Vibrations are caused by electrodynamic forces and the effect of a fault can be detected by measuring the vibration level. Vibration-based diagnostic method which is used in Latvenergo and measured vibration results as case studies are presented in this paper.

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Reliability of a power system depends on its equipment. As power transformers are essential elements in substations, reliability of power transmission system largely depends on their operating and technical conditions. Therefore complicated maintenance strategies are applied including various diagnostic methods and a thorough evaluation of the acquired results. Although diagnostic tests aid the evaluation of the technical condition, the risk assessment of a particular transformer is the next step in order to ensure certain reliability level. The risk assessment approach applicable for transformers in power transmission system in Latvia is proposed in this paper.

Regular evaluation and risk assessment of the technical condition of power transformers is very important. It is known that good condition state guarantee system's reliability. Various parameters of power transformer can be evaluated, and decision can be made manually or with help of mathematical model. With dissolved gas analysis (DGA) method it is possible to detect faults by using dissolved gases in transformer oil. By using DGA as one of the indicators, a new mathematical riskassessment model for power transformers is being developed for power system in Latvia, and the new model will be based on specific features. Development of DGA indicator is presented in this paper and as a case study results of DGA of different transformers are analyzed and discussed.