http://dranetz.com/wp-content/uploads/2014/02/harmonics-understanding-thefacts.pdf

This link may help.

DEAR WELHETESANI

HOPE THIS SLIDE OF PICTURE COULD HELP

Hi Wael,

If you study the nature of third harmonic (and other triplens) between the three legs of the level-3 NPC inverter, you will find that they are cophasors or instant-to-instant identical. They therefore do not mutually add up to zero at the neutral point (unlike positive sequence, or negative sequence current components), rather they add up and become three times the value in each leg.

Consequently, they have no other path than to flow as part of Inp. Such harmonics are classified as zero sequence components.

-Sanjay

Thank you so much Sanjay