English title

Total harmonic distortion (THD) is the most popular power quality index. Two problems with THD are ambiguities in its definition and utilization of it in some conditions. In order to solve these problems, a number of modifications have been suggested for THD definition and some new indices have been introduced. In this paper, these indices are briefly reviewed, then are investigated, analyzed and compared in view of different features. Thus, it is found that which ambiguities and problems can be solved by each index and which new problems are created by them. Finally, necessary requirements of a substitution index are described and a vector index including a number of suitable quantities are proposed. Regarding the voltage or current signal conditions and each special application, appropriate combinations of the quantities based on the introduced criterion in this paper can be used to analyze signals.