Evaluation of a Communication-Assisted Overcurrent Protection Scheme for Photovoltaic-Based DC Microgrid

Integration of distributed generations units in distribution systems affects the amplitude and the direction of fault currents. On the other hand, dc fault currents have large amplitude and fast transients. Thus, the setting and coordination of overcurrent relays in dc microgrids will be a challenging issue. This paper proposes a new communication-assisted overcurrent protection scheme for photovoltaic (PV)-based dc microgrids. The proposed scheme has lower cost than the schemes based on the differential protection and also provides the backup protection for the relays. First, possibility of using the overcurrent-based protection is evaluated using a comprehensive strategy for every line section of the system. The strategy determines which relays have to be set for protection of the microgrid in the gridconnected and islanded operation modes. Then, the selectivity and coordination problems of the overcurrent relays in the presence of the PV units are solved by implementing blocking and intertripping schemes using communication infrastructures of the smart grids. Finally, the proposed protection scheme is validated by simulation results on a dc microgrid test system.

I. Introduction

Nowadays, the structure of the power distribution systems has been changed by integrating renewable energy sources and small scale units such as photovoltaic and wind turbines. This resulted in the emergence of “active” distribution systems and microgrids. Microgrids are local networks that involve renewable energy sources and energy storage systems (ESS). They have the capability to be locally controlled. Therefore, they can disconnect from the grid when there is a fault at the main grid, and continue to supply a portion of their local loads in a so called “islanded mode” [1].