What are Harmonics?

A finely distributed stator and field windings when operated in a uniform magnetic field of an ideal generator generates pure sinusoidal voltages. The sine waveform would be a perfect one. But in reality it is not possible to have generation of perfect sinusoidal voltages. The reason is in any working AC machines, the winding distribution or the magnetic field or both are not uniform. This results in some distorted waveform. The distortions are fine and periodic similar to the frequency of the sine waveform. These periodic distortions are called harmonics.

(Fig. Source: from the web)

You may observe from the above illustration that the base sine wave is a perfect wave without any distortion. But because of presence of harmonics, the waveform is distorted. In general harmonics are infinite. All the harmonics are added up and are considered as magnitude of distortion. The cumulative addition of harmonics can be done by Fourier Analysis. However the effect of 5, 7 harmonics would be high. Also note that even harmonics are considered as canceled by themselves because of alternating nature (i.e. positive and negative halves) of the sine wave.

Every wave shape has a Total Harmonic Distortion (THD). When harmonic currents generated by non linear loads gets superimposed on fundamental current, a distorted sinusoidal current wave form results. These harmonic currents flow in the least impedance path from load towards the power source. When the harmonic currents flow through the power systems result in harmonic voltage drops. This voltage drop is nothing but the harmonic voltage distortion.

Harmonic Currents

Harmonic currents are similar to harmonic voltages or in other words these are distorted current wave forms. Harmonic currents are caused by nonlinear loads. What are nonlinear loads? These are nothing but the loads which cause the current to vary disproportionately with the voltage during each cycle or simply we can say that nonlinear loads are any loads which draw current which is not sinusoidal. Some of the examples of nonlinear loads are solid state drives, UPS systems, battery chargers, computers, etc. 

Nonlinear loads on electrical power systems are common phenomena now. Nonlinear loads are not new to the power systems. But the increased usages of these are creating a concern to the power system stability. The reason is simple. As said above these loads draw non-sinusoidal current thereby generating harmonic currents.

Controlling Total Harmonic Distortions

Now the whole issue comes down to the following:
·         Can non-linear loads be eliminated?
·         THD of currents and voltages affect what in a power system?
·         Can we control THD?

The answer to the first query is a NO. With increase in electronic equipment usage even at residential places the question of eliminating non-linear loads does not arise.
If we cannot eliminate can we live with it? What happens if THD exists as such? Harmonic currents effect each and every component of power system adversely. This includes over heating of transformers, motors, line conductors, etc. They create additional dielectric, thermal and mechanical stresses. These some times result even in flawed tripping of circuit breakers. Severe voltage distortion may result when the power system is in parallel resonance (inductive reactance equals to capacitive reactance) at one of the nonlinear loads harmonic current frequencies. Even without resonance, harmonic currents causes losses in system components. 
Whenever nonlinear loads are applied on three phase system, triplen harmonics create a big adverse impact. Triplen harmonics are third (only odd) multiples of fundamental frequency (3, 9, 15, etc). The impact of triplen harmonics is such high in some cases it may increase neutral currents by 140% -170% of fundamental frequency phase current magnitude. These triplen harmonics are transferred to the primary of the system transformer there by increasing losses of transformer. This results in excessive heating of transformer and transformer is subjected to the cascading ill effects of these high losses.

How to control THD of currents?

Many different methods, strategies are applied to mitigate harmonics in power systems. Still no particular standard is made by the international community on this aspect. Only own cost effective solutions are being followed by different plants to suit their requirement. Few of such solutions are using K- rated transformers, multi pulse rectifier transformers, phase shifting transformers, harmonic filters, etc.