In this chapter, the first section shows the results of lifetime survey of the existing equipment and estimates the number of the expected equipment exceeding their expected average lifetime in Thailand’s transmission network and also the case of forty-eight pilot high voltage substations which are in utility’s short-term replacement planning. The second section presents the results of substation performance evaluation and importance priority evaluation of forty-eight high voltage substations. The third section demonstrates the risk analysis results based on ‘d’ distance technique. Subsequently, the results of total replacement numbers of aged equipment evaluated by proposed replacement procedure and by expected average lifetime are shown and discussed. In the last section, risk treatment of the highest risk substation is determined as a guideline for lessening the total risk on the system.