The concept of energy storage has become a practical reality. Utility companies are now installing energy storage systems on their distribution circuits to gain experience with these new distributed power resources and to determine how best to use them for both planned applications and contingency events.

Recently, S&C Electric Co. (Chicago, Illinois, U.S.) installed and commissioned a Distributed Energy Storage System (DESS) for American Electric Power Co. (Columbus, Ohio, U.S.) at its Citizens Substation in Bluffton, Ohio. The DESS consists of three major components: a sodium-sulfur battery assembly, manufactured by NGK Insulators Ltd., provides the energy storage; an S&C Smart Grid Storage Management System performs the dc-to-ac energy conversion, and also controls charging and discharging algorithms; and a padmounted step-up transformer converts the ac system output to line voltage.

The Bluffton installation is one of three 2-MW systems that S&C furnished for AEP in 2008. At the most complex site, the energy storage system is installed on a 30-mile (48-km)-long 34.5-kV feeder. Several S&C Scada-Mate Switches, equipped with the S&C IntelliTEAM II Automatic Restoration System, create an area that can be locally served with stored energy if power to the feeder should be interrupted. If an upstream fault occurs, the system is “islanded” from the utility grid and serves as many downstream customers as its capacity will allow. The system provides peak-shaving functionality, as well.

S&C also provided the storage management system, system integration and installation for a 1-MW system for AEP's Chemical Substation in West Virginia. This system — which has been in service since June 2006 — stores energy at night, during off-peak hours, and discharges it into the distribution feeder during peak periods to reduce the load on the distribution transformer. The system has allowed AEP to defer capital investment in a larger-sized transformer for three to five years.