## 5 Conclusions  
This paper has presented two control strategies to achieve a fast and proper fault current limitation for four wire microgrid inverters. Both strategies are supported by a dynamic virtual resistance mechanism that allows for a fast transference between grid-(dis)connected operation modes. Concerning the AC side inverter operation, a seamless transference between grid-(dis)connected modes has been ensured obtaining short transients, below three grid cycles. For this purpose, a combination of an AC droop control based on dynamic phasors when grid-connected and master voltage/frequency control when grid-disconnected has been proposed. It has been demonstrated not only that this is a valid option to maintain the voltage behaviour between operation modes, but also it is able to make the grid-disconnected operation independent from the AC droop control loop constraints. Thus, the transients in grid-disconnected mode are only limited by the settling time of the voltage control loop. Furthermore, a varying virtual resistance is suggested for achieving the mentioned seamless transference, disabling the resistance value progressively when entering in grid-disconnected operation. It has been illustrated that the proper selection of the steady-state virtual resistance and the used variation ratio helps to avoid voltage sags in the transient phase between the operation modes, allows for being less sensitive to the reconnection process and, finally, provides a predominant resistive behaviour for the grid-connected operation. An over-load manager supervisor strategy based on thermal criteria has been introduced. This over-load strategy provides an increase in managing flexibility to a microgrid inverter in a grid-connected mode. The over-size challenge is then delegated to higher current switching devices, a not really cost sensitive component today in power converters. Thus, if the cooling system has enough thermal time lag, it is possible to obtain over-load skills in which the available over-load currents are managed by the proposed strategy. In case of prolonged over-load, the situation is detected and the inverter simply delivers a percentage of the nominal current to relieve the accumulated thermal stress. Finally, a short-circuit proof strategy is introduced as a fault current limiter and voltage regulator. This short-circuit proof strategy is validated though unipolar, bipolar, tripolar and tetrapolar pure short-circuits. As the control in grid-disconnected mode is only based on voltage and current control loops, the suggested strategy only considers voltage and current, but not power. The strategy is mainly based on computing the rms value of the delivered current. Then, the current and voltage can be quickly adapted to handle the fault occurrence and clearance. This is done maintaining sinusoidal waveforms in both cases by a simple calculation of two proportional factors and the use of a filter. In this sense, the time response of protective devices will be affected minimally. The time response has been a priority in this paper, obtaining fault current limitation and voltage regulation in the order of two-three grid cycles even considering low impedance short-circuits, a superior capability compared to similar literature. The feasibility of the proposed converter has been demonstrated not only at control level by simulation using Matlab/Simulink but also experimentally at a CENER-ATENEA four-wire microgrid. Figure 21: Scenario 4 (three-phase to neutral short-circuit). \(u_{x^{\prime\prime}n^{\prime}}\) (100 V/div) and \(i_{L2x}\) (150 A/div). **Author Contributions:** Conceptualization, D.H.-P., M.P.-G. and C.C.-A. ; Software, D.H.-P., M.P.-G. and C.C.-A. ; Validation, M.S., D.R. and M.A. ; Formal Analysis, D.H.-P.; Investigation, D.H.-P., M.P.-G. and C.C.-A. ; Methodology, D.M.-M.; Resources, M.S., D.R. and M.A. ; Writing--Original Draft Preparation, D.H.-P.; Writing--Review D.H.-P., C.C.-A. **Funding:** This research received no external funding.