Reliability of the electricity supply plays an important role in energy sector. The number of power consumers and amount of consumed power energy increase on yearly basis. As a result of consuming losses of electricity power increase too, which are incurred to consumers, transmission operator and power producer in case when power supply is interrupted. Electricity supply mainly depends on the safety of equipment, which appears at each stage of electricity chain: from generators to electricity consumers. Safety of the equipment depends on several factors, but in this work attention is mainly focused on the technical condition of high power transformers. High power transformer is one of the main elements of power system, which determine safety of electricity supply. Therefore, in this work, attention is drawn to transformer maintenance, diagnostics and prognosis of technical condition, fault detection and risk assessment – there is transformer prevention and maintenance evolution from repair works based on worked out schedule till the technical due diligence of technical condition and maintenance with an acceptable degree of risk and prognosis of the remaining life time. It is extremely important to choose appropriate and qualitative diagnostic method which will suitable to the individual transformer unit, properties assessment of insulating oil and solid insulation and which will applicable to the approved maintenance strategy. The information obtained by the object diagnostic, should be maximally complete and unequivocal. The status of the object insulation should be reflected on the basis of information obtained. Diagnostic method should have a complete conception of the investigated object; otherwise the safety of the object is not properly evaluated. This work dealt with: high-power transformers possible maintenance strategies and its survey, the assessment methodology of technical condition (TCI – technical condition index) and parameters of assessment, prognosis methods of the technical condition and techniques, possible transformer failures and fault assessment of future risk determination. Technical condition detection algorithm based on Republic of Latvia Transmission System Operator (further RL TSO) applied diagnostic method is developed in the work.