Question: I'm a mechanical engineer so I don't know much about electric circuits and power generation to begin with. Can somebody explain what a harmonic is in the power grid and what is the reason why it is bad? I understand it comes from non-linear electric loads but what does that mean?

Answer: 
A particular reason why the electricity companies think harmonics are bad is that they have to supply them (which means a generally slightly thicker cable on average) AND they can't usually bill the user for them. There are exceptions of course (for higher energy users) and they are encouraged (by the cost of their bill) to keep harmonics low and power factor as close to unity as possible.

A harmonic is a term that nearly always applies to a non-linear load distorting the normally sineusoidal load current. Basically it's not a higher power consumption but it does mean the infrastructure has to be able to cope with the basic billable currents and the generally non-billable harmonics: -

[![enter image description here](https://i.stack.imgur.com/lJvhY.png)](https://i.stack.imgur.com/lJvhY.png)
If you understand how roller bearing create pulses from irregular surfaces , then you can imagine current gets bumps and harmonics from irregular impedance. It causes more heat and wear. In magnetics once the force reaches saturation like a spring bottoming out, the force rises sharply causing damage from excess current.

The nonlinear behavior of all magnetic cores is just like backlash on a belt drive but perhaps smoother. The hysteresis and stiffness or steep slope of some vs others that are smoother in motors of all kinds affects the amount of harmonics generated. Also SMPS supplies need to be active power factor corrected now if for large AC-DC supplies in IEC standards. 50% THD reduces PF by 10% which adds to the stored energy in the grid and increases conduction losses.

Harmonics are also generated by line frequency current pulses such as old linear diode bridge cap linear supplies that are anything but linear on the front end. THe current only charges ~ 10% or <30% depending on load and rating and then discharges to the regulators the rest of the time between each half cycle. These pulses create more damaging **even** harmonics which add in phase in the transformer neutral wire, while odd harmonics tend to cancel when in a differential mode or delta 3 phase system. (perfect 50% square waves have no even harmonics)

**,Thus the harmonics are like bumps in a roller bearing surface impedance** that add excess heat, stress forces, lower efficiency , insulation capabilities which all results in lower lifetime and higher cost of ownership.