Re: Total Harmonic Distortion

Spir Georges GHALI

paule said:

Thanks, if its exceeds the maximum value of 5 % for n=3 and 6 % for n=5, what will be the impact in the connected instrument or devices, does it will damage the instrument?what precaution or protection we can apply.

Dear Mr. Paule ;


If the Voltage Harmonic Distortion for each harmonics exceeds the value defined by standard, that means the Total Harmonic Distortion of Voltage ” THD U  or  THD V ” will be globaly more than standardrized valeus, that can creats some disturbances in working of instruments, and even the currents for lieanr Loads will be distorted.

By the way, when we have these high percentages of THD U, that means we have big percentages of current distortion ” THD I “, that leads to increase the losses in transformers, generators, cables, and also the heating of these equipments will be more. In some cases when we have a big value of ” THD I ” but the ” THD U ” is less than the stadard value, we should in this case dereat the power of Transformes and Generators, and also over-size the section of cables.

In the same time, the most dengarous harmonics are the Double Third Harmonics, that means : 3, 9, 15, …, because these harmonics have the same vector,so, the sum of them aren't ” 0 ” and they run in the neutral cables, for this reason once we have this kind of harmonics in the network, we should calculate the value of them and do correctly the choice of the neutral cable's section. But most of the maufacters of some elctronic equipments like : Virable Speed Drives, UPS, try to minimize the percentages of them in their euipments. Noting that in some equipments of serious manufacters the percentages of this kind of harmonics are ” 0 “.