Commonwealth Edison (ComEd), a unit of Exelon Corp., provides electrical service to most of northern Illinois, U.S., and is continually striving to improve the overall reliability of its distribution system to provide the highest possible level of service to customers. One effort over the course of the last seven years has been to deploy large numbers of traditional loop-scheme (LS) reclosers on its 12-kV distribution feeders.

As these efforts to improve reliability have progressed over the years, ComEd has recognized the need to use reclosers with advanced technology to further leverage the return on investment of its existing distribution automation (DA) equipment. It has been determined this advanced technology can be deployed effectively within traditional LS recloser schemes to further minimize the number of customers affected when a fault occurs on the distribution system with minimal modification to the existing devices.

Extending Automation

ComEd is in the midst of a major project to improve reliability — by increasing the segmentation of its distribution feeders to reduce the number of customers affected by any given outage — through further use of advanced DA equipment. Wide-scale use of conventional loop schemes has proven successful in improving the utility’s system average interruption frequency index (SAIFI) and customer average interruption duration index (CAIDI) metrics; however, the segmentation limit of these schemes has been reached.

ComEd has prioritized applying DA on feeders with the most customers and those with the worst reliability issues. As the project has progressed, the utility has found that the criteria for what is an acceptable number of customers in a given feeder segment has changed, with a need to further reduce the number of customers in any given feeder segment. To meet the reliability metrics that ComEd has committed to achieving, the utility needs to install additional DA on feeders where a base level of DA is already applied.

At this point, ComEd is deploying advanced-technology DA devices with pulse-closing capability within existing recloser loop schemes to allow the existing assets to continue providing significant return on investment. Ultimately, the utility’s goal is to meet the objectives of Illinois’ smart grid legislation, the Energy Infrastructure Modernization Act, economically.

Typical loop scheme deployed at ComEd

A conventional loop scheme deployed on a large-customer-count feeder at ComEd typically consists of a normally closed recloser at the midpoint of both feeders with a normally open tie recloser between the two feeders. Should a fault occur on either feeder, only half of the total customers fed by the affected circuit would experience an outage as the LS reclosers reconfigure automatically to isolate the faulted section of line and restore the section that is not faulted.

The benefit of LS is to reduce the SAIFI by one-half since only half of the customers would be affected by the outage. Also, CAIDI is improved since fault current magnitude and phase information is available from the devices through supervisory control and data acquisition (SCADA) to determine the approximate fault location.

The deployment of LS over the years has clearly provided improved reliability. The number of avoided customer interruptions (ACI) at ComEd has increased as DA has been added. From 1999 through 2002, ComEd installed distributed intelligence-based DA with peer-to-peer communications on its 34.5-kV circuits. As a result, up until 2007, ComEd’s ACI were predominantly a direct result of the correct operation of the 34-kV DA equipment.

Beginning in 2007, ComEd began another major effort to install DA on 12-kV circuits. As 12-kV DA was added each year since 2007, the annual 12-kV ACI has increased significantly. In 2011 and 2012, more than 1 million customers did not experience an outage because of the proper operation of DA equipment. In 2012, more than half of ComEd’s ACI were a direct result of its 12-kV DA.