Power quality: How to mitigate harmonics

Power quality problems like harmonics are a major cause of unscheduled downtime, equipment malfunction and damage. Reliability and consistency of electricity supply and equipment function are critical to businesses; harmonic pollution is a serious threat.
With so many new technologies and equipment in place, the occurrence of them is even more frequent. Understanding what causes harmonics and learning how to avoid them can help provide a more efficient and reliable power system with a healthy return on investment.
Electrical engineers who specify high-voltage systems must understand how to mitigate harmonics when designing new power systems.

[@bobcocampo]

Will single phase line to line supply mitigate third harmonics and other zero sequence harmonics?

[@KingofFray]

If I want to mitigate harmonic problems with an active filter in my power system, is it %THDI or %TDD that needs to take the consideration? I'm confused here