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].