As the yearly electric energy demand grows, there is a significant increase in the penetration of distributed generation (DG) to fulfil this increase in demand. Integration of a DG into an existing distribution system has many impacts on the system, with the power system protection being one of the major issues. Short circuit power of a distribution system changes when its state changes. Short circuit power also changes when some of the generators in the distribution system are disconnected. This may result in elongation of fault clearing time and hence disconnection of equipments in the distribution system or unnecessary operation of protective devices. In this thesis, the effect of DG penetration on the short circuit level has been analyzed in a distribution system with wind turbine and gas turbine generators. Different cases have been studied. Location and technology of the DG sources are changed to study the effect that these changes may have on the coordination of protective directional over-current relays.