1 Improvement of active power sharing ratio of P /V droop controllers in low-voltage islanded microgrids T. L. Vandoorn, J. D. M. De Kooning, B. Meersman and L. Vandevelde Dept. of Electrical Energy, Systems & Automation, Ghent University, St-Pietersnieuwstraat 41, 9000 Gent, Belgium, e-mail: Tine.Vandoorn@UGent.be. Abstract—Microgrids provide a coordinated integration of distributed generation units in the electrical power system. By operating in islanded mode, they can increase the reliability of the system or electrify remote areas. For the power sharing and voltage control in low-voltage microgrids, active power/grid voltage droop control is highly suitable. In order to optimize the integration of renewable energy sources in the microgrid, a variant of this droop control, the voltage-based droop (VBD) control, has been presented. A well-known concern about droop controllers is the inherent trade-off between voltage control and power sharing. Therefore, in this paper, an additional control loop is included in the VBD control to improve the active power sharing ratio. In this way, accurate power sharing is achieved, i.e., the DG units respond to load changes exactly according to their droops. Although this modification relies on communication, it does not jeopardize the reliability of the microgrid as if the communication is lost, the basic VBD control still ensures a stable microgrid operation, while operating without the need for communication. Index Terms—distributed generation, droop control, micro- grid, power sharing accuracy I. I NTRODUCTION The strong increase of distributed generation (DG) units has a significant impact on the operation of the distribution networks. The networks are increasingly being confronted with congestion and voltage problems. Therefore, the current fit- and-forget strategy of integrating DG in the network is not a sustainable option concerning the reliability of the power systems. A more coordinated approach is required, which can be provided by integrating DG, loads and storage elements into microgrids. Microgrids have a single point of connection with the distribution network, which enables them to operate independently in the islanded mode or stay connected to the distribution network in the grid-connected operation condition [1]. Microgrids are likely to play a key role in the evolution of the smart grid [2]. In this sense, the smart grid can emerge as a system of integrated smart microgrids [3]. This papers focusses on the islanded operation of the mi- crogrid. In the islanded mode, the microgrid is independently responsible for both voltage and power control. As most microgrid elements are connected to the network through inverters, new control strategies for these inverters have been developed. Mirrored with conventional grid control, the droop control strategy has been implemented in microgrids. Both active power/frequency (P /f ) [4], [5] and active power/voltage (P /V ) [5]–[7] droops have been discussed. The latter copes with the mainly resistive lines in the low-voltage microgrids that are considered here. The voltage-based droop (VBD) control strategy of [8] extends the P /V droops to microgrids with a high share of renewables. The droop controllers ensure a stable microgrid operation, which is their key objective. However, sometimes, accurate power sharing needs to be guaranteed, thus, irrespective of the line parameters. In this way, a fair contribution of DG units in the microgrid control can be achieved. A disadvantage of droop control is that there is always a trade-off between the voltage control and the accuracy of the power sharing. The accuracy of power sharing or power sharing ratio reflects the contribution of each unit to cope with load variations compared to the other units. Perfectly accurate power sharing is achieved when the load variations are picked up by the DG units (ΔP ) exactly according to their droops (K). These droops are dependent on the ratings of the units and the controllability of the energy source. For example, gas- fired power stations contribute more to the primary control to cope with load variations than nuclear power plants. This is analogous for DG units, where fully-controllable DG units contribute to the power sharing proportionally with their rated power, while less controllable units (such as many renewables) will contribute less. The grid voltage V is a local parameter and can be different in different network locations, which can affect the power sharing ratio. Therefore, in the P /f - Q/V droop control, the reactive power (Q) sharing ratio may differ from the droop ratio, i.e. inaccurate reactive power sharing. Similarly, the active power sharing ratio can be inaccurate in the P /V - Q/f droop controllers. Several solutions to increase the power sharing accuracy have been presented in literature, focussing on the P /f - Q/V droop controllers. In [9], a small high- frequency signal is injected in the system as control signal for the output active and reactive power. However, the circuitry required to measure the small real power variations in this signal adds to the complexity of the control [10]. In [10], each unit regulates its terminal voltage based on the reference voltage that is obtained from, firstly, the conventional Q/V droops and, secondly, a correction term based on the measured load voltage. An analogous method to achieve accurate power sharing by introducing load voltage feedback is presented in [11]. In this paper, the method of [10] is modified to improve the active power sharing ratio in low-voltage networks. The mod- ifications are twofold. Firstly, [10] focusses on Q/V droops in the conventional P /f - Q/V droop control, while here, the P /V droops in the VBD control are adapted. Secondly, in microgrids, there is not a single load voltage. The loads and DG units are distributed in the network and the line impedances in between cannot be neglected. Hence, this paper suggests to communicate the active power output of the units instead of the load voltage to achieve accurate power sharing. In § II, the power sharing of the Q/V and VBD controllers are analysed. The power sharing ratio is improved in § III. This is achieved by, firstly, communicating the load voltage analogously to [10], and secondly, by communicating output