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: 
The following may help understand non-linear loads and how they generate harmonics.

![schematic](https://i.stack.imgur.com/rhzSX.png)

[simulate this circuit](/plugins/schematics?image=http%3a%2f%2fi.stack.imgur.com%2frhzSX.png) – Schematic created using [CircuitLab](https://www.circuitlab.com/)

[![enter image description here](https://i.stack.imgur.com/dIkNs.gif)](https://i.stack.imgur.com/dIkNs.gif)

*Figure 1. A simple bridge rectifier, smoothing capacitor and resistive load.*

The circuit of Figure 1 should be easy enough to understand. When the top AC input is positive current flows through D2, R1 and D3 back to the other AC line. C1 charges up and maintains voltage on the load. For a mechanical engineer this would be analogous to having two pulsed hydraulic supplies, four non-return valves (D1 - 4), a pressure reservoir (C1) and the load (R1).

Form the above it should be seen that current (flow) on the first pulse will follow the supply pressure but that on subsequent pulses no current will flow until the reservoir pressure is exceeded by the supply pressure. The result is that the required current flows in pulses - see the black curves in Figure 2. 

*Figure 2. Rectified AC (red), capacitor voltage (blue) and rectifier current (black). The AC current (to the left of the rectifier on Figure 1) will look similar to the black curve except that each half cycle will alternate above and below the line.*

It should also be intuitive that since the rectifier provides current in pulses that the peak current may be many times the average current.

Since any periodic wave can be formed by the addition of a sine wave of the fundamental frequency and its harmonics (in varying amounts) the distorted current demand creates harmonics in the current supply.

[![enter image description here](https://i.stack.imgur.com/27HVo.gif)](https://i.stack.imgur.com/27HVo.gif)

*Figure 3. This fabulous illustration of the Fourier Transform by Lucas V. Barbosa on Wikipedia's [Fourier transform](https://en.wikipedia.org/wiki/Fourier_transform) page shows the transformation of a periodic waveform from the time domain to the frequency domain. The frequency plot shows the relative strength of the harmonics with clarity that could not be obtained from staring at the time plot.*
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.