Distributed generation expands scope of protection at downstream of distribution network

Distributed generation (DG) is a rising power electrical source technology, which brings considerable economic benefits and society benefits. So DG is applied widely in power distribution network. But primary current protection couldn’t meet need of the selectivity and sensitivity because it was affected by change of the system operation mode in power distribution network. The distance protection was affected little by the system operation change. The effect of the distributed generation access to radial distribution network on distance protection was studied in this paper. Matlab/simulink was utilized to establish the simulation model of power distribution network with the distributed generation, which was configured three-step distance protection on each line. Through changing DG capacity and DG different positions access to the distribution network, the effect of the distributed generation on the distance protection was analyzed roundly. When DG was located in the downstream of fault spot, distance protection would not be affected on this line. When DG was located in the upstream of fault spot, the measure impedance of distance protection which was located in the upstream of DG would be smaller and the protection scope of distance protection would be longer when the DG positions was closed to fault spot. Then the distance protection could operate correctly. But the measure impedance of the distance protection located in the downstream of DG would be invariable, the distance protection could operate correctly. So the distance protection would not be affected, which located in the downstream of DG. The measure impedance of distance protection located in the upstream of DG would be larger and the protection scope of distance protection would be shorter when the DG positions access to the distribution network was unchanged and the DG capacity was becoming larger. Likewise the measure impedance of distance protection was unchanged, which located in the downstream of DG. The distance protection would not be affected. But when fault happened in the protection scope of distance Ⅱ protection, the measure impedance of distance protection located in the upstream of DG would be larger as the DG capacity was becoming larger, which led to the distance Ⅱ protection couldn’t operate correctly. So the access of DG would be influence on the distance Ⅱ protection. The simulated test results show that the access of DG would be influence on the sensitivity and selectivity of distance protection located in the upstream of DG, but the distanceⅠand Ⅱ protection could operate correctly. The distance protection would not be affected when it located in the downstream of DG. When the DG positions access to the distribution network was changed, the measure impedance of distance protection was smaller as the DG was nearer to fault spot, which led to influence on the selectivity. When the DG capacity was changed, the measure impedance of distance protection was larger as the DG capacity was larger, which led to decrease the sensitivity. So some measures should be taken to decrease the DG influence on the distance protection and enhanced power supply reliability, which restricted the capacity of the access DG or adopted the adaptive distance protection equipment.