الفهرس 
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Abstract
Abstract
This thesis addresses the voltage sag problem in distribution systems. A part of the medium voltage network in the region of North Cairo Electricity Distribution Company (N.C.E.D.C) has been monitored for a period of 171 days to record the voltage sag events occurring in the system. The parameters and measurements of the system under study have been used for developing a simulation model using MATLAB/Simulink/SimPowerSystem program.
The developed computer model enables the simulation of different causes of voltage sag. Namely, distribution/transmission network faults, transformer energizing, load switching and induction motor starting. The study considers the system configuration (overhead lines and/or underground cables) and loading scenarios (loading values, load switching,………..).
A main objective of this study is to propose a methodology to realize the origin of voltage sag depending on the event characteristics (magnitude and duration of voltage sag). This methodology relies on comparing the site measurements with the simulation results. The comparison reveals on overlapping between the characteristics of voltage sag due to distribution system faults and that due to transformer energizing.
Accordingly, the adopted methodology proposes the detection of the second order harmonic current as a means of resolving the overlap between transformer energizing and distribution faults.
Another main objective of the study is the application of selected single-event indices and site indices to the recorded measurements as an assessment of the voltage sag of the studied system.
Measured voltage sags have been allocated on ESKOM chart.
The assessment study proves that the number of recorded voltage sag on the studied system exceeds the limit proposed by certain standard.
Finally, the thesis suggests a new feature to be added to the power quality analyzer to account for a comprehensive voltage sag assessment and analysis.