Contents I. Introduction
The reduce of fossil energy and environmental concerns, accompanied by technical and economic reasons, gave rise to increased interest in penetration of distributed generators which consists of renewable energy generators and clean energy generators such as wind generator, photovoltaic generator and fuel cell. [1]–[4]. As the integration of distributed generation, energy storage systems, e.g. battery, super capacitor and flywheel energy storage and so on, and novel loads such as heat pumps and electric vehicles, and improving the utilizing efficiency of renewable energy, the concept of the microgrid was proposed [5]–[6]. According to the types of the voltage and current in the power system, the microgrid can be classified as AC microgrid, DC microgrid and AC-DC hybrid microgrid. The superiority of DC microgrid can be summarized as: (1) It is no need to control frequency and reactive power. (2) The conversion losses among dc loads, output sources and inverters are reduced. (3) The concept of synchronization with the AC power utility grid can be neglected. On account of above reasons, DC microgird is being spotlighted as system to achieve the aim of efficiency utilizing distributed energy and reliability of power supply for loads [7]–[11]. In the DC microgrid, the balance of power is only reflected by the DC bus voltage, thus keeping DC bus voltage constant is the hinge to realize instantaneous power balance in the system [12]–[17].
