## 2 DC Microgrid Faults  
A DCMG is defined as a MG in which the power interchange is given in the DC bus [19; 38]. A DCMG may be powered from AC sources, although these sources must be connected to the DCMG through AC to DC interfaces (power inverters) [17]. Figure 1 shows a conceptual diagram of a DCMG with the location of the protection devices (PDs) that are covered in this document.  
Faults in DCMGs can be divided into two principal types: the short-circuit fault current, and arc fault current [11]. This paper focuses on the short-circuit fault current.  
Figure 2 depicts the types of short-circuit faults that may take place in DCMGs produced by over-current and short-circuit events. The following two types of faults are considered: line-to-line fault and line-to-ground fault. A line-to-line fault occurs when an undesirable connection is established between the positive and negative lines, creating a short circuit that connects the supply voltage terminals. In line-to-line faults, the wires are directly connected to each other; therefore, line-to-line faults are of low impedance, which are more dangerous, though they are easier to detect. In line-to-ground faults, one or both conductors are connected to the ground. Therefore, line-to-ground faults are high-impedance faults in most cases; nonetheless, they can be low-impact depending on the  
Figure 1: Conceptual diagram of a DCMG.  
grounding setting used for the DCMG [1; 39; 40]. The ground fault is the most common type of fault in industrial systems [19].  
Concerning the behavior of the fault in DC systems, in steady-state operations, if the current ripple is small, it can be affirmed that the effect of the inductance is negligible. However, under a short circuit, the CB must act by opening the circuit which produces a quick change in the current; under a fault, the effect of the inductance is considerable, and over-voltages are generated. These over-voltages may produce arcs in terminals of the CB. It is important to mention that the arc is easily extinguished in AC systems where the voltage waveform crosses through zero; however, this does not occur in DC systems in which the voltage waveform is constant and not crossing through zero. In consequence, arc-extinguishing devices are required to effectively clear faults in DCMGs [41; 42]. Therefore, protection systems for DCMGs must be faster than those of AC systems. In DCMGs, protection must be configured for all faults on the upstream side of the microgrid because all resources feed the faults, which can have different characteristics in terms of magnitude, wave-front, fault current direction, and operation mode (island or grid-connected modes).  
Fault response characteristics of DC systems can be divided into (a) transient state and (b) steady-state. These correspond to the transient part of the fault injected from the DC connection capacitors, the converter cable discharge, and the steady-state part injected from the power sources [43]. The transitory part of the fault currents can also be split into slow, medium, and fast front transitory. Voltage-dependent charges, converter control, and batteries cause slow front transients. Over-current in capacitors used as filters cause medium front transients, while recovery voltage transients at the opening of the protective devices (PDs) cause fast front transients [44; 45].