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This paper presents the applications of regression models to predict the future growth trend of total harmonic distortions (THD) of 40 power transformers from field measurement data. The prediction values are important and can be used for transformer planning. Those transformers which feed different types of customers reveal different harmonic characteristics. While the linear regression model is suitable for harmonic voltage distortion, the nonlinear exponential regression model is better for current. All transformers' harmonic voltage distortions follow with harmonic current distortions, that is, feeder loads are harmonic sources. The prediction harmonic values of regression analysis are lower than that from the method of average spread rate. Keyword: Harmonics, power transformer, regression analysis, prediction, statistics

This paper presents a survey of voltage and current harmonic distortion levels at 40 power transformers and 10 capacitor units in Northern Taiwan. Through one year measurement, about 12,000 sets of harmonic voltage and current data are recorded and analyzed. The types and levels of harmonics are showed in each power transformer which supplies different residential, commercial, and industrial customers with various loads. At the same time, a suggestion is given to the representative value of background level of harmonic voltages with 95% cumulative probability which is adopted by Taipower. At last, effects of harmonics on capacitors and power transformers are discussed. Keywords: harmonics, distribution substation, survey, electric power quality.