27-09-2010, 02:45 PM
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A voltage sag
abstraction
A voltage sag is a momentary decrease in the rms voltage, usually caused by a fault in the utility transmission line or distribution system within customer facility. It is a temporary voltage drop below 90% of the nominal voltage level. Voltage sags and momentary power interruptions are probably the most important power quality problems affecting industrial and large commercial customers.
A voltage sag is a sudden reduction in the supply voltage magnitude followed by a voltage recovery after a short period of time. Voltage sags are mainly caused by short circuits, overloads and starting of large motors. The importance of voltage sags has increased due to problem sags cause to many types of equipment. To be able to estimate the damage voltage sags cause to customers also new tools and analysis programs are needed.
Network information systems are widely used in power distribution companies. These systems include tools for case-specific steady state analysis for different purposes, e.g. programs for load flow calculations, fault current analysis, load growth estimation and reliability analysis. Also voltage sag analysis could be included in this list of programs.
This paper presents typical voltage sag distributions calculated for Finnish rural and urban networks. Voltage sags propagate throughout the power system and a sag sensitive customer can experience sags caused by faults at the same, higher or lower voltage levels. In this paper the main interest is distribution companies and their possibilities in decreasing the impact of voltage sags. Thus only voltage sags caused by short circuit faults on MV radially operated networks and experienced by LV customers are studied. Network characteristics have influence on voltage sag distribution.
for more :-
http://www.leonardo-energyrepository/Library/PQGuide/5.Voltage%20Disturbances/5.5.1%20Voltage%20Sags.pdf