Biru, Getachew(PhD)Tesfay, Kiros2018-07-062023-11-282018-07-062023-11-282014-06http://etd.aau.edu.et/handle/12345678/7084Electricity is produced and delivered to customers through generation, transmission and distribution systems. A stable and reliable electric power supply system serve customer loads without interruptions. Distribution systems deliver power from bulk power systems to customers. Distribution reliability primarily relates to equipment outages and customer interruptions. Electric power interruption in Addis Ababa region distribution is becoming a day to day phenomenon. Even there are times that electric power interruption occurs several times a day, not only at the low voltage but also at the medium voltage distribution systems. This thesis work attempts to thoroughly identify causes for power interruptions and customer dissatisfaction and discusses the design, reliability and operation and maintenance of Addis Ababa distribution system. Furthermore, based on load forecast for 25 years, a model distribution substation design have been carried out to help EEPCO use the model in its design practice, as many of the substations have been designed without considerations of future power load growth and are suffering from overload operation. The model design is carried out for the 250MVA, 132/33kV distribution substation for Addis Center distribution system to upgrade the existing 63MVA, 132/15kV distribution substation. Distribution reliability indices such as SAIFI, SAIDI, CAIDI, and ASAI have been analyzed thoroughly on the Addis Ababa distribution substations (Addis Center) using on data collected from EEPCO. The analyzed and calculated distribution reliability indices values have been compared with standard benchmark values and comparison clearly indicates that Addis Ababa distribution system is extremely unreliable. The values for SAIDI, SAIFI, CAIDI and ASAI are 23 minutes/year, 0.5 interruptions/customer, 112 minutes/year and 99.97% respectively in a Germany standard, whereas the corresponding values for the existing distribution system of Addis Center are 252 minutes/month, 3.65 interruptions/customer, 2280 minutes/month and 99.425% respectively. In addition to the model design, the study discusses the measures to be taken in terms of operational and maintenance tasks to improve the serious reliability problem of Addis Ababa distribution system. Key words: Power Distribution System, Voltage Drop, Distribution Substation Design, Feeders, Reliability Indices, Reliability Indices Analysis, Operation & MaintenanceenPower Distribution SystemVoltage DropDistribution Substation DesignFeedersReliability IndicesReliability Indices AnalysisOperation & MaintenanceThesis