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Circuit Breakers (CBs) of transmission and Super distribution are considered to be the most important equipments of power systems. CBs should operate safely to protect power systems against faults and perform tasks such as trip and connect load. The purpose of this paper is to present reliability block diagram of transmission and Super distribution CBs and evaluation level of reliability of transmission and Super distribution substations of Khorasan Regional Electric Company (KREC). To achieve this goal, a four-step approach has been proposed. In the first step, collecting information about the failures and events occurred in the KREC has been investigated. Failures and events lead to an emergency exit and outage of CBS are studied and categorized. In the second step, has been formulated, the failure rate has led to the outage of CBs, Then the failure rate, the repair time them are calculated also. In the third step, the reliability block diagram of CB is drawn based on the failure rate calculated in the second step. Based on the failure rate calculated from the reliability block diagram and with the help of the Reliability Calculation of Power System (RCoPS) program is written under the MatLab software, indexes of reliability of the substation terminals are calculated. In the fourth step, with regard to the output energy of each terminal from the substation, the Expected Energy Not of Supplied (EENS) and unreliability cost are calculated.

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Abstract: By creating electricity markets, regional electrical companies are working to reduce their current costs for greater profit and maintain Competitiveness strength. In this way, the reduction of maintenance costs, which is a significant part of the cost of operating the transmission network, has a much importance. On the other hand, electricity network equipment such as transformer, transmission lines and circuit breakers that have been exploited over the last few decades, naturally, With longer lifetime, have been aging. Consequently, regional electrical companies are now faced with a major challenge, such as aging equipment that to maintain the reliability of the system to an acceptable level and to escape the heavy costs associated with outages, they require a coherent planning for maintenance on the network. In the new competitive environment, regional electrical companies prefer to continue to use aging equipment as much as possible instead of replacing them with high costs by performing appropriate maintenances. In this paper, researches on determining the optimal rate of preventive maintenance, determining the priority of equipment for preventive maintenance and determining the importance of network equipment have been performed. In addition, statistical studies of the failure of khorasan regional electrical company substations for 16 years (2001-2017) have been carried out. By investigation the failure of the transmission and super distribution substations equipments, can be proposed a more comprehensive planning of maintenance of substation equipment, it is also possible to identify the critical points of the substations and prevent the outage of the equipment by performing proper maintenance. Finally, evaluation of reliability of substations with the use of smart grids and preventive maintenance models has been investigated with the help of Markov equations.