Distributed energy resources (DER) are in transition from the lab to the marketplace. The defining characteristic of DER is that they are active devices installed at the distribution system level, as opposed to the transmission level. While no specific size range has been defined, most distribution systems would have difficulty accommodating distributed generating resources larger than 10 MW/MVA at any single location and many systems may have even lower limits. Distributed energy resources include generation resources such as fuel cells, micro-turbines, photovoltaics, and hybrid power plants or storage technologies such as batteries, flywheels, ultra capacitors and superconducting magnetic energy storage. They may also consist of dynamic reactive power control devices and possibly customer end-use load controls.