This book presents theoretical aspects of short-circuit performance of power transformers, transformer testing experiences, short current testing laboratories and high-voltage thyristor valves for electrodynamic testing. The questions of the original application experience of LVI-testing, Frequency Response Analysis (FRA) to check the condition of transformer windings, infra-red control, ultraviolet control, and Partial Discharges (PD) for the insulation monitoring of electrical equipment are examined in this book. The LVI method and short-circuit inductive reactance measurements are sensitive for detecting such faults as radial and axial winding deformations, a twisting of low-voltage or regulating winding, a loss of winding pressing, etc. The most important elements of intellectual networks (Smart Grid) are the systems for monitoring the parameters of electrical equipment. Information-measuring systems (IMS), which are described in this paper, were proposed to be used together with rapid digital protection against short-circuit regimes in transformer windings. Short-circuit performance of power transformers, transformer testing experience, short current testing laboratories and high-voltage thyristor valve for electrodynamic test of power transformer are examined in this book. High-voltage electrical equipment with the SF6 (sulfur hexafluoride) gas insulation, analysis of accident rate and service experience are represented. Estimation of the technical condition state of substation electrical equipment with the use of software and information tools allows for increasing the effectiveness of technical diagnostics and reliability of electrical equipment in service. The following diagnostic models for evaluating the technical condition state of electrical equipment are examined: model of the electrodynamic stability (mechanical state) of the transformer and reactor windings, multi-frequency diagnostic model of n-layer paper-oil insulation and model of the drift of temperatures of the bus arrangement of transformer. Monitoring, control and analysis of breakdowns of overhead transmission lines (6-500 V) in the Middle Volga region have been analysed. The book is based on the previous author's researches. The author has more than 200 scientific and technical publications. The main research interests concentrate on the field of transformer short-circuit testing, transformer winding fault diagnostic, Frequency Response Analysis, mart Grid and information-measuring systems.