Hz, 50 A goes down the neutral.* At 150 Hz 2 x 50 A goes down the neutral. So now we have 100 A in two of the line conductors and 150 A in the neutral.


Doubtless the likes of Mike or OMS will comment if I am talking drivel. ?


* see Annex A722.