This is a small comment I made on a linked in discussion on Harmonics that I think is worth sharing.
Harmonics are a result of currents generated by non-linear loads (ballasts, Adjustable Frequency Drives, Power Supply's, Some UPS's, Inverters, all loads that change from AC-DC-AC are non linear), these currents modify the fundamental AC Sine Wave (60Hz), 3rd, 5th and 7th Harmonics, being the most harmful, and common. Harmonics can produce, heating on the transformer windings and reduces its capacity in kW, also false trips on the circuit breakers (nuisance tripping), high neutral currents, etc. All harmonics currents add for a %THD (Total Harmonics Distortion) per IEEE 519, these levels should be below 10%. IEC Norms are a bit more tough on Point of Common Coupling (PCC) Levels (see EC 61000-3-2 Ed. 3 2005). Solutions vary depending on a case by case basis. I recommend a Power Quality Survey of minimum 1 week (depending on the size of the plant or building). Also many people confuse Harmonics with Surges, remember always that Harmonics are a low frequency power quality problem (Hz), and Surges are a High Frequency (kHz and MHz) power quality issue. There is vast information online and I recommend the Dranetz PQ Book and also Mark Wallers "Harmonics: A Field Handbook for the Professional and the Novice", it's a great book for novice, Mr. Waller explains Harmonics with a down to earth approach. Also I am referring to Low Voltage Harmonic problems (up to 600VAC), High Voltage Harmonics can cause other issues and should be handle accordingly.