Power Distribution System Failure and Vulnerability Analysis in Ethiopia Case Study : Kotebe Distribution System
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Date
2021-11
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Addis Ababa University
Abstract
Distribution systems are usually the main areas where power outages occur. Electric
power interruptions in the capital city of Ethiopia,i.e. Addis Ababa, are usual
problems. Even there are situations that electric power interruption occurs several
times a day.
This thesis tries to identify the causes of power interruptions for the Kotebe distribution
system and the vulnerability of the existing components. It is a componentbased
failure analysis on a selected feeder of the distribution system. It is selected
considering the total annual frequency of interruptions for each feeder of the distribution
system. Feeder 6 of the Kotebe substation is the most frequently interrupted
line. The existing conditions of the components on the feeder are inspected. Distribution
poles, conductors, insulators, transformers, protection and other auxiliary
devices are the inspected components. The probable effect of the most frequent
causes of interruptions identified in the distribution on the poor distribution components
is studied.
Interruption during operation and maintenance is the common cause identified in
the distribution. But this is through the order of the maintenance workers of the
utility. Sudden faults are usually due to weather influences. It accounts for 48.66%
of the total known faults excluding interruption due to operation. The vulnerability
to failure of the existing distribution poles is identified through modelling the
distribution poles, gravitational force of tilted poles and weather influence by COMSOL
software. Decay depth of wooden poles considering the selected area weather
condition is studied using TimberLife software, and its effect for failure is simulated
using COMSOL. Poles will deflect upto 3.050 and 1.7490 with and without considering
decay depth at the worst condition of a certain weather influence, respectively.
Poles resilience condition is also identified by analyzing deflection and stress on their
surface.
Many of the existing distribution transformers are working overloaded. The loss of
life of distribution transformers and their vulnerability is studied using the IEEE
standard (C57.19-1995) guidelines. The hot spot temperature, ageing acceleration
factor, and percent loss of life of each transformer are calculated. 83 of 157 transformers
are expected to sustain below the IEEE standard expected life (i.e., 20.55
years). The effect of lightning considering soil resistivity of the area is also studied
by modelling the feeder 6 of Kotebe distribution system using Matlab/Simulink.
Metal Oxide Varistor(MOV) Surge arresters with grounding resistance above 25Ω would have higher surge magnitude that could damage the transformers and surge
arresters.
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Keywords
Distribution components, Distribution system, Failure analysis, Loss of life of transformer, Resilience, Soil resistivity, Vulnerability