| In existing AC PV systems, PV generates DC voltage, this DC voltage is inverted to AC through a grid-tied inverter and on the load side the AC voltage is converted to DC once again. This AC power system requires two energy conversion steps, DC/AC conversion at the PV side and AC/DC conversion at the DC load side. Approximately 10% of PV generation is lost in these conversion steps. Renewable energy resources like solar and fuel cells generate DC power, on the other hand a large and increasing portion of loads use DC voltage internally such as LED lights, VFDs, battery chargers, and power supplies. There is a possibility to eliminate the costly and inefficient power inverters and install a DC network linking DC loads to the DC sources. DC microgrids and DC distribution power systems offer efficiency improvement, higher reliability better expandability and stability over their equivalent AC systems.;The aim of this work is to accelerate the deployment of DC systems by quantifying the benefits of DC systems compared to AC ,removing the technical barriers and addressing the issues related to safety and protection.;First, the two most popular architectures of DC microgrids are presented and the pros and cons of each configuration are discussed. Then, an emerging architecture for PV plus storage DC microgrids for commercial buildings is studied, which provides power to the DC loads from a local PV source without the need for a dedicated PV Maximum Power Point Tracking inverter or DC/DC converter.Moreover, control and topologies of DC microgrid's individual components including: DC/DC and AC/DC converters are discussed. Finally, three possible power balancing strategies for DC microgrids including: centralized, distributed and distributed with higher level of supervision are explained.;Also, a comparative study between AC PV systems and a specific architecture of DC PV systems are presented. The study examines the DC system energy performance in different locations across the U.S. for several commercial building types and operating profiles. The results from this comparative study are expanded to the generic DC systems.;DC microgrid control algorithm has to maintain stable power balance between generation and consumption. While ensuring power balance, the control algorithm should also control power sharing among sources and allow for multiple slack sources for redundancy Also it should be able to reliably mitigate the impact of PV and load variations in the microgrid from transferring to the AC power grid, when the microgrid is interacting with the main AC grid. This work introduces two power balancing methods that satisfies the requirements mentioned above. Then, the feasibility and effectiveness of the proposed strategies are verified in either lab scale system setup or using control hardware-in-the-loop simulations.;One of the purposes of this work is to quantitatively examine the stability and performance of the DC system. For the mentioned purpose, the model of the DC system including: all the components and their controllers are described in terms of mathematical models. Then, the mathematical model is used to properly set the response time of the components controller to ensure stability of the system in all different operation modes.;One of the limitations in using DC networks is the issues related to the current-limiting devices and circuit breakers. In this work, six configurations of DC circuit breakers from three categories are evaluated and the results are compared in terms of the time required by the breakers to interrupt the fault current, maximum DC breaking current, rated voltage, efficiency and current state of development. These six configurations include solid state circuit breaker, hybrid solid state circuit breaker with mechanical dis-connector, hybrid solid state circuit breaker with fast mechanical switch, mechanical circuit breaker with LC resonance path and a hybrid fault current limiting circuit breaker. Also a control approach for protection of voltage source converters when the fault happens at close to the converter's terminal is proposed. In the remainder, we examine the different grounding methods and system architectures and discuss the design trade-offs in terms of safety, reliability, detection, mitigation, noise, and cost. Moreover, impedance grounding, isolation, and bi-polar architectures are examined and their benefits with respect to these criteria are discussed. |
