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Currently, inverter-based stand-alone microgrids are gaining interest due to the advantages of obtaining energy from renewable sources. To manage the operation, these microgrids include storage systems connected in par- allel to the PCC through electronic inverters that are controlled as voltage sources in order to support the fre- quency and voltage at the PCC. For the purpose of ensuring P and Q sharing among inverters and also the synchronization stability of the microgrid, droop control is widely used, achieving a satisfactory performance in normal operation. Nevertheless, in the presence of overloads or short-circuits, the inverters must limit the current for self-protection, thereby modifying the performance of the system that then becomes prone to suffer transient stability problems. In this paper, first the performance of the inverter-based stand-alone microgrids with the conventional P-f and Iact-f droops is analyzed, obtaining the stability boundaries during current limitation. In order to always ensure the synchronization stability of the system, this paper then proposes the G-f droop that consists in employing the equivalent conductance seen by each inverter for its frequency droop control. Furthermore, as this variable always correctly represents the inverter power angle, the system dynamics are not affected by the operating conditions. The theoretical results have been validated by means of simulation and Hardware-In-the-Loop results, showing the superior performance of the proposed G-f droop This work has been supported by the Spanish State Research Agency (AEI) under grant PID2019-110956RB-I00/AEI/ 10.13039/501100011033, and by the Public University of Navarre through a PhD scholarship. Open access funding provided by Public University of Navarre.

The rapid uptake of renewable energy sources is causing synchronous generators (SG) to be replaced by power electronic inverters meaning these inverters need to offer the characteristics traditionally associated with SG. As a result, it has been proposed that the inverters should be controlled in grid-forming mode in order to support the voltage of the microgrids. Given that these inverters are controlled as a voltage source, temporary events such as short-circuits or overloads could cause currents that are far higher than the rated current. As the semiconductors used in power electronics are highly sensitive to overcurrents, this paper proposes a dual voltage-current control that provides the grid-forming inverters with the capability to quickly limit the current under any overload or short-circuit condition. The proposed method has been validated through experimental tests in stand-alone mode. This work hasbeen supported by theSpanish State Research Agency (AEI) under grantPID2019-110956RB-I00/AEI/10.13039/501100011033, and by the Public University of Navarre through a doctoral scholarship.

In order to decide whether passive or active damping is required in a three-phase inverter, a previous step is to assess the intrinsic damping of the system. However, few works focus on modeling this damping for DC/AC operation. This paper proposes two models to reproduce the damping sources: the simulation model, used to validate the system in large-signal, and the small-signal model, which can be employed for the controller design. Both models have been validated by means of experimental results for a 1.64 MVA inverter. This work was supported by the Spanish State Research Agency (AEI) under grants PID2019-110956RB-I00/AEI/10.13039 and DPI-2016-80641-R, and by the Public University of Navarre through a doctoral scholarship.

Due to the replacement of synchronous generators, grid operators are currently demanding to control grid-connected inverters in grid–forming mode to make them participate in the maintenance of the grid. To carry this out, the traditional droop controls based on the active and reactive powers are usually adopted, achieving a satisfactory performance in normal operation. Nevertheless, the power-frequency (P-ω) droop may become transiently unstable under voltage dips. This is because of the modification of the active power response caused by the inverter current limitation together with the voltage reduction. To enhance this, the power angle-frequency (δinv-ω) droop is proposed, consisting in employing an estimation of the inverter power angle as input to obtain the inverter frequency. The proposed method provides the inverter with the same performance as the P-ω droop in normal operation, while enhancing considerably the transient stability margins under current limitation. This is thanks to the higher variation of the inverter power angle with the phase difference between the inverter and the grid. Simulation results show the transient stability problems of the P-ω droop as well as the superior performance of the proposed δinv-ω droop control, which has also been verified by means of HIL results. This work has been supported by the Spanish State Research Agency (AEI) under grant PID2019-110956RB-I00 /AEI/ 10.13039/501100011033, and by the Public University of Navarre through a doctoral scholarship.

Energy-storage inverters under stand-alone operation must control the output voltage. However, existing methods for instantaneous voltage regulation are not suitable for high-power inverters due to their limited sampling frequency. To avoid this problem, a single feedback voltage loop with feedforward compensation is proposed in this paper, making it possible to actively damp the LCL filter resonance and thus facilitating the controller design. Simulation results demonstrate the fast reference tracking and the high-quality voltage obtained with non-linear loads. This work was supported by the Spanish State Research Agency (AEI) and FEDER-UE under grant DPI2016-80641-R and by the Public University of Navarre through a doctoral scholarship. The authors acknowledge the financial and continuous support of Ingeteam Power Technology.