This thesis presents an assessment of voltage sags when faults are happened in the 115 kV transmission line of the Provincial Electricity Authority (PEA) when distributed generation (DG) is installed in the system. The computer program PSCAD/EMTDC (Power Systems Computer Aided Design / Electromagnetic Transient in DC system) version 4.2 is applied to provide electrical parameters for the modeling and simulation and to analyze the fault occurred which resulted in voltage sag. Test results from PSCAD/EMTDC show the values of voltage sag when 5 types of fault are occurred (Single Line to Ground Fault Double Line to Ground Fault Three Phase to Ground Fault Line to Fault and Three Phase Fault). For all cases the value of root mean square voltage before fault is 100% or 1.0 pu. It can be seen that for both radial line and closed circuit when DG are installed in the system the value of phase to ground RMS residual voltage and phase to phase RMS residual voltage at the bus of Bang-pa-in 2 substation are more than the values in case of the system without DG.

This paper presents an assessment of the quality of electricity transmission and distribution system to supply power to the industrial area. The objective is to analyze the power quality that customer in the industrial area need high level. By analyzing the power quality of Provincial Electricity Authority (PEA) in transmission system 115 kV distribution system 22 kV and 400/230 volt in Rojana industrial park from power quality meters. The study concluded that the phenomenon occurs continuously consist of RMS voltage power frequency voltage unbalance short term flicker long term flicker Harmonic meet the standards applied to all. The phenomenon occurs temporary consist of transient voltage dip voltage swell interruption in the conclusion the power quality problems caused by voltage dip most especially with loads which are sensitive to voltage. From the moment the measurement showed that the voltage dip on transmission system 115 kV maximum of 19 times. The voltage dip that occur including three voltage level of 30 times there are events that do not pass SEMI F47 standard 12 times or 40%.