Towards a Fault-Tolerant grid interface system for DC power distribution networks in Green Buildings

DC power distribution is becoming more prevalent in buildings, starting in niche applications such as data centers and gradually moving into more mainstream applications. There is need for the DC network in the building (termed as nanogrid) to interface with the external AC utility grid for energy exchange, security and reliability. In this study, we examined the power electronic converter topologies available for this function, as well as how to integrate the secondary objectives of metering, protection, communications and energy efficiency optimization. A three-phase bidirectional buck rectifier is chosen as a case study to interface the external AC grid to the internal DC nanogrid. The output of the bidirectional rectifier is chosen to be 380Vdc, while having to fulfill the requirements of smooth DC output voltage, unity power factor and low THD. The implementation of space vector PWM for this rectifier gives good dynamic response during the power flow change-over. The increase in number of power semiconductor switches may however lower the reliability of this topology. To assess the reliability of this converter, the possible internal faults of this topology are visualized using a polar plot of output voltage against phasor of the source voltage from the AC grid. The fault mode can be then identified easily from the fault pattern in polar plot. Simulation results are presented to support the analysis and recommendations.