Protection Issues in Microgrids and Multi-microgrids

Abstract

Conventional power generation from fossil fuels contributes to the problems of climate change and low energy efficiency. These problems can be partly addressed by interconnecting distributed generators (DGs) with the distribution system to supply electric power from renewable energy resources such as photovoltaic (PV), wind, biomass, and fuel cells. One way of integrating a large number of DG sources with the distribution system is to interconnect them with low-voltage (LV) loads to form a microgrid (MG). Multi-microgrids (MMGs) increase the load capacity of the distribution system by integrating several MGs, DGs, and medium-voltage (MV) loads with the distribution system. This chapter discusses a number of challenges in the protection of active distribution systems, MGs, and MMGs. The existing protection schemes and coordination strategies to address these challenges are briefly reviewed in this chapter. This is followed by the presentation of a new protection scheme for MMGs. The existing protection scheme for MMGs originally proposed by the EU More Microgrids Project disconnects all the DG sources when fault conditions occur. As a result, this protection scheme prevents the continued operation of the MMG during fault conditions. The MMG also experiences varying fault current levels due to contributions from DGs and MGs compared to single grid-connected MGs. A new adaptive overcurrent protection scheme for MMGs containing MGs with varying generation and load is proposed in this chapter. It automates the setting of relay characteristics to deal with the variations of generation and load as well as adjust the tripping time delays based on the fault current levels. It also addresses the problems due to the presence of DGs such as the blinding of protection, failed reclosing, and false tripping. The effectiveness of this protection scheme is evaluated by simulation for the MMG scenarios containing different combinations of net load (NL-) and net generator (NG-) MGs.