Overcurrent Protection Schemes for Inverter-Based Islanded Microgrids

Date of Award


Publication Type


Degree Name



Electrical and Computer Engineering


M. Azzouz


Distributed generators, Active distribution networks, Inverter-based islanded microgrids



Creative Commons License

Creative Commons Attribution 4.0 International License
This work is licensed under a Creative Commons Attribution 4.0 International License.


Integration of distributed generators (DGs) into distribution networks results in active distribution networks (ADNs) characterized by bidirectional power ow and can evolve into microgrids. Microgrids could host synchronous-based DGs (SBDGs) or inverter-interfaced DGs (IIDGs). These networks have many advantages, including power loss reduction, deferring network upgrades, and backup for the main grid. Despite these advantages, IIDGs have limited fault current contributions, adversely impacting the protection coordination.

This dissertation investigates the overcurrent protection challenges faced by inverter-based islanded microgrids (IBIM). The aim is to devise reliable overcurrent protection schemes for IBIM in the fundamental and harmonic domains taking advantage of the exibility of the IIDG controllers. Optimal protection coordination (OPC) is achieved by developing a new short-circuit current calculation (SCC) algorithm for IIDGs in the fundamental domain and using a harmonic short-circuit current calculation (HSCC) algorithm.

An effective method is proposed by modifying the IIDG controller to include a virtual impedance-fault current limiter (VI-FCL) in the positive-sequence frame. The new SCC algorithm incorporates VI-FCLs to enable modeling droop-based IIDGs as a voltage source behind an impedance and protect inverter switches from overcurrent. The VI-FCL is implemented as an additional control loop in the inverter control scheme to limit IIDG fault currents and achieve 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. An optimal value for the adaptive VI-FCLs and relay currents is calculated in Stage I. Stage II aims at obtaining optimal DOCRs settings by solving the OPC problem as a constrained nonlinear programming problem. Time-domain simulations are used to demonstrate the effectiveness of the proposed adaptive VI-FCL and the accuracy of the proposed SCC algorithm.

OPC is usually solved for the original network topology with all lines, loads, and generation intact. However, power grids may experience contingencies due to transient events, e.g., generation or line outages. Low fault currents of IIDGs necessitate a sensitive and reliable protection scheme. The protection scheme utilizes adaptive VI-FCLs to limit IIDGs fault currents and achieve protection coordination. The two-stage OPC algorithm is modified to include the islanded topology and each possible topology following an N-1 contingency.

Limited fault currents in IBIM impose immense challenges on conventional overcurrent protection schemes. Therefore, a sensitive and selective protection scheme is proposed for islanded microgrids using a third harmonic voltage generated by IIDGs. The generated harmonic voltage results in a harmonic layer formed during short-circuit faults and is decoupled from the fundamental fault current, i.e., limited by IIDGs. Further, the generated harmonic voltage is adaptively adjusted based on fault severity. The proposed protection scheme utilizes harmonic directional overcurrent relays (HDOCRs) equipped with a dual time-current-voltage setting that senses the generated harmonic voltages and currents at the relay location to ensure an OPC of islanded microgrids. The OPC with the proposed dual setting is formulated as a constrained nonlinear program to determine the optimal forward and reverse relays' settings. The results ensure the ability of the proposed scheme to protect islanded microgrids without communication and its capability to reduce relays' operation times.

Lastly, the adaptive harmonic generation is utilized to develop a new harmonic-based overcurrent protection scheme for IBIM considering N-1 contingency. The proposed scheme employs HDOCRs with only forward trip characteristics. The OPC problem is formulated as a constrained nonlinear program to obtain the HDOCRs' setting. A two-stage OPC algorithm is developed to include the main topology and each possible single IIDG and line outage. The results confirm that a single set of relay settings can achieve OPC up to the maximum resistive fault on the main network topology while satisfying the N-1 criterion.