Figure 1 : Fundamental and waveform

Figure 1 : Fundamental and waveforms containing harmonics
(a) Normal fundamental 50Hz Waveform.
(b) Waveform containing the fundamental plus third and fifth harmonics.
Power system voltage and currents are sinusoidal waves (See Figure 1a) Harmonics are sinusoidal components of a periodic wave that are integral multiples of the fundamental frequency.
Superposition of harmonics with fundamental will distort the original waveform (See Figure 1b).
2.2 Harmonic Sources
Harmonics are generated in power systems by:
• Arcing Devices - Equipment that creates arcs as part of normal operation, such as electric arc welders.
• Magnetic Circuits - Magnetic devices such as transformers, produce distorted wave shapes when they are operated in their nonlinear regions. 
• Power Electronics - The switching of such devices as Silicon Controlled Rectifiers (SCRs) and thyristors produce electrical wave shapes that are not sinusoidal in nature. These devices include Variable Speed Drives (VSD) or electronic power supplies.
2.3 Consequences of Harmonics
• Excessive Equipment Heating - Increased heating is the result of increased copper and iron losses due to the increased frequencies present. Equipment ratings must take the presence of harmonics into account. For example, the International Regulations requires that the size of neutrals be increased in circuits that serve loads that contain significant harmonics. The larger neutral is required to carry the added high frequency currents.
• Electrical Interference - Interference may take the form of loss of data, communication interference, or misoperation. Many electronic devices count on regular sinusoidal voltage waves for detection of peaks and/or zero crossings used in timing circuits.
• Over voltage - An over voltage is a voltage above the normal rated or maximum operating voltage of a device or circuit. Harmonic over voltages are caused by local circuit resonant conditions that can overstress equipment insulation. One most common form is the tuning of a circuit due to the addition of a capacitor. If such a harmonic resonance occurs capacitor banks can even be exploited.
The following equation can be used to determine if
the system is tuned to a power system’s characteristic
harmonic: