Short story about harmonic research

Harmonics were always of special concern in power system studies. In the past the power system was comprised of mainly passive components with relatively linear operating range as well as synchronous generators. Harmonic analysis of such systems is state-of-the art right now. Nowadays modern power systems include more and more power electronic components than in the past. The most significant power electronic components are different types of flexible alternating current transmission system (FACTS) devices such as static synchronous compensator (STATCOM) or static var compensator (SVC). Also renewable energy sources (e.g. wind turbines, photo voltaic installations, tidal steam generators) as well as high-voltage, direct current (HVDC) electric power transmission systems become more popular. Power electronic equipment in modern power systems is obviously a source of additional harmonic components not seen previously. On the other hand, the application of advanced and fast control in grid-connected power converters introduces possibility of control higher than the fundamental frequency components. Appropriately used power electronics can definitely improve the quality of power.

To show how harmonics have being been more and more important in electrical engineering and power system studies a popular electrical engineering database was investigated. Institute of Electrical and Electronics Engineers (IEEE) is considered by many as one of the biggest professional association gathering electrical and electronics engineers from many fields. The association has an extended digital library comprising more than 3 million technical and scientific documents. Unfortunately access to the documents if not open for all interested and therefore knowledge sharing is limited especially for small research units all around the world.

Analysis of such database form harmonic and wind power research perspective can give a good overview how advancement of technology in both areas has been developed in the space of the years. At the begging let us see how publications regarding harmonics have appeared.

Publications concerning harmonics
Figure 1 Total number of publications concerning harmonics.

The first paper regarding harmonics in the ieee database is from 1899 but the most significant development within the area of harmonics started in late 80's. In Figure 1 one can see that the interest of harmonics is still increasing. Up to now there are more than 55000 of publications only about harmonics. Obviously harmonic analysis can be applied in many fields of electrical engineering. Let us additionally see how in the same period the interest in wind turbines and wind farms development increased.

Publications concerning wind turbines and wind farms
Figure 2 Total number of publications about wind turbines and wind farms.

In case of research associated with harmonics the tendency is rather linear, but regarding both wind turbines and wind farms there is an exponential growth of interest directly reflected in number of publications. This tendency can be easily seen in Figure 2. This general interest in wind power can be also easily seen in increasing number of commercial projects. Therefore harmonics analysis in this particular field of electrical engineering seems to be important also from market tendencies perspective.

Wind power capacity
Figure 3 Existing world capacity of wind power.

Figure 3 shows that increasing interest of wind power technology is correlated with increasing world capacity of wind power [1]. As it can be concluded this tendency will be increasing within next few years. Therefore new research projects are required to extend knowledge and experience in the area of renewable energy sources. This also implies an increasing interest of harmonic research within this area of power industry. As it can be seen also in Figure 2 wind turbine studies are more popular than wind farm. It seems to be a natural way of research. Although it also indicates that there is much less experience with wind farm analysis as a whole system comprising many wind turbines and other components. The first observed publication in the IEEE database regarding wind turbines is from 1958 but regarding wind farms is much later (1982). Even less experience is gained in offshore wind power solutions. This field is newbie (first mention in 2000) in comparison to onshore installations. Anyway nowadays knowledge sharing capability and technology development allow deriving knowledge from other fields and continuous improvement.

Publications concerning offshore wind farms.
Figure 4 Number of publications regarding strictly offshore wind farms.

In Figure 4 one can see even clearer tendency of exponential growth of interest and research within the field of offshore wind farms. I hope that based on presented above facts one can see that there is a great interest in offshore wind power solutions which creates new challenges to the wind industry from harmonic analysis perspective.

[1] J. L. Sawin and E. Martinot, "Renewables 2011 Global Status Report," Renewable Energy Policy Network for the 21st Century, Annual Report, 2011.


Harmonic analysis - historical background

I would like to mention at the begging that for more than last three years I have been working on various aspects of harmonic in power system and power electronics. Almost all of my research activities were focused mainly on the application of harmonic analysis in large offshore wind farms. Since I started there have been few observations and ideas that I would like to share with you. From my perspective such knowledge sharing approach can bring closer harmonic analysis in electrical engineering and allow to establish a dialog between engineers and physicians from different fields of science. Let me start from the historical background of harmonic analysis.

First well documented appearance of harmonics in relation to mathematics and science is from ancient Greek. The science which pursued the investigation of multiple and epimoric (ratios of the form [n+1]:n) ratios in music was concerned with the mathematical 'fitting-together' (harmonia) of the constituent notes and intervals of music [1].

Harmonics were described by Claudius Ptolemy, who related musical harmonies to the properties of mathematical proportions derived from the production of sounds themselves. Ptolemy argued for basing musical intervals on mathematical ratios, in agreement with the followers of Pythagoras, backed up by empirical observation. Ptolemy wrote about how musical notes could be translated into mathematical equations and vice versa [2]. Those harmonies he considered to be distributed in all aspects of the physical universe. In particular, they were there in the phenomena of the planets and the human soul [3].

Nowadays, harmonic analysis is the branch of mathematics that studies the representation of functions or signals as the superposition of basic waves. It investigates and generalizes the notions of Fourier series and Fourier transforms, mathematical tools significantly useful in different areas of engineering. The basic waves are called harmonics, but the name harmonic in this context is generalized beyond its original meaning of integer frequency multiples.

It can be said that the beginning of nowadays harmonic analysis reaches back a French mathematician and physicist, Joseph Fourier, who originally defined the Fourier series for real-valued functions of real arguments, and using the sine and cosine functions as the basis set for the decomposition. Fourier series could be used only for analysis of periodic functions.

Many different approaches of defining and understanding the concept of Fourier series have been developed since that time. All are consistent with one another, but each of which emphasizes different aspects of the topic simultaneously extending significantly the basic concept of harmonic analysis. Some of the more powerful and elegant approaches based on mathematical ideas and tools not available at the Fourier time extended his original work and are successfully applied in harmonic analysis at present.

Wavelet transform of EMI
Wavelet transform of electromagnetic interference.

Many other Fourier-related transforms (e.g. Laplace transform, Fourier transform, short-time Fourier transform, wavelet transform) have since been defined, extending the initial idea to other applications. This general area of inquiry is now by physicists and engineers called harmonic analysis.

I hope this short introduction appears to be interesting for you and can encourage you to check later posts regarding various aspects of harmonic studies in engineering and large offshore wind farms. If you have any questions or comments do not hesitate to drop me a line below.

[1] D. Creese, Monochord in Ancient Greek Harmonic Science. Cambridge University Press, 2010.
[2] J. Solomon, Ptolemy Harmonics: Translation and Commentary. Brill Academic Publishers, 1999.
[3] A. Barker, Scientific Method in Ptolemy's Harmonics. Cambridge University Press, 2001.