Improving directional decision algorithm of directional protective devices based on fault current analysis of distribution system with distributed generation
The integration of DG (Distributed Generation) into distribution system brings several advantages in aspect of economic, environment, and grid operation. On the other hand, it causes several protection coordination problems. This is because the integration of DG changes the power flow of distribution system from uni-direction to bi-direction. Furthermore, it is because different reverse fault current from DG sides is contributed according to the type of interconnection transformer and DG. Therefore, conventional protection scheme cannot be applied for protection coordination of distribution system with DG. Thus, directional decision algorithm is operated for protection coordination of distribution system with DG.
However, the algorithm cannot determine directional decision when zero-sequence voltage is very low during ground fault. This may happen problems such as sympathetic tripping or blinding of protection. To prevent the problems, the algorithm should be improved so that directional decision is made without zero-sequence voltage when the voltage is comparatively low during ground fault.
First of all, this thesis investigates the reverse fault current contributed toward fault location according to the type of interconnection transformer and DG. For this, the methodology to calculate the fault current according to the type of transformer and DG is introduced. Using the methodology, the fault current contribution from each transformer and DG is analyzed separately to eliminate the mixed contribution from both transformer and DG. Furthermore, the mixed fault current contribution from the different combination of transformer and DG is analyzed.
Based on the fault current contribution analysis, the thesis introduces improved algorithm to determine correct directional decision without zero-sequence voltage during ground fault. For this, the existing algorithm and its problem are investigated. The reliability evaluation of the algorithm is performed based on real distribution system with Inverter-Based Distributed Generation (IBDG) happened sympathetic tripping and blinding of protection. Furthermore, in the real distribution system, the characteristic of fault current contributed toward directional protective device is analyzed. Based on the analysis, the improved algorithm is introduced. Finally, it is verified that the introduce algorithm can determine correct directional decision although zero-sequence voltage is lower than the threshold value.
