<p>Power transformers are important components of electrical systems. Their failures are very costly, mainly because of the interruption of electrical service they cause. Rapid and accurate diagnostic of transformer internal faults are key factors of efficient and safe operation. This diagnostic is usually carried out by experts. Several techniques of diagnostic of power transformers windings exist. The online diagnostic, the implementation cost, the interpretation of results, the complexity, the location of faults, the detection of mechanical and electrical faults, the accuracy are some important criteria to consider when choosing a diagnostic technique. In this paper, we firstly present some popular diagnostic techniques of power transformer windings, such as: negative sequence currents, frequency response analysis, Phase difference between voltage and current, flux-based technique, voltage-current diagram, vibration analysis, differential protection, and dissolved gas analysis. Next a discussion on limitations of each technique is presented. Then based on the above-mentioned criteria a comparative analysis of these techniques is proposed.</p>