## 2 Islanding Detection Principles and Methods  
### Islanding Operation in Distribution Grids  
Nowadays, there are three types of distribution grids: passive, active, and microgrid. Figure 1 displays part of the passive, 10 kV distribution grid. The direction of electric energy flow is from the transmission grid (where power plants are connected) through the distribution grid to consumers. Assume that a fault has occurred on the 10 kV transmission line connecting busbars W1 and W0 (the location of the fault is represented by the lightning symbol in Figure 1). Then, the protective device will activate the CB6 switch, and the faulted line will be switched off. Simultaneously, the local load (see Figure 1) will be disconnected and left without electricity until the faulted line is repaired. In passive grids, the occurrence of islanding operation is not possible. If the DGs (i.e., a photovoltaic PV and a synchronous generator GEN) are connected to bus W0, the passive grid becomes active--Figure 2. Bus W0 is now a point of common coupling (PCC) between the DGs and the distribution grid. Let us assume the same event as in the case of Figure 1, i.e., line fault and its disconnection. Even though the faulted line is disconnected from the rest of the distribution network (via CB6), the DGs can still supply the part of the network connected to the PCC and the faulted line. The protection schemes and devices need to be updated to ensure the disconnection of the faulted line (CB 7 needs to be installed and connected to the protection device). Such a situation is an example of an islanding operation (or islanding), and in this case, islanding is unplanned and dangerous since the faulted line is energized; workers who want to repair the line may get hurt. Many scientific papers deal with protection coordination in distribution gridswith DGs; some examples are [8; 9]. However, when the faulted line is disconnected, part of the grid connected to PCC (see red circle in Figure 2) is still an unplanned island. To ensure the sustainability of the islanding operation within the grid island, the balance of active and reactive power between generation and consumption needs to be maintained, which requires regulation and flexibility of the DGs. Since the flexibility of some types of DG (i.e., PV and wind) is limited, it can lead to voltage (magnitude and angle) and frequency distortions [5]. Therefore, such an unplanned grid island must be quickly detected and eliminated (by shutting down all DGs in the grid island), which imposes the need for fast and efficient IDM. Figure 1: Passive distribution grid. Figure 2: Active distribution grid. The possibility of the planned islanding operation is part of the microgrid definition, so an additional device, such as an energy storage system, is needed (see Figure 3). When planned islanding operation in a microgrid is present, local load needs to be supplied in a reliable manner by the energy produced by DGs or by the energy stored in the energy storage system. Although microgrids can operate in both grid-connected and island mode, they mainly work in grid mode due to high costs. If an unplanned islanding operation of the microgrid occurs (i.e., due to the faults), it needs to be detected quickly, so effective IDM is also inevitable in the microgrid. Many authors [6; 7; 10; 11] agree that quality IDM needs to satisfy several criteria concerning detection time, accuracy, implementation cost, influence on power quality, etc. There are many technical standards worldwide in which restrictions such as the voltage range, frequency range, detection time, and quality factor are indicators for IDM [8]. Detection time differs from 0.2 s (defined in VDE 0126-1-1 [12]) to 2 s (defined in IEEE 1547 [13]). The main goal of all researchers is to create an IDM that can detect islanding as soon as possible. Regarding accuracy, IDM must reduce the non-detection zone (NDZ). The NDZ represents the zone where the IDM cannot detect islanding (illustrated in Figure 4).