## 7 Conclusions  
This paper presents the difference of operation applied to the anti-islanding detection methods (AIDM) when an inverter is assumed as a voltage-controlled source (VC-VSI) or current-controlled source (CC-VSI). The existence of these two types of inverters limits the conventional resonant load concept required for island detection. Thus, the concept of zero and non-zero power flow (ZPF/NZPF) was introduced to extend the worst case for island detection for either CC-VSI or VC-VSI.  
The paper also clearly exposed the problem of several (passive methods, four active methods based on positive feedback strategies and impedance measurement) AIDMs operating towards VC-VSIs, especially focused on GSCIs (inverters under AC droop control strategies widely used to parallelize several units). Moreover, the effects of using AIDM methods based on voltage and frequency displacement, passive and active (positive feedback), for which the detection is not feasible, have been presented. The analysis was extended to impedance measurement (IM) AIDM, and its limitations have been detailed. This applicability limitations' analysis was conducted to present the detection frequency bandwidth (DFBW) as a new tool that allows determining which external conditions are compatible with the detection feasibility; in other words, which kind of loads and grids (weak/strong) will be in the detection framework. In this sense, the optimal frequency to determine the impedance can be calculated.  
The use of an AIDM (IM or other) in VC-VSI inverters requires a certain computational burden that usually is limited for such inverters. Thus, an improved IM method was proposed as a hybrid option based on existing methods, but focusing on the detection change and not on the impedance value accuracy. Thus, the proposed IM AIDM was split into high and low computation requirements, allowing it to be more time efficient.  
The proposed IM AIDM anti-islanding method has been designed, simulated, and tested in a 90-kVA four-wire three-phase inverter. The selection of the optimal frequency to be injected for the detection method has been selected using the proposed DFBW concept. All the conducted tests have concluded that the detection strategy can be implemented with low computational costs; less than 4% for the high frequency calculations and less than 0.5% for the low frequency ones. Furthermore, the IM AIDM applied is able to generate an island detection flag below 100 ms. The detection time obtained with this method is below the minimum detection time fixed by standards of 160 ms. It is important to remark that for an IM, the consideration of a resonant load under zero power flow conditions at the PCC does not add time to the detection. Thus, for the present IM proposal, the strictest time requirements have been obtained over ZPF or NZPF situations, and CC-VSI or VC-VSI became valid, general, and efficient AIDM with a reduced computational cost.  
Figure 22: Experimental results under non-zero power flow. (**a**) Zero power injection and 4.6-kW resonant load per phase. (**b**) Two-kilowatt power injection and 4.6-kW resonant load per phase. (**c**) Five-kilowatt power injection without local load. (**d**) Ten-kilowatt power injection and 4.6-kW resonant load per phase.  
**Author Contributions:** Conceptualization, M.L.-M. and D.H.-P.; methodology, M.L.-M. and D.H.-P.; software, M.L.-M.; validation, M.L.-M. and D.H.-P.; formal analysis, M.L.-M. and D.H.-P.; investigation, M.L.-M., D.H.-P., C.C.-A., R.V.-R., and D.M.-M.; resources, D.M.-M.; writing, original draft preparation, M.L.-M. and D.H.-P.; writing, review and editing, M.L.-M., D.H.-P., and C.C.-A.; visualization, M.L.-M., D.H.-P., C.C.-A., and R.V.-R.; supervision, [PERSON]-P., [PERSON], and [PERSON]-M.  
**Funding:** This research received no external funding.  
**Conflicts of Interest:** The authors declare no conflict of interest.