This shows up in systems with a lot of power electronics, data centers, rectifiers, VFDs. Switch mode supplies inject harmonic currents. Sometimes it matters. Sometimes it does not.
In industrial plants you will often exceed IEEE 519 recommendations. A big reason is that much of the load is motors and pumps. They are usually less sensitive to harmonic voltage distortion than electronics. When harmonics do become a problem, you will see shunt capacitor bank filters and detuned banks added. The goal is simple. Reduce the impedance seen by harmonic current so the harmonic voltage stays lower. V equals I times Z.
The part that confuses people is sequence behavior.
The fundamental acts like positive sequence because whatever phase rotation you define as the fundamental, ABC or CBA, that becomes the definition of positive sequence. Harmonics then fall into a pattern. Harmonics of order 3k + 1 tend to be positive sequence. Harmonics of order 3k − 1 tend to be negative sequence. Harmonics of order 3k are triplen harmonics, and they behave like zero sequence. Positive and negative harmonics want to go to neutral. Zero sequence harmonics want to go to ground.
Triplen harmonics are in phase on all three phases. That means they add in the neutral if a neutral exists. If there is no neutral they will try to return through whatever stray path exists, usually system capacitance to ground. That path is often high impedance. High impedance means higher voltage deviation. A delta winding, of a delta-wye, can trap triplens and keep them from flowing upstream, but it can also mean extra heating in that delta.
Even harmonics are usually uncommon in power systems. They tend to imply half wave behavior or a badly biased converter. In practice you mostly deal with odd harmonics.
Power electronics produce these components because their switching is synchronized to the same phase angle of their phase. It is not random between phases. That repeatable switching creates a repeatable harmonic current injection profile, and therefore sequence components.
In the toolkit, the takeaway is that you should care about the path from the load back to a ground source. You can switch between delta-wye grounded and delta-delta with no ground reference transformers. Then adjust the harmonic profile. If triplen harmonics are present, voltage distortion jumps when there is no ground reference because the triplen current is forced into the high impedance capacitive path. This goes to show that this ground source path is essential for maintaining power quality with high triplen harmonic loads.
Bottom line. Triplen harmonics are not just a number on a report. They are a wiring and transformer configuration problem. Pick the wrong transformer connection for a system full of power electronics, and voltage distortion can get ugly fast. An alternative is to use shunt filters, series reactors, and STATCOM, but these are more complicated mitigations.