Improvement of energy efficiency is the principal requirement of the power market. The expansion of the transmission network and maintenance strategies has direct influence on the availability of the transmission network, as well as operating and maintenance costs. The paper defines the criteria for the maintenance of transmission network components taking into consideration the technical criteria, the importance of particular installations in terms of energy, the life of the components and the statistics of events occurring during the operation. The quantities that influence the availability of the transmission network are specified. The models and methods required for calculation of transmission network reliability and availability are described. The required availability of electric power installations is defined. Guidelines for the development of the transmission network are proposed, and problems related to planning, construction and operation of electric power installations are briefly discussed. In choosing optimal voltage level according to technical and economic criteria, the actualisation method was applied. The optimum single-line diagram and the HV equipment necessary for various voltage levels are specified. The life of individual components in a particular HV installation is analysed. A model for the assessment of techno-power indicator of a particular transmission area is proposed. Estimates of relative costs of time-based maintenance of the HV bay and transformers are given taking into account different voltage levels. Possible strategies and a categorisation of transmission network maintenance are defined. Based on the importance of particular process signals, a categorisation of interventions in a substation is proposed. A list of priorities in time-based maintenance is specified taking into account the power indicator and the condition of the particular component in the substation. Both the economic justification of the application of the new technique of diagnostics and monitoring of the condition of components of HV installations, and the possibility for redefining the criteria for transmission network maintenance are analysed. Based on the estimate of fixed and variable costs for various maintenance strategies, optimisation of investments in time-based and/or condition-based maintenance was performed.