Harmonics are multiples of the fundamental frequency of a wave. In a balanced 3-phase wyesystem, the currents going into the neutral node are supposed to cancel, so that there is no current on the neutral wire. Harmonic waves may not behave this way, so we should investigate… Harmonics can be modeled as current sources as we have seen, but equivalently, they can be modeled as voltage sources. Lets look at the 2nd & 3rd Harmonics…
The system will still be balanced in the presence of the 2ndharmonic (except negative sequence) and the current from the 2ndharmonic sources will still cancel in the neutral wire! It can be shown that this is true with ANY even harmonic. There is something interesting that is immediately visible aboutthe third harmonic voltage equations we just found. All the waves are in phase with each other.
This is the 3rd harmonic problem
In a similar derivation as the previous slide, with current instead of voltage we can get in the neutral wire:
Ian=Ibn=Icn for 3rd harmonic
In the 2ndharmonic, the current still cancels in the neutral wire, but the 3rd harmonic current adds, producing a very large neutral current. So we need to get rid of the neutral wire. By using a delta configuration, we can trap the current in the delta loop and also take advantage of one useful property. can easily see that the power across the load in a delta loopdue to the third harmonic is zero.
Source: Derek Grant
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