The integration of distributed generation (DG) units into traditional distribution grids causes several significant changes in their characteristics like power flow direction, voltage profile and short circuit level. Therefore, the currently used control and protection strategies can no longer work properly and have to be revised and modified. The most important protection problems are e.g. blinding of protection, false tripping, unsynchronized reclosing. For a reliable and efficient protection system, both transient and steady states of fault current contributions should be considered. In this paper, in order to study the real impact of DG units on a given protection scheme, the fault current contributions generated with exact models of DG units including their interfaces with the grid and control system (photovoltaic generator, PSMG with full size converter, DFIG with partial size converter, commonly met on Belgian grids, and directly connected IG) are presented and compared with the ones that are generated by ideal models of DGs in the same conditions. The PSCAD software is used for the simulation of transient contributions of DGs under several faulty conditions in a tested medium voltage distribution grid. (C) 2014 Elsevier Ltd. All rights reserved.