A New Directional Element for Microgrid Protection

With protective relays exclusive for microgrid applications yet to be developed, microgrids are currently limited mostly to directional overcurrent relays (DOCRs) for protection. This paper unveils that the long-known small fault current of electronically coupled distributed generation (DG) units is merely one of several phenomena that DOCRs suffer from; the current magnitude in microgrids with DG units may even provide misleading indications of fault conditions. More important, commercial-grade torque- and impedance-based directional elements are shown to be adversely affected by the fault behavior of DG units. The misoperation may include incorrect fault direction identification or deactivation of directional elements by false detection of loss-of-voltage condition, ruling out existing impedance relays as workable solutions for microgrid protection as well. This paper also devises a new directional element that addresses these problems. The proposed element detects the direction of asymmetrical faults using the magnitude and angle of the superimposed negative-sequence impedance. For symmetrical faults, the proposed directional element uses the magnitude of superimposed positive-sequence impedance along with the positive-sequence current and torque angle. PSCAD/EMTDC simulation studies verify the performance of this new method.