Figure 5 Representation single line diagram of a grid-connected DG source [19].

ACTIVE Anti-Islanding Methods

Active methods introduce intentional disturbances to the rest of the circuit and then analyze the feedback to decide whether there is an islanding or not [22]. The way of detecting islanding for active techniques cooperate with the power system operation by injecting perturbations. The idea of an active detection method is that this small perturbation will result in a significant change in system parameters when the DG is islanded, whereas the change will be negligible when the DG is connected to the grid [23].

Active Frequency Drift (AFD) Technique

In this technique, some disturbances of the current signal are injected into the Point of Common Coupling (PCC) depending on voltage at point of common coupling \(V_{\text{PCC}}\) which follows the fundamentals of current of the inverter\(\ \text{\ I}_{\text{inv\ }}\)[24-27]. Hence, in the grid-connected mode, this distortion does not disturb the current and voltage. Therefore, the frequency of the system has the same frequency of the grid. In the other hand, the grid-disconnected mode (islanding condition) has distortion leads to a phase difference between the current and voltage. Hence, this difference leads to a drift in frequency that obligates the UF/OF relays to cutoff the DG from the rest of the circuit.As shown in Figure 6, it is a comparison between a waveform of distorted DG output current with undistorted sine waveform. The chopping factor \(C_{f}\) in equation (5) is used to calculate the intensity of the disturbance as in the following equation:
\begin{equation} C_{\text{f\ \ }}=\ \frac{2t_{z}\ }{T}\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ (5)\nonumber \\ \end{equation}
Where, \(T\ \)is the voltage period of the grid and \(t_{z}\) is the dead time. However, this technique can easily be implemented using a microprocessor. But unfortunately It affects the power quality [28, 29, 30, 31].