ABSTRACT
Risk level is chosen in accordance to production maintenance and replacement in case of unexpected loss. Majority of power system equipment is designed for a reliable operation within the 25 year interval. System level failures may range from no system impact at all to widespread outage with significant indirect losses. Halloween blackout in 2003 is characterized as a local power outage. Local power outage could also have damaged transformers as root cause. Power transformers are electrical devices installed on the power plants or substations and are employed in the power network to increase or decrease the voltage level with the same frequency. Normally, the total installed power transformer capacity in the grid is four-five times higher than the generation capacity. The reliability requirements to a power transformer operation are determined by the degree of its influence on the whole power grid security. Transformer replacement is costly and logistically challenging.
