Abstract

DC power distribution systems (especially DC nanogrids) are becoming a great area of interest for researchers, that can lead to a better integration of DERs and supplying local loads in a more efficient way compared to AC systems. DC nanogrids for Net-Zero Energy Homes (NZEHs) are expected to include a number of Distributed Energy Resources (DERs) in relatively close proximity. The control structure can be based on a hierarchical approach with DC bus signaling (DBS). Power electronics interfaces (uni-directional Boost for sources and bi-directional Class-C for storage units) are usually employed as the interface of DERs in DC nano and microgrids. However, these power interfaces are fault-sensitive, meaning that in case of a fault in the DC bus, with a DC bus voltage lower than the source voltage, the upper anti-parallel diode conducts. Thus, one loses control of the current injected into the DC nanogrid. In conventional systems, this current is typically high enough to open the DC Circuit Breakers (CBs). One issue in DC nanogrids is the difficulty in making only the DC CBs close to the fault to open. Various control schemes have been developed for power balance and energy management, but fault protection still remains an issue.
This Thesis discusses the realization of a fault detection and isolation scheme which is based on the coordination of fault-insensitive power electronics interfaces and low cost contactors. A bi-directional 4-switch DC-DC converter is employed in this work which allows the control of the injected current regardless the source voltage to be lower or higher than the DC bus voltage. Fault current limiting and blocking capabilities of the fault-insensitive converter allows the use of low cost and lower rated contactors. The identification of which segment(s) of the DC nanogrid is(are) faulted and which contactor(s) should open is based on peer-to-peer communication between DERs. Following the detection of a fault, the DERs decrease the injected current to a value low enough for safe action of the contactors.
Finally, the proposed concepts are verified with hardware experiments. Experimental results with power electronics interfaces operating with DC Bus Signaling (DBS) with VI curves including droop, current limiting and a CAN communication system are presented. It is shown that the DC grid can be protected against faults by coordinating the action of power interfaces and contactors, and only the faulted segment is isolated keeping the healthy part of the DC nanogrid energized.