Applications of signal processing tools in a power systems course

International Journal of Electrical Engineering Education, Jan 2004 by Jurado, Francisco, Valverde, Manuel

Abstract

The scope of a modern electrical engineering curriculum presents a challenging problem. It is quite difficult to design a well rounded and all-encompassing curriculum in a rapidly evolving field such as electrical engineering. In this paper, signal processing tools are presented that allow the student to see immediately the advantages and limitations of these techniques for electric power 'quality'. Three signal processing techniques are considered: discrete Fourier transforms, wavelet filters, and discrete short-time Fourier transforms. The paper explains how the course material and teaching style respond to various requirements for an integrated design experience.

Keywords harmonics; power engineering education; power quality

The electric power industry is undergoing a major change, both technically and politically. To prepare future engineers for the challenges they will face, educators must upgrade the power engineering curriculum to reflect changing trends in the industry. One way to respond to the needs of industry is to introduce engineering students to technologically current approaches while students are still in the classroom setting. This may be accomplished by combining traditional and new material.

Electric power quality can be loosely defined as a measure of how well electric power service can be utilized by customers.1-6 When wave shapes are irregular, voltage is poorly regulated, harmonics and flicker are present, or there are momentary events that distort the usually sinusoidal wave, and power utilization is degraded. One refers to these conditions as degradation of power quality.

The subject of power quality engineering truly encompasses most areas of electric power engineering, from generation to utilization, and power quality engineering has been a topic of interest from the inception of the power engineering field. Some contemporary factors have made it the subject of more focused interest, however. The advent and widespread use of high-power semiconductor switches at utilization, distribution, and transmission levels has made non-sinusoidal load currents more common. Deregulation of the power industry has made power quality a distinguishing feature of distribution service. Losses in transmission and distribution systems have come under greater scrutiny in recent years, and certain types of power quality degradation result in losses. For all these reasons, electric power quality has become an important topic in power engineering.

At the graduate level, research needs, more advanced application areas, and university hiring have added impact the doctoral programme. At master level, there is a special need in the electric power quality area: this is a subject that relates to maintaining sinusoidal voltage wave shape at all load buses. Increasing reliability and selling power quality related services, such as unbundled services, are specialized niche needs in industry. Equipment manufacturers have also entered the commercial sector in the marketing of new power system components for power quality enhancement. Power quality has special importance in an educational programme because it teaches modelling and interactions of large-scale systems. Measurement and instrumentation also come into play. Power quality issues are relatively new in power education programmes because of commercial interest in these areas and because the advent of high power electronic switched loads has resulted in power quality degradation in some cases.

The role of modeling in power quality assessment and analysis is crucial. For this reason, modelling is one focus of graduate level power quality courses. Overall, the power quality course brings together several areas and disciplines in a way that is informative and motivational to students.

A key element of modernization is the use of computer programs, similar to those used by industry. Available programs include: PSCAD, MATLAB, Electromagnetic Transients Program (EMTP), and TOP. They are user friendly and need only a PC. The TOP program is freeware (http://www.pqsoft.com/top/) allowing students to study and to experiment outside the university teaching laboratories.

Signal processing tools

In signal processing, the time-frequency domain has often been exploited to analyse signals with fast-changing spectral contents. Wavelet analysis7-9 can be used for similar purposes, and has been exploited recently for several types of voltage and current disturbances10-15 and for power system protection.16 Several possible power system applications for wavelet analysis have been proposed: automated disturbance classification;13 recognition using a wavelet-based neural classifier;10 propagation of power system transients;13 detection of faults;16,17 and visualization of time-varying harmonics.18 The stated advantage of using wavelets compared to Fourier transforms19-21 is the tradeoff between frequency and time resolution at different frequencies.

The wavelet transform has received a lot of attention in the literature, at the expense of the short-time Fourier transform (STFT). The advantage of the latter is its ease of interpretation. Power engineers are used to thinking in terms of sinusoidal signals and the STFT fits closest to this. Most of the results obtained using wavelets can equally well be obtained using STFT.