Grid Integration of Distributed Energy Storage Devices in DC and AC Distribution Systems

Energy storage plays a vital role in solving many challenges experienced by the power system, such as the integration of renewable energy sources, load leveling, power quality control, etc. Energy storage systems suitable for distributed storage applications typically produce a DC voltage. Thus the focus of this dissertation is the efficient control and integration of DC energy storage devices in DC and AC distribution systems.;DC distribution systems are still in their infancy; however, a number of applications are ideally suited for DC power distribution, including data centers, fast charging stations for electric vehicles, more electric ships and aircrafts, etc. Among these applications, the fast charging station is of particular interest because of the widespread adoption of plug-in hybrid electric vehicles (PHEV) and pure electric vehicles (EV). A network of gas-station-equivalent fast charging stations is of great importance for solving the so-called "range anxiety" issue. As with gas stations, it is expected that multiple chargers will be co-located to form a charging station. This layout allows for the fast charging station to make use of a common AC/DC rectifier stage. After studying the power demand of the fast charging station, the power delivery architecture is proposed with DC power distribution and energy storage integration. With the energy storage system to provide peak power, the AC/DC frontend for the charging station can be sized based on the average power demand which is substantially lower than the peak power demand.;Following the design of the fast charging station, the AC/DC frontend plays a vital role in the sense that it provides the DC bus that powers all the chargers. It is justified that the 12-pulse diode rectifier is a suitable choice for the AC/DC frontend because of its high reliability and low cost. However, the 12-pulse diode rectifier suffers from relatively high level of current harmonics on the AC side. Traditional way to mitigate harmonic issues is to add separate filters (active or passive). In this dissertation, a novel approach is proposed to use the same DC/DC converter to integrate energy storage while simultaneously improving the power quality on both sides of 12-pulse diode rectifier. The first implementation of this approach utilizes the DC/DC converter to shape the DC side current drawn from the rectifier and thus indirectly eliminate AC side current harmonics. During this study, a new way to design the LC filters of the 12-pulse diode rectifier is developed, which results in substantially lower value of inductance and capacitance for the LC filter and lower harmonics on the AC side. Based on this result, the second implementation utilizes the DC/DC converter to inject virtual resistance into the LC filter and thus shape the rectifier output current. This implementation provides even better results in terms of harmonics elimination and minimizing VA rating of the DC/DC converter. It also provides the third functionality of the DC/DC converter, which is compensating the voltage ripple of the DC bus. Experiment and simulation results are given to verify all the presented statements.;For energy storage integration in AC distribution system, the goal is to find an efficient way to integrate dissimilar batteries into the grid. Since these batteries can be very different, they cannot be directly connected together to form a high voltage high capacity battery pack. The approach is to design DC/AC power converter that interface with each of the low voltage battery modules and form a module that includes energy storage device and power converter. A number of these modules can be linked together on the AC side of the power converter, to reach the required AC output voltage and directly interface with the grid. This approach holds the promise of higher system level efficiency and simplicity than the two-stage solution typically used. The well-known H-Bridge topology is used for the DC/AC power conversion. When the H-Bridges are cascaded together, the independent control of each H-Bridge becomes challenging because all of them are linked together. Existing control strategies all need a central controller to directly control all the H-Bridges or communicate with local controllers located within the H-Bridge modules, which limits the modularity of the system. In this dissertation, a new control strategy for cascaded H-Bridges is proposed with no central controller. The control strategy for each H-Bridge is completely implemented in the local controller and there is no communication between these local controllers. Experiment results verified the effectiveness of this control strategy and a small scale community energy storage system (CES) is built and integrated into the Future Renewable Electric Energy Distribution and Management (FREEDM) system.