PQ, reliability and DG

The distributed generation (DG) ownerʹs reliability, measured by the sustained interruption indices, should improve markedly if DG has been installed and operated properly. However, because this is just one customer out of hundreds or thousands on the feeder, the improvement does not show up the utilityʹs traditional reliability indices. There can be a positive or negative impact on the utility-level indices in these cases: DG can reduce the number or the duration of sustained interruptions if automated switches are available to restore power in DG-supplied islands or to DG-supported alternate feeds; the loss of fuse-saving on laterals can significantly degrade the utility-level indices. There is little opportunity for DG to affect the PQ of other customers, as measured by the indices for voltage sags, which are related to faults and interruptions. DG can still have adverse impacts on harmonics and steady-state voltage regulation, which are other aspects of PQ. Utilities should consider tracking ASIFI, ASIDI, and other indices that are based on load size rather than the number of customers. As a supplement to the traditional reliability indices, these would better show the positive impacts of DG, assuming DG owners tend to be larger customers.