Frequency Domain Impedance Based Protection for Flexible DC Distribution Grid

The flexible DC distribution grid is suitable for distributed energy resource integration and DC load power supply. However, the fast-increasing fault current has a great impact on the system when a short circuit fault occurs. Besides, the existing main protections are easily affected by high resistance faults and converters. To solve the aforementioned problems, a pilot protection method using frequency domain (FD) impedance was proposed. Firstly, a direction criterion based on the polarity of FD reactance is constructed to identify the reverse fault. Furthermore, the values of FD reactance and resistance are used to distinguish the forward internal and external faults. The method uses only local information to identify the close-up fault, and the pilot protection criterion to identify the remote-end or high resistance faults. This combination of single-end and pilot protection criteria makes the method have high reliability and fast speed. Moreover, no data synchronization is needed. Finally, extensive simulations were utilized to verify the effectiveness of the proposed method.