Protection of bi-directional AC-DC/DC-AC converter in hybrid AC/DC microgrids

This paper presents an investigation on the various faults that a bi-directional AC-DC/DC-AC converter linking a DC microgrid to the main AC grid may encounter. In addition, the protection of this converter against line-line, line-ground, one phase open and DC bus faults is developed, presented and discussed. The converter proved to be a very effective tool to transfer any desired amount of power from either of the sides to the other quickly. However, due to its importance, it has to be well protected. The fault current through the converter for any fault on the AC side was found not to exceed 2-4 p.u. This is because of the impedance of the filter, which limits the fault current. This increases the complexity of detecting this fault using the amplitude of the current. However, another quantity such as the zero sequence current can be used for this fault detection. On the other hand, for a fault on the DC side, the charged capacitor discharges in a very short time. Hence, the derivative of the current can be used for detecting the fault. A detailed discussion about these concepts is presented in this paper.