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In recent years, the need to characterize the feeder-level power quality performance has emerged. It is driven by the interest to compare the PQ performance among utility companies or among various feeders. This is a very challenging task, however, since the voltage sag profile along a feeder is needed for calculating the feeder-level PQ indices. It is impossible to measure the voltage sag level at each customer site. In response to this emerging need, we propose to estimate the sag profile of a feeder based on limited number of metering points. The proposed sag estimation method has the following characteristics: 1) it makes use of the radial connection characteristic of a distribution feeder, 2) it requires only a limited number of metering results, 3) it employs a least-square method to predict the sag profile along a distribution line. One of the potential applications of the proposed method is to calculate the feeder power quality performance indices such as System Average RMS Frequency Index (SARFIx).