Using voltage sag and interruption indices in distribution planning

A traditional approach to distribution planning calls for the most economical system upgrades, timed to meet projected capacity needs. In a more competitive environment there is also value in improving the power quality. Power quality impacts, especially in the areas of rms voltage variations and sustained interruptions, can alter the economic evaluation of investment options for capacity expansion. While it may appear attractive to feed new load from a particular substation, an increased sag rate may result, favoring a different solution. This paper describes a planning process that includes power quality impacts. The process is dependent on service quality indices that measure the performance of the system. Methods for estimating the indices from measurements and simulations are discussed. An example is presented that illustrates how consideration for the cost of rms variations and sustained interruptions can alter the planning decision.