The integration of converter-interfaced distributed generation in microgrids has raised several technical issues, including the successful operation of protective devices during faults. The protection issue is associated with the lack of large current injection during a fault, due to limits imposed by the semiconductor switches. This paper proposes a fault-detection and clearing control strategy method for symmetrical and asymmetrical line faults in a looped microgrid. The protection devices are simple overcurrent devices with the same settings, due to the looped microgrid topology. The proposed method is applied without using any kind of physical communication. The fault is detected by measuring indirectly the microgrid impedance. After the fault identification, the distributed energy resources (DERs) adjust their control in order to inject a current proportional to the measured microgrid impedance, according to a droop curve. This means that the DER closer to the fault injects a relatively larger current, achieving by this way a selective coordination of the protection means. The effectiveness of the proposed control strategy is evaluated through detailed simulation and experimental tests.

The high proliferation of converter-dominated Distributed Renewable Energy Sources (DRESs) at the distribution grid level has gradually replaced the conventional synchronous generators (SGs) of the transmission system, resulting in emerging stability and security challenges. The inherent characteristics of the SGs are currently used for providing ancillary services (ASs), following the instructions of the Transmission System Operator, while the DRESs are obliged to offer specific system support functions, without being remunerated for these functions, but only for the energy they inject. This changing environment has prompted the integration of energy storage systems as a solution for transfusing new characteristics and elaborating their business in the electricity markets, while the smart grid infrastructure and the upcoming microgrid architectures contribute to the transformation of the distribution grid. This review investigates the existing ASs in transmission system with the respective markets (emphasizing the DRESs’ participation in these markets) and proposes new ASs at distribution grid level, with emphasis to inertial response, active power ramp rate control, frequency response, voltage regulation, fault contribution and harmonic mitigation. The market tools and mechanisms for the procurement of these ASs are presented evolving the existing role of the Operators. Finally, potential barriers in the technical, regulatory, and financial framework have been identified and analyzed.

In this letter, a new methodology is proposed to extend the applicability of conventional power flow methods to islanded microgrids (MGs), operating with the droop control method. A slack bus is considered to be connected at a random MG node. The operational conditions of this slack bus as well as of the rest MG nodes are determined through an iterative procedure. The objective of this procedure is to force the active and reactive power flowing through the slack bus to zero, representing the islanded MG operation. The proposed approach has a similar accuracy with time-domain simulations, while it reduces significantly the required computational burden.

In this paper, a centralized control strategy for low-voltage networks is proposed, aiming to coordinate the droop characteristics of the distributed renewable energy sources. The main objectives are the overvoltage mitigation and the optimal network operation in terms of maximum power injection and uniform power curtailment. The distinct features of the proposed method are the incorporation of droop-based control techniques into the conventional optimal power flow formulation and the use of the sensitivity theory as a means to significantly decrease the computational burden. The validity of the developed method is justified using a genetic algorithm, while extensive comparisons with conventional centralized and decentralized control strategies indicate its superior performance in the operation of low-voltage networks.