1 Transition from Islanded to Grid-Connected Mode of Microgrids with Voltage-Based Droop Control T. L. Vandoorn, Student Member, IEEE, B. Meersman, Student Member, IEEE, J. D. M. De Kooning, Student Member, IEEE and L. Vandevelde, Senior Member, IEEE Abstract—Microgrids are able to provide a coordinated inte- gration of the increasing share of distributed generation (DG) units in the network. The primary control of the DG units is generally performed by droop-based control algorithms that avoid communication. The voltage-based droop (VBD) control is devel- oped for islanded low-voltage microgrids with a high share of renewable energy sources. With VBD control, both dispatchable and less-dispatchable units will contribute in the power sharing and balancing. The priority for power changes is automatically set dependent on the terminal voltages. In this way, the renewables change their output power in more extreme voltage conditions compared to the dispatchable units, hence, only when necessary for the reliability of the network. This facilitates the integration of renewable units and improves the reliability of the network. This paper focusses on modifying the VBD control strategy to enable a smooth transition between the islanded and the grid-connected mode of the microgrid. The VBD control can operate in both modes. Therefore, for islanding, no specific measures are required. To reconnect the microgrid to the utility network, the modified VBD control synchronises the voltage of a specified DG unit with the utility voltage. It is shown that this synchronisation procedure significantly limits the switching transient and enables a smooth mode transfer. Index Terms—Distributed generation, droop control, microgrid, synchronisation I. I NTRODUCTION The environmental goals and liberalisation of energy markets have led to a huge increase of distributed generation (DG) units (often with renewable energy sources) in the electrical power system. This has a large impact on the power system planning and operation [1]–[4]. With a high penetration of DG, the voltage and congestion problems can significantly reduce the hosting capacity of the networks for DG [5]. Also, as the ratio DG power versus centrally-generated power increases, the DG units will need to assist in ancillary services such as reserve provision. Therefore, the fit-and-forget strategy for integrating DG is not a sustainable option and a coordinated approach with active control of these units will be required. With respect to the coordinated integration of DG, the micro- grid concept has been developed. Microgrids are power systems that consist of an aggregation of loads, sources and storage elements [6], [7]. An important characteristic of the microgrid is that it is regarded as a controllable entity from the utility networks This work is financially supported by the FWO-Vlaanderen (Research Foun- dation - Flanders, Belgium). T. Vandoorn thanks the FWO for the Fellowship received. The research was carried out in the frame of the Inter-university Attraction Poles programme IAP-VII-43, funded by the Belgian Government. The research of J. D. M. De Kooning is funded by the Special Research Fund (BOF) of Ghent University (Belgium). T. L. Vandoorn, B. Meersman, J. D. M. De Kooning and L. Vandevelde are with the Electrical Energy Laboratory (EELAB), Department of Electrical Energy, Systems and Automation (EESA), Ghent University, Sint-Pietersnieuwstraat 41, B-9000 Ghent, Belgium, e-mail: Tine.Vandoorn@UGent.be point of view. This is made possible because the microgrid has a single point of common coupling (PCC). Dependent on the state of the PCC switch, the microgrid can operate in grid-connected and islanded mode. Microgrids are likely to play a key role in the evolution of the smart grid [8], [9]. It is expected that the smart grid will emerge as a system of integrated smart microgrids [10]. As most DG units are connected to the network through a voltage- source inverter (VSI), proper microgrid operation requires proper inverter control. The inverter control of the DG units in grid-connected micro- grids is related to the delivery of a certain amount of power to the network. Generally, grid-following units, with current controllers that track the measured terminal voltage, are used [11]–[13]. The grid stability and power quality remain a task of the transmis- sion system. In the islanded mode, the inverter controllers are responsible for the microgrid stability and power quality. Grid- forming, thus voltage-controlled, units are required because of the lack of a utility network forming the reference voltage. Generally, droop-based control algorithms are used for the primary control in islanded microgrids, to avoid communication and single points of failure for a reliable system operation. Droop control can be classified in P /f and P /V g droops, with P the active power, f the grid frequency and V g the terminal rms voltage. The former focusses on mimicking the P /f controllers of the synchronous generators connected to the transmission networks [7], [14]–[16]. Low-voltage microgrid are considered in this paper, which generally lack the rotating inertia the conventional grid control is based upon. Also, the lines are predominantly resistive such that there is a linkage between P and V g , not frequency (through phase angles). Therefore, P /V g droops have been used in [17], [18]. The P /V g droop controller focusses on dispatchable DG units. A variant of these P /V g droops, called the voltage-based droop (VBD) control, has been presented in [19]. This control strategy takes into account the less dispatchable nature of various DG units. It also enables to use the voltage as trigger for primary load and storage control in [20]. An important benefit of the VBD control strategy is that it can be used in both the grid-connected and the islanded mode of the microgrid. Hence, in the event of islanding, i.e., transition from grid-connected to islanded mode, the microgrid will remain online without changing the control strategy. During the transition from islanded to grid-connected mode on the other hand, synchronisation of the microgrid voltage to the utility volt- age is required. Otherwise, large switching transients in voltage and current may occur. Hence, in this paper, a synchronisation procedure is included in the VBD controller, while retaining the same control strategy in the grid-connected and islanded mode. In [21], synchronisation of a DG unit with a modified control strategy in both modes is presented. In [22]–[24], the synchronisation of a multiple DG microgrid is achieved by