# An Islanding Signal-Based Smooth Transition Control in AC/DC Hybrid Micro-Grids
## 1 Introduction  
The electricity demand has greatly increased with the rapid development of society and the economy. This exposes some problems of the fossil fuel-enabled power supply system--for example, the overuse of fossil and air pollution. Thus, it stimulates the utilization of renewable energies in large-scale centralized power plant and distributed power generators. Due to the intermittent feature of renewable energy, large-scale integration may bring instability of the grid. The micro-grid as a union of distributed power sources, energy storage systems, energy conversion devices, loads, supervisory systems, and protection has attracted more attention for its highly efficient energy utilization and flexible control [1; 2; 3]. The operation, control, and protection of alternating current (AC) micro-grids has been conducted by the researchers for a long time [4]. Recently, with the increase of direct current (DC) power equipment, such as mobile phones, computers, electric vehicles, frequency conversion devices, and so on, the concept of the DC micro-grid has been proposed to reduce redundant AC/DC converters [5; 6]. To further improve the power transfer efficiency, an AC/DC hybrid micro-grid has been proposed [7]. A hybrid micro-grid has both an AC bus and a DC bus, and they are interconnected through an interlink converter (ILC). A hybrid micro-grid provides an integration of all kinds of distributed generators (DGs) and loads. The number of converters can be reduced, and thus the power quality is improved. As a result, it becomes a prospective structure of distributed energy integration. A micro-grid has two operation modes: grid-connected mode and islanding mode [8; 9; 10]. In most cases, the micro-grid operates in grid-connected mode. When an external fault happens or the power quality cannot meet the requirements, the micro-grid disconnects from the utility grid by switching atthe point of common coupling (PCC), and enters islanding mode to provide continuous power supply for the local load, in order to have a reliable power supply. After the external fault is cleared, the micro-grid switches back to grid-connected mode [11; 12]. The transition may occur from grid-connected mode to islanding mode, or inversely from islanding mode to grid-connected mode. During the transition from grid-connected mode to islanding mode, due to the different control strategy, a new power management will be achieved; thus, the transition impacts occur. During the transition from islanding mode to grid-connected mode, besides the power management problem, synchronization transition impacts occur as well. Impulse current affects current quality, and may cause damage to electrical equipment [13; 14]. It has a bad effect on power grid stability, and in some extreme situations, it may even lead to splitting of the power grid. Therefore, it is important to provide a smooth transition control between different operational modes of the micro-grid. In [15], transient impulses during the smooth transition between on-grid and off-grid status are investigated regarding a demonstration project of the micro-grid at Polytechnic of Bari, Italy. This micro-grid is integrated into a low voltage distribution network, including programmable or nonprogrammable generators, loads, and storage systems. To reduce the impacts of impulses, it employs a supervisory control and data acquisition system to optimize the inner micro-grid power production, in order to minimize the exchange power with utility grid [15; 16]. Thus, a smooth transition may be achieved. However, most of the time, exchange power between micro-grid and utility grid is not easily controlled, as expected. In various power exchanges, a smooth transition is always required. In [17], voltage control in islanding mode and current control in grid-connected mode are performed, while an extra synchronous control strategy is used during the transition period to suppress transient bumps.