## Main Anti-Islanding Detection Methods  
Anti-islanding detection methods (AIDM) have been evolving during the last few decades. A summary picture of the main AIDM can be seen in Figure 2, in which two categories can be defined: local or remote, according to if the AIDM is or is not part of the inverter itself. Then, three sub-categories can be considered; passive methods (PM), active methods (AM), and methods based on communications (C).  
The first adopted solution, called passive methods (PM), is based on the observation of the point of common coupling (PCC) (refer to Figure 1) to detect the loss of mains by means of monitoring the voltage or the frequency variations [31; 32; 33]. Passive detection methods (PM) operate by monitoring the voltage amplitude and the frequency of the PCC defining a non-detection zone (NDZ). The NDZ is essentially a window defined by the voltage amplitude and frequency thresholds where the inverter is not able to detect any island event. When the mains are off, due to any external occurrence (intentional or non-intentional), the voltage or the frequency could be pushed enough to exceed NDZ limits, as is described in Figure 3.  
However, if a local load, usually called resonant load, is consuming all the power (active and reactive) that the inverter is delivering when the utility goes off, neither the voltage nor the frequency will change significantly. This fact hinders the island detection. Thus, active methods (AM) appeared to overcome this challenge. Their aim is to observe, as well as to provoke a perturbation, boosting the variation of some electric variable, consequently enhancing that observation. According to their operating principle, they can be classified into three main groups:  
* Positive feedback (PF): These are active methods injecting some perturbation in order to generate a voltage or frequency drift to make evident the islanding transition. Several examples can be  
Figure 3: Non-detection zone (NDZ).  
Figure 2: Anti-islanding detection methods’ classification. PM, passive methods; AM, active methods; C, communications; IM, impedance measurement; PF, positive feedback.  
found in the literature: active frequency drift (AFD) [31; 32], slip mode frequency shift (SMS) [31; 32; 33; 34], Sandia voltage shift (SVS), and Sandia frequency shift (SFS) [31; 32; 35].
* Impedance measurement (IM): An impedance measurement mechanism detects the utility disconnection thanks to the impedance change of the PCC. Some authors started using external switching impedances devices [36; 37], but the increase of the computational capabilities of the inverters microcontrollers makes them able to inject harmonics under different approaches: simple harmonic injection [31; 38], double harmonic injection [31; 38], and also using inter-harmonics [39; 40; 41]. Finally, as indirect harmonic injection, PLL (phase locked loop) can also be found [29; 31], as well as system identification-based methods [42].
* Based on communications [25; 26]: Some signal generator based on different carrying means aims to provide the state signals of the mains. Some examples of such possibilities are radio, power line carrier communication, power signals, or wireless alternatives. They usually need a remotely located transmitter and receiver, and this responds to why it is considered a remote AIDM. However, it is possible to find some examples based on a power device itself under a local conceptualization [25].