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Power Quality & Harmonic Distortion on Distribution Systems

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Power Quality & Harmonic Distortion on Distribution Systems

Power quality is deteriorating with the increase of non-linear loads on utility distribution systems. The voltage and current waveforms are becoming more distorted. Since this is becoming a wide spread problem today, and new, more strict, distortion guidelines are under development, utility engineers are having to deal with analyzing and planning for the control of the distortion. This paper introduces some common harmonic analysis t·echniques and applies them to voltage waveforms recorded on a typical REA transmission and distribution system to increase power quality.

Joseph Fourier first used this idea to solve the heat conduction equation in 1807. But certain mathematicians in the French Academy of Sciences in Paris, such as Laplace, Lagrange, and Legendre, who reviewed Fourier's work rejected it because of lack of rigor and proof of convergence. Finally, in 1822, after two more revisions, Fourier published his Theorie Analyrigue de la Chaleur that contained, among other things, the Fourier series, and the Fourier integral.

The definition of power system harmonics is based on the application of the Fourier transform and superposition to voltage and current waveforms. As implied earlier in equation (1), an ideal power system contains only the first harmonic - 60 Hz. When non­linear load conditions exist, the distortion of the voltage and current waveforms can be explained and analyzed much easier using Fourier transforms.

There are many ways to obtain the Fourier transform of a voltage or current waveform on a power system. The easiest way is to use a spectrum analyzer and measure the harmonics directly in an on-line mode. Another popular method is to sample and store the time-domain waveforms as discrete data points and compute the harmonic components digitally using a microprocessor in an off-line type mode. With either method it is necessary to condition the signal properly by reducing the relative magnitudes of the waveforms to levels appropriate to the testing equipment being used and filtering out any high frequency components above the Nyquist frequency so that aliasing errors are not introduced into the output. Aliasing is a higher frequency component, above twice the sampling frequency, taking on the appearance of a lower frequency component. A popular technique for computing the harmonic magnitudes and phase angles of a sampled time domain waveform is a Discrete Fourier Transform.

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Power Quality & Harmonic Distortion on Distribution Systems