An adaptive virtual impedance fault current limiter for optimal protection coordination of islanded microgrids
Talal Elemamali Sati, Maher A. Azzouz
Abstract
Abstract Fault currents of inverter‐interfaced distributed generators (IIDGs) depend on inverter controllers. Thus, IIDGs fault currents are different than those of synchronous‐based DGs, both from the magnitude and waveshape perspectives. In the event of short‐circuit faults, droop‐based IIDGs switch between a voltage source and a current source, which increases the complexity and non‐linearity of short‐circuit current calculation (SCC). This paper proposes a new SCC algorithm that incorporates virtual impedance‐fault current limiters (VI‐FCLs) to enable modelling droop‐based IIDGs as a voltage source behind an impedance. The VI‐FCL was implemented as an additional control loop in the inverter control scheme to limit IIDG fault currents and achieve optimal protection coordination (OPC). Further, the VI‐FCL is adaptively adjusted to enhance overcurrent protection sensitivity. A two‐stage OPC algorithm for directional overcurrent relays (DOCRs) is developed. In Stage I, an optimal value for the adaptive VI‐FCLs and relay currents are calculated. Stage II aims at obtaining optimal DOCRs settings. Time‐domain simulations are used to demonstrate the effectiveness of the proposed adaptive VI‐FCL and the accuracy of the proposed SCC algorithm. The proposed SCC algorithm and the OPC program are successfully validated using an islanded microgrid that is part of a Canadian distribution system.