Medium voltage distribution network with distributed generation line ground fault protection method
Aleksandrs Dolgicers, Ivars Zālītis, Jevgeņijs Kozadajevs

The invention pertains to the electric power engineering. When considering medium voltage distribution networks with distributed generation, there can be cases when due to a ground fault a power system connection line is disconnected only from one side leaving a network section with insufficient zero sequence current in order to completely disconnect the fault, which can result in damage to the network and risk to human lives. The proposed method tests the trip criteria of a directional ground fault protection and simultaneously determines the frequency and the rate of change of frequency of an analogue positive sequence voltage signal obtained with an analogue positive sequence filter. The change of voltage frequency can be expected because the active power balance of the separated system most probably would not be ensured. Using the rate of change of frequency as indicator of system link disconnection and zero sequence voltage magnitude as ground fault indicator the protection trip for the faulted line and distributed generators is issued after specified time delay. A time delay before reset of trip timer has been added to provide stable protection operation during intermittent ground faults. The proposed method can be used for creation of new medium voltage distribution line ground fault protection devices, and a skeleton diagram for its implementation is shown in fig.6.



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