# Maintenance Management of a Transmission Substation with Optimization
## 1 Introduction  
Maintenance is a combination of technical, administrative, and managerial actions during the lifetime of a device, the purpose of which is to keep it in, or bring it back to, the condition that enables performing of its functions. It is, therefore, a usual process needed by every device for normal operation. The maintenance in transmission substations (TS) is crucial for secure and reliable operation of the electric power system (EPS). The maintenance terminology covers two types of maintenance: preventive and corrective. In the field of electricity transmission devices, preventive maintenance still prevails [1]. This type of maintenance can be either time-based maintenance or related to the state of devices, condition-based maintenance (CBM) [2]. From 2009 onwards, the health index [3, 4, 5] has been used in the field of condition-based maintenance to evaluate indicators of maintenance. However, new trends in the field of maintenance, i.e., reliability -centered maintenance (RCM), are being introduced in the wider area of engineering [6, 7], as well as in the field of maintenance of devices in the electricity transmission system [1, 8, 9, 10]. These trends are also accelerated by the standardization in the field of maintenance and asset management [11, 12]. The reliability of operation and associated maintenance is, in the majority of cases, based on the reliability calculation using the Markov model [9, 13, 14, 15]. An adequate efficiency of the determination of system criteria for maintenance can be achieved with the selection of various algorithms, such as the best-worst method (BWM) [16], where numerous possibilities are assessed on the basis of determination with regard to various attributes, and the best maintenance criteria are selected. In the maintenance process, authors have also included optimization procedures dealing with economy and reliability [10, 17, 18, 19]. The authors usually deal with individual elements, such as overhead lines [5, 20] and transformers [21, 22]. References [9, 23] dealwith maintenance of devices in a TS. The authors describe a maintenance method that is based on the technical condition of the devices. Certain authors have also investigated time scheduling of maintenance tasks [14; 24] or determination of the optimal inspection interval of equipment in the TS, taking into account its age [25]. The authors of the presented paper, in [26], discussed the strategic maintenance of switching substations where reliability indices were not optimized. The contribution of this paper is in the calculations of reliability indicators using the failure effect analysis (FEA) method, and not with the calculations of Markov chains for an individual device. In this method, any change in the state of an individual device affects the entire TS, which can, consequently, change the state of other devices as well. The maintenance processes of the devices in the substation are closely related to the reliability indicators (one of them is importance) for the entire TS, and, at the same time, to the condition of the individual devices, which is one of the article's contributions. The novelty presented in the present paper lies in the integrated treatment of TS maintenance based on the RCM concept, using an optimization algorithm. The modified differential evolution optimization algorithm with self-adaptation (SA) of the control parameters was used to select the optimal maintenance period of TS devices (revisions are carried out every year, every two years, or every three years), while the maintenance is carried out when the significant operational state of the EPS is the most suitable for it. This state is defined by generation, consumption, and power flows in the EPS. The method of EPS elements' maintenance used until now is TBM, where their maintenance periods are defined by the TSO's internal rules. The main objective of this approach is to reduce maintenance costs while keeping the operational reliability level and improving the maintenance system constantly.