30 IEEE power & energy magazine march/april 2015 Digital Object Identifier 10.1109/MPE.2014.2379935 Date of publication: 24 February 2015 D DURING THE LAST FEW YEARS, MANY COUNTRIES AROUND THE WORLD HAVE SEEN a massive deployment of distributed energy resources (DERs) in their distribution systems. In certain regions, penetration has reached levels that increasingly challenge traditional power system manage- ment, affecting the overall stability, reliability, and eficiency of grids. The uncoordinated response of large numbers of DERs may even put overall grid security at risk. This fact was clearly highlighted by the famous 50.2 Hz problem in Europe: it was discovered that the simultaneous tripping of several gigawatts of DERs due to a minor overfrequency event could potentially lead to an undersupply in the European power system so large that it could not be compensated for by using conventional reserve capacities. This ongoing development has led to a fundamental paradigm shift regarding the approach for interconnecting DERs, especially in Europe. There has been a move away from a “it it and forget it” practice and to the active integration of DERs. Today it has become common consensus among the stakeholders involved, including grid and system operators as well as industry, that DERs must be fully integrated into power system operation to maintain the stable and reliable operation of distribution grids with high penetra- tions of DERs. Recent developments in national grid codes and interconnection speciications relect this fundamental paradigm change: new revisions of the interconnection speciications now deine requirements for advanced DER functions, including active and reactive power control, voltage and frequency control, and dynamic grid support, including fault ride- through (FRT). While these requirements were mainly imposed on large-scale DERs con- nected to high- and medium-voltage (HV/MV) grids today, the most recent grid codes demand similar capabilities from small-scale DERs con- nected to low-voltage (LV) grids. To overcome the fragmentation of national DER connection requirements, a new Europe- wide technical speciication is currently being inal- ized by the European Association of Transmission System Operators (ENTSO-E). The Requirements for By Roland Bründlinger, Thomas Strasser, Georg Lauss, Andy Hoke, Sudipta Chakraborty, Greg Martin, Benjamin Kroposki, Jay Johnson, and Erik de Jong Lab Tests 1540-7977/15©2015IEEE