In the power generation sector, the key priority has always been to ensure sufficient power quality efficiency. The generation, control and compensation/reduction of time and spatial harmonics through the entire power system is a significant element in this sense. In the mid-1800s, electrical generation and distribution systems were first developed, and since then enhancement and mitigation of harmonics have progressed from a local generator design issue to a globally controlled supply characteristic that needs to be considered at every point of the electrical distribution system.
Harmonics and their effects
Harmonics develop when the current or voltage waveform is not following the pure sine wave shape. As a result, harmonic distortion is defined as the variation of current or voltage waveform from a pure sine waveform by the incorporation of additional waveforms, typically with frequencies that are some multiple 
The yellow waveform illustrates a harmonic signal. Source: Adobe Stockof the fundamental frequency. This harmonic distortion leads to events that result in major deterioration of power quality. These events could include more transmission line, motor and generator losses manifest as a reduction in performance, and these components might need to be de-rated. Further, there can be problems with sensitive electronics and communication systems when working at high frequencies. Another major challenge is the reduction of insulation life and thus damaging capacitors in case of overvoltage spikes that are usually produced by harmonic resonance.
Causes of harmonics
A key factor contributing to the growing prevalence of harmonics in power systems is that in recent years, the number of power electronic circuits causing non-linear currents has increased. This has also intensified the efforts of authorities to meet expectations and specifications for power efficiency. The new focus on renewables, energy storage and distributed generation systems has continued this trend. Such technologies rely on power electronics and thus can affect the power system further by generating electromagnetic interference and harmonics. However, power electronic devices are considered the major source of harmonic distortion only for lower order harmonics, and when the power electronics converters are poorly designed and cannot be controlled. It has been claimed that apart from power electronic converters, magnetically saturated devices and arcing are two other major harmonic producing loads.
There are also many other sources of harmonics in the power system. For example, in standard rotating machines generating power, ripples in the excitation system’s current and in the torque being produced from generator prime-mover generate time harmonics in the flux, which does not follow sinusoidal mode. Moreover, the windings’ spatial distribution and generator’s geometry also cause additional harmonics to appear at the electrical output. The subsequent harmonic components then spread to other attached loads through the power system.
In power generators, harmonics are categorized into zero-sequence harmonics, reverse harmonics and forward harmonics. This classification is dependent on the effect that they produce in terms of generator torque. They generate no torque if the harmonics are zero-sequence, negative torque if the harmonics are reverse-sequence and positive torque if the harmonics are forward-sequence. The reverse and forward sequence harmonics can produce oscillations in the shaft, leading to critical failure because of enhanced mechanical wear and vibration. Harmonics can also be classified by frequency in which typical harmonics occur in the input frequency’s integer multiples, and in non-integer multiples there are inter-harmonics. Such harmonics are introduced by the latest asynchronous switching converters and may become the main source of flicker.
Reduction of harmonics
Regulatory standards implemented to mitigate harmonics and problems with power quality are possibly costing billions of dollars to industries every year. For example, in 1913 the AIEE standard for the total harmonic distortion of the generator voltage signal at no-load was mandated to be no more than 10%. Nowadays, requirements have gone much further than this and offer broad coverage under varying loading conditions for issues triggered by poor power quality (harmonics). The requirements now provide appropriate harmonic voltage and current levels for various power distribution network points. The best known and most widely used efficiency variables are the total value of total harmonic distortion and a sequence of limits for different harmonics. For continuous signals, tighter limitations have been set in both cases.
Since the beginning of the 20th century, harmonics have been mitigated by improving machine designs. Researchers are developing ways to enhance a machine’s main alternator designs with the goal of minimizing total harmonic distortion at no load. Thus, this is in connection with the total harmonic distortion limit of 10%, which led to an important increase in the output waveform quality of machines developed at the beginning of the 20th century. The more recent emergence of power electronics converters shifted the focus of researchers toward removing the harmonics at the load side. Nonetheless, the latest integration of renewable energy sources and power electronics converters with the power grid again demands harmonics mitigation at the generation side.
Nowadays, harmonic reduction via sharing electrical generation is also a hot topic of research, as this is particularly crucial for microgrids functioning in islanded mode. In such a scheme, harmonic loads are commonly used by many generating units where active filter systems improve total voltage quality. The energy storage units that are dealing with the intermittent nature of renewable energy sources are also reducing harmonics, in addition to smoothing power demand curves and improving power factors.
