Stochastic prediction of voltage sags in an industrial estate

The paper presents the stochastic prediction of voltage sags in an industrial estate situated in southeastern zone of metropolitan electricity authority (MEA) service area. The stochastic prediction of voltage sags provides information on the sag frequency per year in each range of sag magnitude at a particular location in the power system. The method of fault positions is applied for the evaluation of voltage sag performance in both transmission and distribution systems. The actual line configuration, distribution substation bus scheme, actual line and equipment parameters, and operation of power system are used in this study in order to determine the real behavior of voltage sags in this service area. The annual expected numbers of voltage sags are calculated in terms of the product of area of vulnerability at different percentage of voltage sag magnitudes and the expected line fault performance. Calculations of area of vulnerability are made for: both the transmission and the distribution systems, three-phase faults and single line-to-ground faults. The expected lines and feeders fault performance is derived from the statistic records of the power supply in this service area during 2002-2003. According to this particular study, if the sensitive level of the equipment of the particular customer is set at the voltage sag magnitudes below 0.85 per unit, it is tripped out 21.45 times per year. However, if the sensitive equipment´s ride-through capability is improved to the voltage sag magnitudes below 0.75 or 0.65 per unit, the number of trip outs decreases significantly to 15.27 or 7.02 times per year, respectively.