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Voltage distortion is not a constant value On a typical system, the harmonic distortion follows daily, weekly, and seasonal patterns An example of daily patterns of total harmonic voltage distortion for 1 week is shown in Fig 84 This is typical for many residential feeders where the voltage distortion is highest late at night when the load is low A useful method of summarizing the THD samples of trends like that in Fig 84 is to create a histogram like that shown in Fig 85 Note the two distinct peaks in the distribution, which reflects the bimodal nature of the harmonic distortion trend
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Downloaded from Digital Engineering Library @ McGraw-Hill (wwwdigitalengineeringlibrarycom) Copyright 2004 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website
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Figure 84 Trend of voltage total harmonic distortion demonstrat-
ing daily cycle for 1 week
300 250 Count of Samples 200 150 100 50 0 00% 04% 08% 12% 16% 20% 24% 28% 32% 36% 40%
40% 30% 20% 10% 0%
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Figure 85 Histogram of voltage total harmonic distortion for 1 month
demonstrating bimodal distribution
Once the histogram is prepared, the cumulative frequency curve is computed This is shown overlaying the histogram in Fig 85 and has been pulled out separately in Fig 86 to demonstrate the computation of the 95th percentile value, known as CP95 In this example, a voltage THD of 317 percent is larger than 95 percent of all other samples in the distribution CP95 is frequently more valuable than the maximum value of a distribution because it is less sensitive to spurious measurements Usually an electric utility will collect measurements at more than one location and compute a different CP95 value for each monitoring location Figure 87 shows a histogram of CP95 values compiled from different sites, which serves to summarize the measurements both
Downloaded from Digital Engineering Library @ McGraw-Hill (wwwdigitalengineeringlibrarycom) Copyright 2004 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website
Cumulative Frequency
100% 90% 80% 70% 60% 50%
Power Quality Benchmarking Power Quality Benchmarking 341
100% 90% 70% 60% 50% 40% 30% CP95 = 317% 20% 10% 0% 00% 04% 08% 12% 16% 20% 24% 28% 32% 36% 40% Cumulative Frequency 80%
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Figure 86 Demonstration of graphical method of calculating the CP95 of a distribution
7 6 Count of Sites 5 4 3 2 1 0 00% 04% 08% 12% 16% 20% 24% 28% 32% 36% 40% 44% 48% 52%
40% 30% 20% 10% 0%
VTHD
Figure 87 Histogram of CP95 values for voltage THD at 54 moni-
toring sites
temporally and spatially A CP95 value can also be determined from this histogram, which is a statistic of a statistic that can be used to provide a reference value for an entire utility system
842 Characterization of three-phase harmonic voltage measurements
Many distribution systems in the United States supply single-phase and other unbalanced loads Therefore, the harmonic content of each
Downloaded from Digital Engineering Library @ McGraw-Hill (wwwdigitalengineeringlibrarycom) Copyright 2004 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website
Cumulative Frequency
100% 90% 80% 70% 60% 50%
Power Quality Benchmarking 342 Eight
primary phase voltage is generally different This presents a problem in characterizing the harmonic distortion of a three-phase measurement which has varying distortion levels on each phase There are two possible methods: 1 Consider each phase to be a separate measurement The potential problem with this method is that a count of how often distortion levels exceed a specified level could be 3 times too large 2 Average the distortion levels on the three phases Each three-phase steady-state measurement contributes a single distortion level to the samples A possible drawback is that a high distortion level on one phase is obscured if the other two phases exhibit low distortion levels The latter method has less potential for inaccuracy and is used for calculating the harmonic distortion indices presented here
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