05-07-2012, 12:37 PM
Harmonic Distortion in Power Systems
thesis on Harmonic Distortion.pdf (Size: 1.22 MB / Downloads: 140)
Definitions and Terminology
This chapter gives an overview of methods for the analysis of
distortion in power system and some related, commonly used, indexes
that gives information of the waveform deviation in condensed form.
Window size of the analyzed signal
Different window sizes give different frequency spectra for a
fluctuating distorted signal, i.e. the signal is not periodic or the
window size is not chosen to cover the whole period.
If the window size is 20 ms (one cycle at 50 Hz) the resolution in the
frequency domain is 50 Hz. Thus, no frequency components can be
found between multiples of 50 Hz. If the window size is 10 cycles the
resolution is 5 Hz.
Circuit analysis methods
The most important aspects for the study of harmonic distortion
propagation in power system are given below.
The power system, i.e. generators, transformers, lines, cables is
assumed to be linear.
Loads can be either linear or non-linear.
Assume there is no cross coupling between harmonics with
different frequencies. In reality there is a cross coupling between
harmonics with different frequencies. A strong coupling is present
between the fundamental voltage component and the current
harmonics from a non-linear load.
Frequency domain: Easy method. Consider each component and
use superposition. Iterate to reach steady state.
Telephone psophometric current
Harmonic currents, mainly zero sequence, in distribution and
transmission lines cause a magnetic field that can induce voltages in a
nearby located telephone line. Each harmonic frequency, including
the fundamental, is weighted with a factor [64], figure 2.4.1, taking
into account the audio response of the human ear and a microphone.
A similar curve exists in the USA, called TIF (Telephone Interference
Weighting Factor), see [59].