Microgrid is a new concept for future energy distribution system that enables renewable energy integration. It generally consists of multiple distributed generators that are usually interfaced to the grid through power inverters. For the islanding operation of ac microgrids, two important tasks are to share the load demand among multiple parallel connected inverters proportionately, and maintain the voltage and frequency stabilities. This paper reviews and categorizes various approaches of power sharing control principles. Simultaneously, the control schemes are graphically illustrated. Moreover, various control approaches are compared in terms of their respective advantages and disadvantages. Finally, this paper presents the future trends.

Active power decoupling methods are developed to deal with the inherent ripple power at twice the grid frequency in single-phase systems generally by adding active switches and energy storage units. They have obtained a wide range of applications, such as photovoltaic (PV) systems, light-emitting diodes (LEDs) drivers, fuel cell (FC) power systems, and electric vehicle (EV) battery chargers, etc. This paper provides a comprehensive review of active power decoupling circuit topologies. They are categorized into two groups in terms of the structure characteristics: independent and dependent decoupling circuit topologies. The former operates independently with the original converter, and the latter, however, shares the power semiconductor devices with the original converter partially and even completely. The development laws for the active power decoupling topologies are revealed from the view of “duality principle,” “switches sharing,” and “differential connection.” In addition, the exceptions and special cases are also briefly introduced. This paper is targeted to help researchers, engineers, and designers to construct some new decoupling circuit topologies and properly select existing ones according to the specific application.

Virtual impedance, angle droop, and frequency droop control play important roles in maintaining system stability, and load sharing among distributed generators (DGs) in microgrid. These approaches have been developed into three totally independent concepts, but a strong correlation exists. In this letter, their similarities and differences are revealed. Some new findings are established as follows: 1) the angle droop control is intrinsically a virtual inductance method; 2) virtual inductance method can also be regarded as a special frequency droop control with a power derivative feedback; and 3) the combination of virtual inductance method and frequency droop control is equivalent to the proportional-derivative type frequency droop, which is introduced to enhance the power oscillation damping. These relationships provide new insights into the design of the control methods for DGs in microgrid.

This paper presents an active topology for power decoupling in single-phase ac-dc converters, featuring the effective suppression of low-frequency power ripple which is an inherent problem in single-phase energy conversion systems. This topology is composed of an H-bridge circuit and a ripple power-decoupling circuit. The ripple power-decoupling circuit shares one bridge arm with the H-bridge circuit so it has less additional components: just one IGBT, one diode, and an energy-storage inductor. Its modulation strategy is properly devised to coordinate the operation of decoupling circuit and H-bridge circuit. To enhance the power-decoupling performance, a direct ripple power-cancellation method based on energy-storage inductor is proposed. A multiresonant controller with feed-forward control is introduced for fast and precise current tracking. The effectiveness of this topology has been verified by detailed simulation studies as well as the laboratory prototype experiment results.