Current derivative based fault detection and localization in a ring bus DC microgrid

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Bai, Yunfei
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This thesis investigates the protection aspects of a 400 V DC microgrid with a ring bus architecture. Based on the existing literature, a model of a simple ring bus DC microgrid was developed in PSCAD simulation software to understand the fault behavior and protection requirements. Protection related features such as grounding circuits and DC circuit breakers were also included in the model. Analytical expressions for the initial fault currents were developed considering simplified equivalent circuits. The expressions indicate that challenges of the DC microgrid protection are operation speed and selectivity. Based on these results, a combined protection scheme with two different protection concepts and settings procedures were proposed. This proposed protection scheme divides DC microgrid faults into two categories: low resistance faults and high resistance faults. A current derivative protection method using local measurements was designed to detect the low resistance faults. A unit protection method using the ratio of current derivatives observed at two ends of the protected line was designed to detect the high resistance faults. The unit protection algorithm also allows the determination of fault locations. Various simulation results verified the operation of the proposed protection scheme as well as the fault location performance and showed that it satisfies speed and selectivity requirements of a typical low voltage DC microgrid. This research highlighted that using multiple protection methods cooperated together can significantly reduce the protection challenges and constraints in DC microgrids.
DC microgrid protection