Electricity is produced and delivered to customers through generation, transmission andrndistribution systems. A stable and reliable electric power supply system serve customer loadsrnwithout interruptions. Distribution systems deliver power from bulk power systems torncustomers. Distribution reliability primarily relates to equipment outages and customerrninterruptions.rnElectric power interruption in Addis Ababa region distribution is becoming a day to dayrnphenomenon. Even there are times that electric power interruption occurs several times a day,rnnot only at the low voltage but also at the medium voltage distribution systems.rnThis thesis work attempts to thoroughly identify causes for power interruptions and customerrndissatisfaction and discusses the design, reliability and operation and maintenance of AddisrnAbaba distribution system. Furthermore, based on load forecast for 25 years, a modelrndistribution substation design have been carried out to help EEPCO use the model in itsrndesign practice, as many of the substations have been designed without considerations ofrnfuture power load growth and are suffering from overload operation. The model design isrncarried out for the 250MVA, 132/33kV distribution substation for Addis Center distributionrnsystem to upgrade the existing 63MVA, 132/15kV distribution substation.rnDistribution reliability indices such as SAIFI, SAIDI, CAIDI, and ASAI have been analyzedrnthoroughly on the Addis Ababa distribution substations (Addis Center) using on datarncollected from EEPCO. The analyzed and calculated distribution reliability indices valuesrnhave been compared with standard benchmark values and comparison clearly indicates thatrnAddis Ababa distribution system is extremely unreliable.rnThe values for SAIDI, SAIFI, CAIDI and ASAI are 23 minutes/year, 0.5rninterruptions/customer, 112 minutes/year and 99.97% respectively in a Germany standard,rnwhereas the corresponding values for the existing distribution system of Addis Center are 252rnminutes/month, 3.65 interruptions/customer, 2280 minutes/month and 99.425% respectively.rnIn addition to the model design, the study discusses the measures to be taken in terms ofrnoperational and maintenance tasks to improve the serious reliability problem of Addis Ababarndistribution system.rnKey words: Power Distribution System, Voltage Drop, Distribution Substation Design,rnFeeders, Reliability Indices, Reliability Indices Analysis, Operation & Maintenance