Analysis and design of overcurrent protection for grid-connected microgrid with PV generation

This paper aimed to demonstrate the reliability of the Over Current protection (OCP) scheme in protecting microgrids with inverter interfaced RES for low voltage distribution networks. To prove this reliability, the PSCAD/EMTDC simulation software was used to conduct simulations for the OCP scheme, while comparing throughout grid-connected mode with and without PV generation, as well as in island mode. The computations are carried out using a model of a CIGRE low voltage distribution system. The OCP average relay tripping time for SLG faults through grid mode without PV has been 0.131 s, & 0.121 s for LLL faults. With regards to PV generators, the average relay tripping time increased to 0.199 s & 0.135 s, including both. This is due to the fault current contributed by PV generation inclusion, which restricts the current seen by the predefined OC relays. The findings revealed that some OC relays failed to trip in island mode causing a loss of coordination and a decrease in fault currents. The system was further tested for different generation levels (15%, 57%, and 81%) in island mode and gave a negligible difference in average tripping time for different generation levels.