Statistical Analyses of the Distribution and the Annual Trend of Harmonic Voltage in Japan

Transcription

1 Statistical Analyses of the Distribution and the Annual Trend of Harmonic Voltage in Japan Kenji Yukihira Central Research Institute of Electric Power Industry (CRIEPI)

2 First conclusion In Japan, harmonic voltage levels have decreased in recent years because of harmonic current limits.

3 Two kinds of limits prescribed in 1994 For electric appliances Nearly identical with IEC For large customers at MV/HV Unique to Japan Proportional to contract power (A/kW) Not affected by short circuit power

4 Measurement for annual trend Since 1994 when the limits prescribed 6.6 kv bus bars Same 10 substations for residential and business area respectively 5 successive days in October Average and average plus 2 times standard deviation Each plot shows1200 data (24 hours, 5 days, 10 substations)

5 THD (%) Ave. Residential area Business area Ave. + 2 S.D Limits prescribed Year

6 5 The trend seems to increase prior to 1999 and decrease after THD (%) Increasing Decreasing Limits prescribed Year

7 All estimated slopes changed from positive to negative. Year Area Graph Slope (%/year) Residential Business Average Ave.+2 S.D. Average Ave.+2 S.D Increasing Residential Business Average Ave.+2 S.D. Average Ave.+2 S.D Decreasing

8 Statistical test These data can be randomly obtained even if the THD does not actually increase or decrease. Statistical tests were performed by using the method of t-ratio to check if the tendency was significant. The absolute value of t-ratio becomes larger when the THD actually increase or decrease.

9 The possibilities that the THD actually increased or decreased are more than 95 %. Large absolute value of t-ratio Year Area Graph T-ratio Residential Business Average Ave.+2 S.D. Average Ave.+2 S.D Residential Business Average Ave.+2 S.D. Average Ave.+2 S.D

10 Second conclusion The distribution of THD is similar to a logarithmic normal distribution that has a long tail and high THD are observed only in the area to the right of the distribution.

11 Special extensive survey In different points all over Japan. 6.6 kv electric power distribution line. Every 10 min for 6 weeks. 1 week data in this study data/point (6 data/hour, 24 hours, 7 days) data for all points

12 Frequency (%) Distribution of THD Logarithmic normal distribution (Solid line) Measurement data (Line graph) Total harmonic distortion (%)

13 Distribution of logarithm of THD Frequency (%) Measurement data (Line graph) Normal distribution (Solid line) Average : 0.474, Standard deviation : Logarithm of total harmonic distortion

14 Distribution in location Each THD : xij i : 1 to 1008 for time j : 1 to 102 for location Average at each point : 1008 a j = x ij i= The distribution of a j includes only the variations with location.

15 Distribution of average THD at each point Ferquency (%) Distribution of measurment data(line graph) Logarithmic normal distribution(solid line) Average THD at each point (%)

16 Distribution in time y ij = x ij a j The distribution of y ij includes only the variations with time.

17 Distribution of THD shifted by the average at each point Ferquency (%) Measurement data (Line graph) Normal distribution (Solid line) Average : 0 Standard deviation : THD shifted by the average at each point (%)

18 Distribution of average THD at each point Ferquency (%) Distribution of measurment data(line graph) Logarithmic normal distribution(solid line) Areas where failures can occur Average THD at each point (%)

19 Conclusions The existing limits of harmonic currents are effective and sufficient. Harmonic voltage distortion has become a local problem. The problem is going to be resolved in Japan.