Contents I. Introduction
The DC grid evolved in the past twenty years from ideas into real applications. Two main factors where important for this. First of all the rise of power electronics in distribution systems, and second, the use of the voltage level in the DC grid as a power flow signaling method [1]–[3] The application of power electronics in distribution systems [4] made it possible to not only control the voltage level, but also to control power flow and include electronic protection [5]. Droop control was adopted from the beginning, but there remained a lack of stan-dardization [6]. Improved droop control using a low bandwidth communication using the DC bus voltage and also restoration of the voltage is discussed in [7] as well as current sharing in DC grids. Droop control, not only in stand-alone microgrids, but also grid-tied DC microgrids, is discussed in [8], where a converter including droop control with multiple outputs is presented in [9]. Standardization of droop control depends on the voltage level used in the DC grid [10]. However there are no international standards on the voltage level at the time of writing. There are many different ways to improve the control, for example adaptive droop control which is required in battery management [11]. Protection based on the Rate of Change of Current [RoCoC] [12] [13] has the ability to detect short-circuit faults within microseconds. This improved fault analysis [14] makes the DC grid inherently safer.
