## 2 Islanding Detection Principles and Methods
### IDM Method Classification  
IDM methods are generally classified into two distinct groups: remote and local methods. Remote methods are based on islanding detection from a distant site (grid operator/dispatcher-based), while local methods detect islanding based on the measurement at the DG site or at the PCC site [3]. The general systematization of the IDMs is shown in Figure 5.  
Figure 4: Illustration of the NDZ. Figure 5: Islanding detection method classification. Remote IDM usually requires communications between the distribution network management center and local DGs. The main advantage is that the accuracy of islanding detection is high, but additional communication equipment can be costly. Some of the usually investigated remote methods are based on the supervisory control and data acquisition systems (SCADA) [14], power line carrier communication [15], transfer trip method [16], the signal produced by disconnect [17], etc. On the other hand, local methods detect islanding operations based on locally available devices and their measurements. In most cases, they do not require additional equipment cost, but their disadvantage is lower accuracy than remote methods. Local IDMs are further divided into three main groups: passive, active, and hybrid methods (Figure 5). Passive local methods are based on local measurements of variables such as voltage, frequency, active power change, reactive power change, etc. In the case of an islanding operation, the above-mentioned variables will be changed (some to a lesser extent and some to a greater extent), and in the case that observed variables change beyond or below the predefined threshold value, the passive method will detect islanding. Since they rely on simple variable changes for detecting islanding conditions, they are easy to implement, and they are not expensive. Commonly observed variables are voltage (voltage unbalance [18], voltage harmonics [19], over/under voltage [20], rate of change of voltage--ROCOV [20], phase jump detection [19]), frequency (over/under frequency and rate of change of frequency--ROCOF [21]), and active or reactive power (rate of change of power--ROCOP [20]). On the other hand, defining appropriate variable thresholds and large NDZ are disadvantages of passive methods. A large NDZ occurs when local generation and consumption (in the islanded part of the network) are similar. Then, the change in the observed variables in the case of islanding is too low, and the passive method can fail in detecting the islanding operation. To overcome the limitations of passive methods, researchers began to use active methods. Active methods operate on the external disturbance signal injection principle, and islanding detection is based on the system response. Although these methods are slower than passive methods, they are more accurate and have lower NDZ. As disadvantages, active methods interfere with power quality, as the signal injection can distort the voltage waveform, which affects harmonic distortion. Some of the commonly used active methods are impedance measurements [21], active frequency drift [22], active frequency drift with positive feedback [23], frequency jump [24], Sandia frequency shift [24], Sandia voltage shift [24], virtual capacitor/inductor [25], variation in active and reactive power [26], and high-frequency signal injection [27]. To combine the benefits of both passive (fast detection and simple implementation) and active (small NDZ) methods and to alleviate their disadvantages, researchers created hybrid methods that engage both passive and active detection principles. The foundation of hybrid approaches is the presence of both active and passive components. Island detection is initially performed by the passive component. If successful, the procedure stops, and the islanding operation is detected quickly. If the passive component is unable to detect islanding, the active component is activated and injects signals to perform islanding detection.