## 1 Introduction  
Utility deregulation has sped up the possibilities for distributed energy resources (DER). The high penetration level of DERs is not only a well-consolidated reality today, but is also experiencing a steady rise thanks to a global environmental awareness of clean energies. A clear picture of this fact is their percentage share of the energy mix [1]. Although most DER inverters were conceived of in the past to behave as CC-VSI (current-controlled voltage source inverters), this situation is evolving. New control techniques, the use of information technologies, and the progress of power electronics have boosted alternative options for either grid-connected or grid-disconnected operation. Other behaviors like voltage-controlled (VC)-VSI inverters enhance grid flexibility services such as power sharing or non-zero voltage crossing between operation modes [2; 3; 4]. Currently, it is possible to distinguish between three main types of grid integration inverters [5]:  
* Grid supply inverters (GSI) are unidirectional CC-VSI when grid-connected, and their aim is to deliver the maximum power to the mains by means of maximum power point tracking(MPPT) algorithms [6]. They are the most common inverters installed in grid-connected systems, but always require a voltage master source for the energy exchange. As the situation of always delivering its maximum power is excessively rigid, some regulations like VDE 4105 [7] (Verband der Elektrotechnik, Elektronik und Informationstechnik) permit the grid-operator to manage the exchanged power by modifying the present AC system frequency. * Grid constitution inverters (GCI) are ideal VC-VSIs and act as the voltage reference source for GSIs in grid-disconnected operation. In grid-connected operation, it can be used as a passive back-up system, with no exchange of power, prepared to act in case of a black-out. In other words, during grid-connected operation, it acts as a mirror of the mains. However, this means that most of the time, a GCI is just wasting its internal losses. * Grid support and constitution inverters (GSCI) [8; 9; 10] are non-ideal VC-VSIs based on the AC droop control. They can respond in both operation modes (grid-connected and grid-disconnected) as voltage source, minimizing the impact on the transference in between modes. A GSCI manages the exchange power by establishing a delta of the amplitude and phase with respect to the mains in grid-connected mode. In grid-disconnected mode, they continue operating as VC-VSI, self-generating the amplitude and phase with the so-called secondary control. Any of the previously listed inverter types (GSI, GCI, or GSCI) can still be operating in grid-connected mode when an unintentional grid disconnection occurs, a situation called islanding in the literature. Islanding situations put the involved installation (inverter included) in a vulnerable position in terms of safety because of the possibility of the inverter holding the energization of a grid section, so much so that anti-islanding (AI) regulations can be found such as VDE 4105, IEEE 1547, or IEC 61727 [7; 11; 12], facing how to respond to this situation. These regulations define the threshold times to detect the islanding occurrence in order to stop feeding the fault and the times for considering a stable recovery of the mains to proceed with a following reconnection. The corresponding detection times are summarized in Table 1.  
Putting AI detection methods (AIDM) into historical context, they appeared when the renewable integration did not consider islanded mode feeding options. Therefore, it is no wonder that AI standards were targeted for CC-VSIs, being the control type most widely used in the case of conventional photovoltaic farms or wind farms. However, new grid integration roles have emerged, demanding more VC-VSIs. In the next few years, electrical distribution grids are expected to be smarter due to the current changing energy scenario. New concepts related to efficiency, reliability, robustness,