Ieee std 1159-1995, ieee recommended Practice for Monitoring Electric Power Quality

8.4.6.2.3 Impedance spectrums
The last technique to be examined is the impedance spectrum. An example is shown in Þgure 21. This
method takes both voltage and current harmonic data and graphs the impedance vs. frequency of the power
system. It provides useful information regarding system frequency response, resonant points, and potential
problems due to harmonic distortion. See [B13].
To generate the impedance spectrum, the desired current harmonic data and the difference in voltage har-
monic data at the point of interest needs to be measured. The difference in voltage harmonic data is the dif-
ference between the no-load and full-load voltage harmonic data resulting from the load(s) in question. The
no-load data can either be obtained from turning the loads off, or possibly using the harmonic data from
some point near the source, say, at the source transformer or service entrance.
With this data, the impedance can be calculated at each harmonic frequency and plotted. The subsequent
graph will provide insight into the frequency characteristics of the power system seen at the point of mea-
surement. Should a high impedance exist due to resonance at a harmonic frequency, for example, then care
should be taken to reduce any harmonic currents of that frequency, and so reduce possible voltage distortion.
8.4.7 Pattern recognition
8.4.7.1 Scope
Becoming familiar with particular electromagnetic signatures is helpful in quickly interpreting graphical
data, but many times it is not the graphs that are the most important issue. Other patterns, such as time of
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IEEE
Std 1159-1995
IEEE RECOMMENDED PRACTICE FOR
58
day, often provide the key to data interpretation. Patterns do not have to be graphical, but can also be textual
in nature. Recognizing these additional patterns will be of great beneÞt in solving the power system puzzle.