1 Abstract--The increasing amount of renewable power gene- ration systems is a challenging issue for the control and operation of the electrical networks. One of the main issues is their lack of inertia, which is becoming a greater problem as much as the share of the power plants based on traditional synchronous generators gets reduced. In this regard the new grid codes ask these plants to provide new functionalities such as the frequency support and inertia emulation. In this paper, a synchronous power controller for grid- connected converters is proposed as a good solution for the renewable generation systems with energy storage. It provides inertia, damping and flexible droop characteristics. Different from the faithful replication of the swing equation of synchronous machines, an alternative control structure is proposed, by which the damping and inherent droop slope can be configured independently to meet the requirements in both dynamics and frequency regulations. Analysis and experimental results are both shown to validate the proposed controller. Index Terms--DC-AC power conversion, Inertia emulation, Power generation control, Synchronous power controller. I. INTRODUCTION raditional generation plants based on renewable energy sources (RES) act as grid-feeding systems, which deliver the maximum power from the primary source to the grid [1]. As much as the penetration of the RES generation plants increases, the insufficient inertia in the whole network could undermine its operating stability. Therefore, the control objectives and dynamics of the grid-connected converters need to be adjusted to take more responsibilities in grid supporting issues, such as inertia emulation, frequency regulation and voltage support. The droop control strategy has been implemented in the control of grid-connected converters as seen in [2]–[6]. Even though the outer droop loops allow the grid-connected converters to adjust the steady-state power injection according to the demand of the grid, the transient behaviors of these This work has been partially supported by the Spanish Ministry of Economy and Competitiveness under the project ENE2014-60228-R. W. Zhang, A. M. Cantarellas, J. Rocabert, A. Luna and P. Rodriguez are with the Department of Electrical Engineering, Technical University of Catalonia, Barcelona 08222 Spain (e-mail: weiyi.zhang@estudiant.upc.edu). A. M. Cantarellas and P. Rodriguez are also with Abengoa Research, Abengoa, Seville 41014 Spain (e-mail: pedro.rodriguez@abengoa.com). converters are not good enough. The lack of inertia is still a drawback, which cannot be improved by droop control without jeopardizing the stable operation of a grid-connected con- verter. A solution to improve the dynamics of the converters is to specify the properties of the grid-connected converters in such a way that it acts like a synchronous generator (SG), as is proposed in [7]. It is an approach that has been drawing a lot of interests in the recent years. This trend is initiated by the fact that conventional grid synchronization algorithm like Phase-locked loop (PLL) presents not inertia characteristics, and the dynamics of any supporting strategy is affected by the inherent dynamics of the PLL. Moreover, a PLL might have a negative impact on the control performance under weak ac grids [8]. A control implementation scheme for the emulation of SG is proposed in [9], in which the loop filter of the conventional PLL is modified to emulate the inertia and damping character- istics. Other similar design or proposals incorporating inertia and damping in a PLL can be found in [10] and [11]. Based on the strategy in [9]–[11], the inertia effect is only linked to the grid frequency, and does not essentially exist while reacting to the power input variations. Then the perturbations in the dc side will be directly transmitted to the ac side without inertia. In addition, the inertia effect does not exist in island operation based on this type of design. Another implementation strategy for emulating SG is proposed and analyzed in [8], in which the PLL is substituted by an active power synchronization loop. Even though this strategy has shown advantages in the interconnection of weak ac grids [12], the inertia and oscillation damping are not specifically addressed, and it has to be switched to a PLL- based vector current control under severe ac faults. In [13]–[15], a torque synchronization loop is designed considering inertia and damping characteristics. A similar strategy is also adopted in [16]. The authors in [17]–[19] propose a synchronous power controller presenting inertia and damping characteristics, and particularly a virtual admittance structure is proposed. The authors in [20] and [21] indicate that the inertia can also be implemented in the microgrid droop controller, making use of the first-order low-pass filter which is traditionally used only for damping the measurement noise. Synchronous Power Controller with Flexible Droop Characteristics for Renewable Power Generation Systems W. Zhang, Student Member, IEEE, A. M. Cantarellas, Student Member, IEEE, J. Rocabert, Member, IEEE, A. Luna, Member, IEEE, and P. Rodriguez, Fellow, IEEE T