I. Introduction
GLOBAL targets on reducing carbon dioxide emissions have been leading to the growth of Distributed Energy Resources (DER) for electricity production. Wind power and photovoltaics (PV) are two of the fastest growing renewable energy resources. In contrast to traditional power plants these units are often of small scale and distributed located thus also mentioned as Distributed Generation (DG). As a main difference to larger power plants DG units are used to be connected to the medium voltage (MV) or low voltage (LV) level of the distribution networks (DN).