Islanding detection method for an inverter-based distributed energy resource, ibder

An islanding detection method for an inverter-based distributed energy resource, IBDER, designed for connection to a power grid having a predetermined nominal harmonic grid voltage with a pre-determined nominal grid frequency, the method comprising:

• injecting a periodic perturbation in the output voltage or current of the IBDER;
• measuring a signal representing a physical quantity occurring at the IBDER as a result of the perturbation;
• analyzing the signal to derive at least one physical quantity from it;
• and
• comparing the or each physical quantity to a respective corresponding pre-determined reference quantity related to a non-islanding condition of the IBDER to determine whether an islanding condition exists for the IBDER;
• wherein the periodic perturbation comprises at least one sub-harmonic perturbation and deriving the at least one physical quantity from the signal comprises recovering a sub-harmonic component that is represented by the signal as a result of the sub-harmonic perturbation.