This book deals with various sources of harmonics, the limit of IEEE-519 for both voltage and current, the effects of harmonics on various electric equipments. The book shows clearly the effects of harmonics on transformers. K-factor which is considered as a modern design for transformers to accommodate harmonic currents is investigated at different loading conditions. This book developed and discussed different cases, including harmonic voltage and current measurements on 11kV level, comparison of these values with the limit of IEEE-519, and calculation of K- factor for choosing the suitable k-rated transformer for each case. At the end of this book, general conclusion and future works are illustrated.