Neutral Conductor Size Selection For Non Linear Loads

Neutral sizing for nonlinear loads: Single phase non-linear loads like personal computers, electronic ballast, and other electronic equipment, generate odd harmonics (i.e. 3rd,5th, 7th, 9th, etc. ). The troublesome harmonics for single-phase loads are the 3rd and odd multiples of 3rd (3rd, 9th, 15th, etc.). These harmonics are called “ triplens harmonics “.

The triplen harmonics are zero sequence harmonics and the phase current containing triplen harmonics will get added algebraically in 3 phase 4 wire systems. If triplen harmonics current is 5 Ampere /phase, then the total harmonics current due to triplen harmonics will be 15 Amp. The total harmonics current finds its path through the neutral conductor. If the system is balanced, no current flows in the neutral conductor because the vector sum of all the phase current is zero.

With balanced non-linear loads, neutral sizing is very important. The conductor size of the neutral conductor for non linear loads must be designed on the basis of triplen harmonics present in the system as well as for handling additional unbalanced current.

Triplen harmonics also cause circulating current on the delta winding of a delta-star transformer configuration. When triplen harmonics current on the neutral of a 3 phase 4 wire system reaches the star winding of the transformer, it gets reflected at the delta connected primary winding. The harmonics current keeps circulating in the delta winding of the transformer. As a result, heat loss in the transformer increased.

Thus the total current in the neutral conductor is equal to the sum of current due to unbalance system voltage and due to triplen harmonics current.

In = Ib + Ih

In = Neutral current
Ib= Current due to system voltage unbalance
Ih= sum total of triplen harmonics current  

Normally 3 and 1/2 wire is considered to take care of current due to unbalance voltage. However, if the loads are non-linear the neutral conductor size must be equal to phase conductor size or even more than the size of phase conductor, depending on harmonics in the system. Thus, neutral sizing for nonlinear loads is very important.  

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