WHAT ARE HARMONICS AND WHAT CAUSE HARMONICS?
Harmonics are currents or voltages with frequencies that are integer multiples of the fundamental power frequency being 50 or 60Hz . Harmonic frequencies from the 3rd to the 25th are the most common range of frequencies measured in electrical distribution systems.Harmonics are caused by and are the by-product of modern electronic equipment such as, adjustable speed drives and variable frequency drives, battery chargers, UPS, and any other equipment powered by switched-mode power supply (SMPS) equipment and due to use of power electronics equipment in different fields.

WHAT PROBLEMS DO HARMONICS CREATE?
In an electrical distribution system harmonics create:-

1. High voltage distortion 
   
2. High neutral-to-ground voltage 
   
3. Poor power factor conditions with a power factor less than 0.9.
   
4. Resonance that produces over-current surges. 
   
5. False tripping of branch circuit breakers.
   
6. Large load currents in the neutral wires of a 3 phase system itself.
   
   

HARMONICS MITIGATION
The majority of electrical nonlinear equipment, especially three-phase types, normally associated with larger powers will often cause the need for the addition of mitigation equipment in order to attenuate the harmonic currents and associated voltage distortion to within the necessary limits.

Depending on the type of solution desired, the mitigation may be supplied as an integral part of non-linear equipment (e.g., an AC line reactor for AC PWM drive) or as a discrete item of mitigation equipment (e.g., an active filter connected to a switchboard). The majority of this Section relates tothe mitigation options available. Neutral current eliminators and phase shift systems (for four-wire systems)

• Standard AC line and DC bus reactors
  
• Wide spectrum (reactor/capacitor) filters
  
• Duplex reactors
  
• Passive L-C (inductance/capacitance) filters
  
• Multi-pulse (phase shifting)
  
• Quasi-multi-pulse (phase staggering)
  
• Active filters
  
• Active front ends (sinusoidal input rectifiers)