I. INTRODUCTION Environmental problems are concerned in recent years, and a lot of renewable energy source such as photovoltaic generations (PV), fuel cells, wind turbines and gas cogenerations have been installed into utility grids. It is well known that many distributed generations (DGs) could cause problems to the utility grids such as voltage rise, frequency variation, etc. Microgrid is one of the solutions to introduce renewable energy sources into the utility grid seamlessly [1]. In most researched microgrids, ac distribution is adopted as same as usual electric power systems [2]. In this case, inverters are essential for dc output type DGs and energy storages. Additionally, some ac output DGs also need inverters to convert the generator frequency to the grid frequency. Therefore, we proposed “low voltage bipolar type dc microgrid”, which is a novel power system for supplying super high quality power [3]. This system is suitable for dc output type DGs and energy storages. The dc power is distributed by 3-wire lines and converted to required ac or dc voltages by load side converters. These converters do not require transformers by choosing proper dc distribution voltage. The distributed scheme of load side converters also contributes to provide supplying high quality power. For instance, even if a short circuit occurs at one load side, it does not affect other loads. We constructed a laboratory scale model of the system and examined the fundamental characteristics [4]. The experimental results showed the system could supply power stably against voltage sag or blackout of the utility grid, etc.
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