Compared with the output power perturbation method, perturbing the output frequency of the inverter voltage is a more effective method for detecting the islanding effect. Active Frequency Drift (AFD) is a common detection method for output frequency disturbance islanding effect at present.
The working principle of the active frequency offset method: by offsetting the frequency of the grid voltage sampling signal at the coupling point, it causes a disturbance to the voltage frequency at the load terminal. Under normal circumstances, the function of the phase-locked loop is to control the frequency error within a small range, but when the power grid fails, the phase-locked loop fails, the frequency of the inverter changes, and the addition of disturbances increases the error and accumulates to a certain range , there will be a passive method to detect it.
The active frequency offset method, because the direction of the disturbance is fixed, may have a counteracting effect on the method due to the nature of the load. For example, capacitive loads are lower in resistance than inductive loads, and inductive loads are higher in resistance. Therefore, if the disturbance direction just cancels out the load impedance characteristics, the disturbance may not accumulate.
In order to prevent this from happening, the active frequency drift method of positive feedback is adopted. The direction of the disturbance is dynamically determined by comparing the changes of the two frequencies before and after. If the frequency is increasing, it will be positive, and if the frequency is decreasing, it will be negative.