Hybrid AC/DC Micro-Grids: Solution for High Efficient Future Power Systems

This chapter titled “Hybrid AC/DC Micro-grids: Solution for High Efficient Future Power Systems” presents a new configuration for future power systems which is the hybrid AC/DC gird for high efficient connection of the inherent AC and DC sources and loads. Three-phase AC power systems have been in dominant position for over hundred years due to invention of transformer and the inherent characteristic from fossil energy-driven rotating machines. However, the gradual changes of load types and distributed renewable generation (DRG) in AC local distribution systems provide food for consideration of adding DC networks. Renewable sources such as fuel cells and solar photovoltaics are DC inherent and should be connected to AC grid through DC/AC conversion techniques whereas some AC inherent renewable sources like wind generators also need DC links in their conversion systems to increase efficiency and mitigate power variation caused by intermittency and uncertainty. The disadvantage of AC grids for connection of DC inherent sources and loads as well as AC loads with DC links is that additional DC/AC or AC/DC converters are required, which may result in efficiency loss from the reverse conversion. In the other hand DC grids are resurging due to the development and deployment of renewable DC power sources and their inherent advantage for DC loads in commercial, industrial and residential applications. The number of power conversions in a DC microgrid has been significantly reduced to enhance system energy efficiency. A more likely scenario is the coexistence of both AC and DC micro-grids, which is so-called the hybrid AC/DC microgrid in order to reduce processes of multiple reverse conversions in an individual AC or DC microgrid and facilitate the connection of various renewable AC/DC sources and loads to power system. Therefore the concept of hybrid micro-grids, which can harmonize both AC and DC sources and loads, has been proposed for future high efficient power systems. Conventional AC and DC grids are interconnected together through the bidirectional AC/DC converter. The component model has been introduced. The control and operation of individual sources and energy storages are presented. The coordination control and power sharing techniques are also introduced.