27. Protective Systems in DC Microgrids

The aging power system causes to several grid problems such as intermittency, power quality issues, and blackouts since a few decades. Therefore, the power infrastructure requires serious troubleshooting studies in a wide manner. The distributed generation and integration of large photovoltaic (PV) plants to the existing utility have led to intensive interest on DC power grid infrastructure. The widespread use of DC based microgrids decreases significant power losses and facilitates operation and maintenance of microgrids. Besides, DC loads are easily supplied by DC microgrids that eliminate the requirement for power inverters. It is noted that elimination of DC-AC power conversion can prevent power losses of entire system between 7 and 15% that is remarkable ratio for a microgrid. In one hand, the DC microgrids provide increased interest due to their advantages such as power density and distribution efficiency comparing to AC power systems. On the other hand, short-circuit current capabilities of DC microgrids lead to significant hazards for users and properties. Moreover, it is not possible to overcome arc faults occurred in a DC microgrid by using regular circuit breakers since DC current do not draw a natural zero crossing waveform. The cost and bulky structure of DC circuit breakers is another important issue in this regard. The actual fault protection systems are based on over current detection for power electronic devices and improved circuit breakers, distributed generation source controllers, and several types of relays. This chapter deals with fault detection methods and protection devices in low voltage DC (LVDC), medium voltage DC (MVDC), and high voltage DC (HVDC) networks. Protection schemes and improved devices with circuit topologies are presented regarding to DC microgrids.