Earthing Arrangements Impacts on Protection Schemes for a Commercial Microgrid

Conventional distribution systems typically utilize TN earthing systems with overcurrent time-trip protection. However, in microgrids overcurrent protection alone is inadequate for fault discrimination. A combination of differential, directional overcurrent and overcurrent time-trip protection is required. This paper represents a practical microgrid model and investigates fault behaviour of the microgrid during one phase to chassis ground fault (L-G), called insulation fault, which is the most common type of fault in distribution systems. The results will be presented for three different earthing systems, TN, TT and IT, in both grid connected and islanded mode. The restrictions of protection scheme for each earthing system is presented and eventually, proper protection scheme will be proposed. All models of the generators and controllers and eventually the grid, are built using PSCAD V6.4.2.